Publications

Estimation of the phase delay between interferometer arms is the core of transmission phase microscopy. Such phase estimation may exhibit an error below the standard quantum (shot-noise) limit, if the input is an entangled two-mode state, e.g., a N00N state. We show, by contrast, that such supersensitive phase estimation (SSPE) is achievable by incoherent, e.g., thermal, light that is injected into a Mach-Zehnder interferometer via a Kerr-nonlinear two-mode coupler. Phase error is shown to be reduced below 1/n¯, n¯ being the mean photon number, by thermal input in such interferometric setups, even for small nonlinear phase shifts per photon pair or for significant photon loss. Remarkably, the phase accuracy achievable in such setups by thermal input surpasses that of coherent light with the same n¯. Available mode couplers with giant Kerr nonlinearity that stem either from dipole-dipole interactions of Rydberg polaritons in a cold atomic gas, or from cavity-enhanced dispersive atom-field interactions, may exploit such effects to substantially advance interferometric phase microscopy using incoherent, faint light sources.

Sub-wavelength arrays of atoms exhibit remarkable optical properties, analogous to those of phased array antennas, such as collimated directional emission or nearly perfect reflection of light near the collective resonance frequency. We propose to use a single-sheet sub-wavelength array of atoms as a switchable mirror to achieve a coherent interface between propagating optical photons and microwave photons in a superconducting coplanar waveguide resonator. In the proposed setup, the atomic array is located near the surface of the integrated superconducting chip containing the microwave cavity and optical waveguide. A driving laser couples the excited atomic state to Rydberg states with strong microwave transition. Then the presence or absence of a microwave photon in the superconducting cavity makes the atomic array transparent or reflective to the incoming optical pulses of proper frequency and finite bandwidth.

We examine spin excitation or polarization transfer via spin chains with long-range exchange interactions in the presence of diagonal and off-diagonal disorder. To this end, we determine the mean localization length of the single-excitation eigenstates of the chain for various strengths of the disorder. We then identify the energy eigenstates of the system with large localization length and sufficient support at the chain boundaries that are suitable to transfer an excitation between the sender and receiver spins connected to the opposite ends of the chain. We quantify the performance of two transfer schemes involving weak static couplings of the sender and receiver spins to the chain, and time-dependent couplings realizing stimulated adiabatic passage of the excitation via the intermediate eigenstates of the chain which exhibits improved performance.

Our goal in this article is to elucidate the rapport of work and information in the context of a minimal quantum-mechanical setup: a converter of heat input to work output, the input consisting of a single oscillator mode prepared in a hot thermal state along with a few much colder oscillator modes. The core issues we consider, taking account of the quantum nature of the setup, are as follows: (i) How and to what extent can information act as a work resource or, conversely, be redundant for work extraction? (ii) What is the optimal way of extracting work via information acquired by measurements? (iii) What is the bearing of information on the efficiency-power tradeoff achievable in such setups? We compare the efficiency of work extraction and the limitations of power in our minimal setup by different, generic, measurement strategies of the hot and cold modes. For each strategy, the rapport of work and information extraction is found and the cost of information erasure is allowed for. The possibilities of work extraction without information acquisition, via nonselective measurements, are also analyzed. Overall, we present, by generalizing a method based on optimized homodyning that we have recently proposed, the following insight: extraction of work by observation and feedforward that only measures a small fraction of the input is clearly advantageous to the conceivable alternatives. Our results may become the basis of a practical strategy of converting thermal noise to useful work in optical setups, such as coherent amplifiers of thermal light, as well as in their optomechanical and photovoltaic counterparts.

A minimal model of a quantum thermal machine is analyzed, where a driven two level working medium (WM) is embedded in an environment (reservoir) whose spectrum possesses bandgaps. The transition frequency of the WM is periodically modulated so as to be in alternating spectral overlap with hot or cold reservoirs whose spectra are separated by a bandgap. Approximate and exact treatments supported by analytical considerations yield a complete characterization of this thermal machine in the deep quantum domain. For slow to moderate modulation, the spectral response of the reservoirs is close to equilibrium, exhibiting sideband (Floquet) resonances in the heat currents and power output. In contrast, for faster modulation, strong-coupling and non-Markovian features give rise to correlations between the WM and the reservoirs and between the two reservoirs. Power boost of strictly quantum origin ('quantum advantage') is then found for both continuous and segmental fast modulation that leads to the anti-Zeno effect of enhanced spectral reservoir response. Such features cannot be captured by standard Markovian treatments.

We put forward the concept of work extraction from thermal noise by phase-sensitive (homodyne) measurements of the noisy input followed by (outcome-dependent) unitary manipulations of the postmeasured state. For optimized measurements, noise input with more than one quantum on average is shown to yield heat-to-work conversion with efficiency and power that grow with the mean number of input quanta, the efficiency and the inverse temperature of the detector. This protocol is shown to be advantageous compared to common models of information and heat engines

Characterization of microstructures in living tissues is one of the keys to diagnosing early stages of pathology and understanding disease mechanisms. However, the extraction of reliable information on biomarkers based on microstructure details is still a challenge, as the size of features that can be resolved with noninvasive magnetic resonance imaging (MRI) is orders of magnitude larger than the relevant structures. Here we derive from quantum information theory the ultimate precision limits for obtaining such details by MRI probing of water-molecule diffusion. We show that currently available MRI pulse sequences can be optimized to attain the ultimate precision limits by choosing control parameters that are uniquely determined by the expected size, the diffusion coefficient, and the spin relaxation time T-2. By attaining the ultimate precision limit per measurement, the number of measurements and the total acquisition time may be drastically reduced compared to the present state of the art. These results are expected to open alternative avenues towards unraveling diagnostic information by quantitative MRI.

In this book, Henry Bar, physicist and the first quantum superhero, guides the reader through the amazing quantum world. His hair-raising adventures in his perilous struggle for quantum coherence are graphically depicted by comics and thoroughly explained to the lay reader. Behind each adventure lies a key concept in quantum physics. These concepts range from the basic quantum coherence and entanglement through tunnelling and the recently discovered quantumdecoherence control, to the principles of the emerging technologies of quantum communication and computing. The explanations of the concepts are accessible, but nonetheless rigorous and detailed. They are followed by an account of the broader context of these concepts, their historic perspective, current status and forthcoming developments. Finally, thought-provoking philosophical and cultural implications of these concepts are discussed. The mathematical appendices of all chapters cover in a straightforward manner the core aspects of quantum physics at the level of a university introductory course. The Quantum Matrix presents an entertaining, popular, yet comprehensive picture of quantum physics . It can be read as a light-hearted illustrated tale, a philosophical treatise, or a textbook. Either way, the book lets the reader delve deeply into the wondrous quantum world from diverse perspectives and obtain glimpses into the quantum technologies that are about to reshape our lives. This book offers the reader an enjoyable and rewarding voyage through the quantumworld.

We propose to implement a quantized thermal machine based on a mixture of two atomic species. One atomic species implements the working medium and the other implements two (cold and hot) baths. We show that such a setup can be employed for the refrigeration of a large bosonic cloud starting above and ending below the condensation threshold. We analyze its operation in a regime conforming to the quantized Otto cycle and discuss the prospects for continuous-cycle operation, addressing the experimental as well as theoretical limitations. Beyond its applicative significance, this setup has a potential for the study of fundamental questions of quantum thermodynamics.

The efficiency of cyclic heat engines is limited by the Carnot bound. This bound follows from the second law of thermodynamics and is attained by engines that operate between two thermal baths under the reversibility condition whereby the total entropy does not increase. By contrast, the efficiency of engines powered by quantum non-thermal baths has been claimed to surpass the thermodynamic Carnot bound. The key to understanding the performance of such engines is a proper division of the energy supplied by the bath to the system into heat and work, depending on the associated change in the system entropy and ergotropy. Due to their hybrid character, the efficiency bound for quantum engines powered by a non-thermal bath does not solely follow from the laws of thermodynamics. Hence, the thermodynamic Carnot bound is inapplicable to such hybrid engines. Yet, they do not violate the principles of thermodynamics. An alternative means of boosting machine performance is the concept of heat-to-work conversion catalysis by quantum non-linear (squeezed) pumping of the piston mode. This enhancement is due to the increased ability of the squeezed piston to store ergotropy. Since the catalyzed machine is fueled by thermal baths, it adheres to the Carnot bound. We conclude by arguing that it is not quantumness per se that improves the machine performance, but rather the properties of the baths, the working fluid and the piston that boost the ergotropy and minimize the wasted heat in both the input and the output.

Interesting effects arise in cyclic machines where both heat and ergotropy transfer take place between the energising bath and the system (the working fluid). Such effects correspond to unconventional decompositions of energy exchange between the bath and the system into heat and work, respectively, resulting in efficiency bounds that may surpass the Carnot efficiency. However, these effects are not directly linked with quantumness, but rather with heat and ergotropy, the likes of which can be realised without resorting to quantum mechanics.

We study the impact of cooperative many-body effects on the operation of periodically-driven quantum thermal machines, particularly heat engines and refrigerators. In suitable geometries, N two-level atoms can exchange energy with the driving field and the (hot and cold) thermal baths at a faster rate than a single atom due to their SU(2) symmetry that causes the atoms to behave as a collective spin-N/2 particle. This cooperative effect boosts the power output of heat engines compared to the power output of N independent, incoherent, heat engines. In the refrigeration regime, similar cooling-power boost takes place.

We review our unified optimized approach to the dynamical control of quantum-probe interactions with noisy and complex systems viewed as thermal baths. We show that this control, in conjunction with tools of quantum estimation theory, may be used for inferring the spectral and spatial characteristics of such baths with high precision. This approach constitutes a new avenue in quantum sensing, dubbed quantum noise spectroscopy.

Diverse models of engines energised by quantum-coherent, hence non-thermal, baths allow the engine efficiency to transgress the standard thermodynamic Carnot bound. These transgressions call for an elucidation of the underlying mechanisms. Here we show that non-thermal baths may impart not only heat, but also mechanical work to a machine. The Carnot bound is inapplicable to such a hybrid machine. Intriguingly, it may exhibit dual action, concurrently as engine and refrigerator, with up to 100% efficiency. We conclude that even though a machine powered by a quantum bath may exhibit an unconventional performance, it still abides by the traditional principles of thermodynamics.

In this paper, we address the question: To what extent is the quantum state preparation of multiatom clusters (before they are injected into the microwave cavity) instrumental for determining not only the kind of machine we may operate, but also the quantitative bounds of its performance? Figuratively speaking, if the multiatom cluster is the "crude oil", the question is: Which preparation of the cluster is the refining process that can deliver a "gasoline" with a "specific octane"? We classify coherences or quantum correlations among the atoms according to their ability to serve as: (i) fuel for nonthermal machines corresponding to atomic states whose coherences displace or squeeze the cavity field, as well as cause its heating; and (ii) fuel that is purely "combustible", i.e., corresponds to atomic states that only allow for heat and entropy exchange with the field and can energize a proper heat engine. We identify highly promising multiatom states for each kind of fuel and propose viable experimental schemes for their implementation.

We investigated peculiarities of absorption, emission and photonic density of states of a cholesteric liquid crystal with an isotropic defect layer inside. The influence of the defect layer position on absorption and emission in the system was studied. It was shown that for non-diffracting circularly polarized incident light absorption/emission is maximum if the defect is in the centre of the system; and for diffracting circularly polarized incident light absorption/emission is maximum if the defect is shifted from the centre of the system to its left border from where light is incident. We also investigated influence of the defect layer thickness and those parameters which characterize loss and gain on absorption and emission. The influence of anisotropic absorption in the cholesteric liquid crystal layer on photonic density states was investigated, too.

We revisit the thermodynamic bounds of work extraction in simple quantum heat machines subject to control by frequent modulations that do not comply with adiabatic assumptions. The laws of thermodynamics are obeyed, yet anomalous deviations from the known bounds are revealed.

We explore means of maximizing the power output of a heat engine based on a periodically-driven quantum system that is constantly coupled to hot and cold baths. It is shown that the maximal power output of such a heat engine whose "working fluid" is a degenerate V-type three-level system is that generated by two independent two-level systems. Hence, level degeneracy is a thermodynamic resource that may effectively double the power output. The efficiency, however, is not affected. We find that coherence is not an essential asset in such multilevel-based heat engines. The existence of two thermalization pathways sharing a common ground state suffices for power enhancement.

The possibility of channeling low-energy relativistic positrons in some ionic crystals with axial symmetry is shown. The annihilation processes of positrons with medium electrons are investigated in detail. The lifetime of a positron in the regime of channeling is estimated; the existence of a long relaxation lifetime has been shown.

We explore, theoretically and experimentally, a method for cooling a broadband heat reservoir, via its laser-assisted collisions with two-level atoms followed by their fluorescence. This method is shown to be advantageous compared to existing laser-cooling methods in terms of its cooling efficiency, the lowest attainable temperature for broadband baths, and its versatility: it can cool down any heat reservoir, provided the laser is red detuned from the atomic resonance. It is applicable to cooling down both dense gaseous and condensed media.

We explore the dependence of the performance bounds of heat engines and refrigerators on the initial quantum state and the subsequent evolution of their piston, modeled by a quantized harmonic oscillator. Our goal is to provide a fully quantized treatment of self-contained (autonomous) heat machines, as opposed to their prevailing semiclassical description that consists of a quantum system alternately coupled to a hot or a cold heat bath and parametrically driven by a classical time-dependent piston or field. Here, by contrast, there is no external time-dependent driving. Instead, the evolution is caused by the stationary simultaneous interaction of two heat baths (having distinct spectra and temperatures) with a single two-level system that is in turn coupled to the quantum piston. The fully quantized treatment we put forward allows us to investigate work extraction and refrigeration by the tools of quantum-optical amplifier and dissipation theory, particularly, by the analysis of amplified or dissipated phase-plane quasiprobability distributions. Our main insight is that quantum states may be thermodynamic resources and can provide a powerful handle, or control, on the efficiency of the heat machine. In particular, a piston initialized in a coherent state can cause the engine to produce work at an efficiency above the Carnot bound in the linear amplification regime. In the refrigeration regime, the coefficient of performance can transgress the Carnot bound if the piston is initialized in a Fock state. The piston may be realized by a vibrational mode, as in nanomechanical setups, or an electromagnetic field mode, as in cavity-based scenarios.

We propose a method of optimally controlling the tradeoff of speed and fidelity of state transfer through a noisy quantum channel (spin-chain). This process is treated as qubit state-transfer through a fermionic bath. We show that dynamical modulation of the boundary-qubits levels can ensure state transfer with the best tradeoff of speed and fidelity. This is achievable by dynamically optimizing the transmission spectrum of the channel. The resulting optimal control is robust against both static and fluctuating noise in the channels spin-spin couplings. It may also facilitate transfer in the presence of diagonal disorder (on site energy noise) in the channel.

Polarizable dipoles, such as atoms, molecules, or nanoparticles, subject to laser radiation may attract or repel each other. We derive a general formalism in which such laser-induced dipole-dipole interactions (LIDDIs) in any geometry and for any laser strength are described in terms of the resonant dipole-dipole interaction (RDDI) between dipoles dressed by the laser. This approach provides a simple route towards the analysis of LIDDI in a general geometry. Our general results reveal LIDDI effects due to nonlinear dipolar response to the laser, previously unaccounted for. The origin of these nonlinear effects is discussed. Our general formalism is illustrated for LIDDI between atoms in a cavity.

We show that frequent nondemolition measurements of a quantum system immersed in a thermal bath allow the extraction of work in a closed cycle from the system-bath interaction (correlation) energy, a hitherto unexploited work resource. It allows for work even if no information is gathered or the bath is at zero temperature, provided the cycle is within the bath memory time. The predicted work resource may be the basis of quantum engines embedded in a bath with long memory time, such as the electromagnetic bath of a high-Q cavity coupled to two-level systems.

We present an approach to monitoring and controlling a free quantum particle by coupling an internal (discrete) state of the particle to a detector (or probe). We consider a sequence of time-dependent, spatially localized interactions of the particle with the probe that are purely coherent (nondissipative), without mean energy-momentum exchange. We show that a sequence of such force-free interactions can freeze or deflect the particle.

We show that nonradiative interactions between atomic dipoles placed in a waveguide can give rise to deterministic entanglement at ranges much larger than their resonant wavelength. The range increases as the dipole resonance approaches the waveguide's cutoff frequency, caused by the giant density of photon modes near cutoff, a regime where the standard (perturbative) Markov approximation fails. We provide analytical theories for both the Markovian and non-Markovian regimes, supported by numerical simulations, and discuss possible experimental realizations.

For quantum systems under manipulations frequent enough to break the Markov approximation, bounds considered fundamental are revisited: the Szilard-Landauer bound on work traded for information, the Carnot engine efficiency bound or Nernst's third law.

A minimal model of a quantum refrigerator, i.e., a periodically phase-flipped two-level system permanently coupled to a finite-capacity bath (cold bath) and an infinite heat dump (hot bath), is introduced and used to investigate the cooling of the cold bath towards absolute zero (T=0). Remarkably, the temperature scaling of the cold-bath cooling rate reveals that it does not vanish as T→0 for certain realistic quantized baths, e.g., phonons in strongly disordered media (fractons) or quantized spin waves in ferromagnets (magnons). This result challenges Nernst's third-law formulation known as the unattainability principle.

We develop a general optimization strategy for performing a chosen unitary or nonunitary task on an open quantum system. The goal is to design a controlled time-dependent system Hamiltonian by variationally minimizing or maximizing a chosen function of the system state, which quantifies the task success (score), such as fidelity, purity, or entanglement. If the time dependence of the system Hamiltonian is fast enough to be comparable to or shorter than the response time of the bath, then the resulting non-Markovian dynamics is shown to optimize the chosen task score to second order in the coupling to the bath. This strategy can protect a desired unitary system evolution from bath-induced decoherence, but can also take advantage of the system-bath coupling so as to realize a desired nonunitary effect on the system.

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network's evolution, the strategy hereby presented derives from the "quantum Zeno effect" and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.

We investigate quantum information processing, transfer and storage in hybrid systems comprised of diverse blocks integrated on chips. Strong coupling between superconducting (SC) qubits and ensembles of ultracold atoms or NV-center spins is mediated by a microwave transmission-line resonator that interacts near-resonantly with the atoms or spins. Such hybrid devices allow us to benefit from the advantages of each block and compensate for their disadvantages. Specifically, the SC qubits can rapidly implement quantum logic gates, but are "noisy" (prone to decoherence), while collective states of the atomic or spin ensemble are "quiet"(protected from decoherence) and thus can be employed for storage of quantum information. To improve the overall performance (fidelity) of such devices we discuss dynamical control to optimize quantum state-transfer from a "noisy" qubit to the "quiet" storage ensemble. We propose to maximize the fidelity of transfer and storage in a spectrally inhomogeneous spin ensemble, by pre-selecting the optimal spectral portion of the ensemble. Significant improvements of the overall fidelity of hybrid devices are expected under realistic conditions. Experimental progress towards the realization of these schemes is discussed.

Decoherence is a major obstacle to any practical implementation of quantum information processing. One of the leading strategies to reduce decoherence is dynamical decoupling-the use of an external field to average out the effect of the environment. The decoherence rate under any control field can be calculated if the spectrum of the coupling to the environment is known. We present a direct measurement of the bath-coupling spectrum in an ensemble of optically trapped ultra-cold atoms, by applying a spectrally narrow-band control field. The measured spectrum follows a Lorentzian shape at low frequencies but exhibits non-monotonic features at higher frequencies due to the oscillatory motion of the atoms in the trap. These features agree with our analytical models and numerical Monte Carlo simulations of the collisional bath. From the inferred bath-coupling spectrum, we predict the performance of some well-known dynamical decoupling sequences. We then apply these sequences in experiment and compare the results to predictions, finding good agreement in the weak-coupling limit. Thus, our work establishes experimentally the validity of the overlap integral formalism and is an important step towards the implementation of an optimal dynamical decoupling sequence for a given measured bath spectrum.

As a rule, the coupling of a quantum system to an uncontrollable thermal reservoir (a "bath") gives rise to the system's decoherence, i.e., the destruction of its unitary coherent evolution [1]. Not less common is the rule that the more complex the quantum system, the more detrimental are the bath effects [2]. Here we point out that the coupling of a complex quantum system to a bath may actually induce advantageous coherent dynamics. Namely, an exact solution for a quantum many-body system characterized by large angular momentum (or spin) that is coupled to a thermal bath reveals a hitherto unexplored general effect: bath-induced effectively nonlinear evolution. This evolution can drive the large-spin system into a macroscopic quantum-superposition ("Schroedinger-cat") state [3, 4]. Such counter-intuitive bath-induced effects should be observable in various setups on long (Markovian) time scales. They change our perspective of non-classicality in open many-body quantum systems. Namely, the bath may cause rather than impede the formation of distinctly non-classical Schroedinger-cat states, despite their fragility in the presence of a bath [2, 5].

Hollow-core photonic-crystal waveguides filled with cold atoms can support giant optical nonlinearities through nondispersive propagation of light tightly confined in the transverse direction. Here we explore electromagnetically induced transparency is such structures, considering a pair of counterpropagating weak quantum fields in the medium of coherently driven atoms in the ladder configuration. Strong dipole-dipole interactions between optically excited, polarized Rydberg states of the atoms translate into a large dispersive interaction between the two fields. This can be used to attain a spatially homogeneous conditional phase shift of π for two single-photon pulses, realizing a deterministic photonic phase gate, or to implement a quantum nondemolition measurement of the photon number in the signal pulse by a coherent probe, thereby achieving a heralded source of single- or few-photon pulses.

We demonstrate through exact solutions that a spin bath leads to stronger (faster) dephasing of a qubit than a bosonic bath with an identical bath-coupling spectrum. This difference is due to the spin-bath "dressing" by the coupling. Consequently, the quantum statistics of the bath strongly affects the pulse sequences required to dynamically decouple the qubit from its bath.

We demonstrate through an exactly solvable model that collective coupling to any thermal bath induces effectively nonlinear couplings in a quantum many-body (multispin) system. The resulting evolution can drive an uncorrelated large-spin system with high probability into a macroscopic quantum-superposition state. We discuss possible experimental realizations.

We introduce a model of a two-dimensional (2D) self-attractive medium embedded into a quasi-1D symmetric double-well potential (DWP), whose depth is subject to periodic modulations (management). The model applies to matter waves in Bose-Einstein condensates (BECs), as well as to the nonlinear transmission of light (in spatial and temporal domains alike). It is known that, in the absence of the management, the DWP induces the spontaneous symmetry breaking (SSB) of 2D solitons, when their norm exceeds a critical value. Above the SSB point, symmetric solitons are unstable, while the system supports stable asymmetric ones. The DWP also admits Josephson oscillations of solitons between the two wells. We study effects of periodic modulations of the DWP's depth on the stability of symmetric, asymmetric, and oscillating solitons. Stability areas for the solitons of these three types are produced in the plane of the modulation frequency and amplitude. The shape of the stability borders is strongly affected by the proximity of the modulation frequency to the frequency of free oscillations of the soliton in the static DWP structure, the solitons being destroyed by the modulations with an arbitrarily small amplitude at the point of the exact resonance. Similar results are obtained for an extended 2D model, which combines the transverse DWP and a longitudinal periodic potential (optical lattice, in terms of BEC). The findings are compared with those reported in other recently studied management models for 1D and 2D solitons, which makes it possible to draw general conclusions about the stability limits of solitons under the resonant management.

We propose a control mechanism for the stabilization of symmetry-broken modes in Bose-Einstein condensates and optical beams trapped in double-well potentials in the presence of symmetric or asymmetric noise. The control, based on periodic π shifts of the phase in one well, is demonstrated for solitons and in the two-mode approximation. The latter setting is considered in both the mean-field and quantum (Bose-Hubbard) regimes.

We study deviations from thermal equilibrium between two-level systems (TLS) and a bath by frequent and brief quantum measurements of the TLS energy-states. The resulting entropy and temperature of both the system and the bath are found to be completely determined by the measurement rate, and unrelated to what is expected by standard thermodynamical rules that hold for Markovian baths. These anomalies allow for very fast control heating, cooling and state-purification (entropy reduction) of quantum systems much sooner than their thermal equilibration time.

We propose a technique that allows to simultaneously perform universal control of the evolution operator of a system and compensate for the first-order contribution of any Hermitian constant noise and the action of the environment. We show that, at least, a three-valued Hamiltonian is needed in order to protect the system against any such noise and propose an explicit algorithm for finding an appropriate control sequence. This algorithm is applied to numerically design a safe gate in an atomic qutrit.

Quantum two-state systems, known as quantum bits (qubits), are unavoidably in contact with their uncontrolled thermal environment, also known as a macroscopic 'bath'. The higher the temperature of the qubits, the more impure their quantum state and the less useful they are for coherent control or quantum logic operations, hence the desirability of cooling down the qubits as much and as fast as possible, so as to purify their state prior to the desired operation. Yet, the limit on the speed of existing cooling schemes, which are all based on Markovian principles, is either the duration of the qubit equilibration with its bath or the decay time of an auxiliary state to one of the qubit states. Here we pose the conceptual question: can one bypass this existing Markovian limit? We show that highly frequent phase shifts or measurements of the state of thermalized qubits can lead to their ultrafast cooling, within the non-Markov time domain, well before they re-equilibrate with the bath and without resorting to auxiliary states. Alternatively, such operations may lead to the cooling down of the qubit to arbitrarily low temperatures at longer times. These anomalous nonMarkov cooling processes stem from the hitherto unfamiliar coherent quantum dynamics of the qubit-bath interaction well within the bath memory time.

A general quantum noisy channel is analyzed, wherein the transmitted qubits may experience symmetry-breaking decoherence, along with memory effects. We examine generalized Bell-basis and find that the optimal basis is not the fully entangled one, but a combination of factorized and partially-entangled states in the presence of memory, asymmetry and the state-bias of the noise. Capacity-maximization is shown to be achievable by combining temporal shaping of the transmitted qubits and optimal basis selection.

An analytical expression is received for the effective interaction potential of a fast charged particle with the ionic crystal CsCl near the direction of axis 100 as a function of the temperature of the medium. By numerical analysis it is shown that the effective potential of axial channeling of positrons along the axis 100 of negatively charged Cl^- ions practically doesn't depend on temperature of the media. The wavefunction and the energy spectrum of the localized state are investigated, and the possibility is proved for the formation of metastable, two-dimensional relativistic positron systems or the positron atoms (PA), i.e. the bound state of a positron in the present case with a negatively charged axis of ions. The problem of one-photon decay of PA is investigated. The possibilities of stimulation of resonant transitions between quantum states of PA by the external hypersound are investigated in detail.

We examine the possibility of coherent reversible information transfer between solid-state superconducting qubits and ensembles of ultracold atoms. Strong coupling between these systems is mediated by a microwave transmission line resonator that interacts near resonantly with the atoms via their optically excited Rydberg states. The solid-state qubits can then be used to implement rapid quantum logic gates, while collective metastable states of the atoms can be employed for long-term storage and optical readout of quantum information.

Given a multilevel system coupled to a bath, we use a Feshbach P, Q partitioning technique to derive an exact trace-nonpreserving master equation for a subspace Si of the system. The resultant equation properly treats the leakage effect from Si into the remainder of the system space. Focusing on a second-order approximation, we show that a one-dimensional master equation is sufficient to study problems of quantum state storage and is a good approximation, or exact, for several analytical models. It allows a natural definition of a leakage function and its control and provides a general approach to study and control decoherence and leakage. Numerical calculations on an harmonic oscillator coupled to a room temperature harmonic bath show that the leakage can be suppressed by the pulse control technique without requiring ideal pulses.

We examine two set-ups that reveal different operational implications of path-phase complementarity for single photons in a Mach-Zehnder interferometer (MZI). In both set-ups, the which-way (WW) information is recorded in the polarization state of the photon serving as a 'flying which-way detector'. In the 'predictive' variant, using a fixed initial state, one obtains duality relation between the probability to correctly predict the outcome of either a which-way (WW) or which-phase (WP) measurement (equivalent to the conventional path-distinguishability-visibility). In this set-up, only one or the other (WW or WP) prediction has operational meaning in a single experiment. In the second, 'retrodictive' protocol, the initial state is secretly selected for each photon by one party, Alice, among a set of initial states which may differ in the amplitudes and phases of the photon in each arm of the MZI. The goal of the other party, Bob, is to retrodict the initial state by measurements on the photon. Here, a similar duality relation between WP and WW probabilities governs their simultaneous guesses in each experimental run.

We consider single photons propagating along two paths, with the polarization correlated to the path. Two information related aspects of this translational-internal entanglement (TIE) are analyzed: a) Using the polarization to record the path (a "flying detector" scheme), we characterize the tradeoff between path- and phase-information. b) We investigate the effects of non-Markovian noise on the two-qubit quantum channel consisting of the photon path and polarization (that are both used to encode information), and suggest noise protection schemes.

We propose a novel protocol for the creation of macroscopic quantum superposition (MQS) states based on a measurement of a non-monotonous function of a quantum collective variable. The main advantage of this protocol is that it does not require switching on and off nonlinear interactions in the system. We predict this protocol to allow the creation of multiatom MQS by measuring the number of atoms coherently outcoupled from a two-component (spinor) Bose-Einstein condensate.

Self-pulsed lasing in the form of a coherent solitary wave train from a periodically amplifying Bragg structure is proposed. It is shown both analytically and numerically that the generated pulse train propagates in the periodically amplifying Bragg structure chip with superluminal group velocity. Its stability and coherence are in contrast with the inherent instability and chaoticity of self-pulsing from a uniform gain medium embedded in a Bragg reflector. Bragg-periodic semiconductor quantum-well heterostructures are candidates for realizing the required periodically amplifying Bragg structure.

We study the effect of noise-induced dephasing on collisional phase diffusion in the two-site Bose-Hubbard model. Dephasing of the quasimomentum modes may slow down phase diffusion in the quantum Zeno limit. Remarkably, the degree of suppression is enhanced by a bosonic factor of order N/log N as the particle number N increases.

We study the two mechanisms of the interplay of long- and short-range interactions in different geometries of ultracold fermionic atomic or molecular gases. We show that in the range of validity of the one-dimensional (1D) approximation, both mechanisms yield similar superconductivity. We show that electromagnetically induced isotropic dipole-dipole interactions in a spin-polarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot 'halo' and a highly degenerate 'core'. Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.

A polarized photon with well-defined orbital angular momentum that emerges from a Mach-Zehnder interferometer (MZI) is shown to seemingly circumvent wave-particle duality constraints. For certain phase differences between the MZI arms, this pattern yields both reliable which-path information and high phase sensitivity.

The aim of this paper is to revisit the implications of complementarity when we inject into a Mach Zehnder interferometer particles with internal structure, prepared in special translational-internal entangled (TIE) intraparticle states. This correlation causes the path distinguishability to be interferometric phasedependent in contrast to the standard case, where distinguishability depends on some external parameters (not interferometric phase). We show that such a TIE state permits us to detect small phase shifts along with almost perfect path distinguishability, beyond the constraints imposed by complementarity on simultaneous which-way and which-phase measurements for cases when distinguishability is uncoupled to interferometric phase.

We present and compare stochastic open-loop techniques aimed at controlling quantum coherence in dissipative environments. One approach describes the evolution time as a random non-Gaussian variable. The other implements dynamical control on non-Markovian time-scales via stochastic modulation of the system-bath coupling.

We investigate conditions under which multiatom absorption of a single photon leads to cooperative decay. Our analysis reveals the symmetry properties of the multiatom Dicke states underlying the cooperative decay dynamics and their spatio-temporal manifestations, particularly, the forward-directed spontaneous emission investigated by Scully et al (2006 Phys. Rev. Lett. 96 010501).

Using quantum channels to transmit classical information has been proven to be advantageous in several scenarios. These channels have been assumed to be memoryless, meaning that consecutive transmissions of information are uncorrelated. However, as shown experimentally, such correlations do exist, and thereby retain memory of previous information. This memory complicates the protection of entangled-information transmission from decoherence. We have recently addressed these fundamental questions by developing a generalized master equation for multipartite entangled systems coupled to finite-temperature baths and subject to arbitrary external perturbations whose role is to provide dynamical protection from decay and decoherence. Here we explore and extend the foregoing strategy to quantum optical communication schemes wherein polarization-entangled photons traverse a bit-flip channel with temporal and spatial memory, such that the two channels experience cross-decoherence. We introduce a novel approach to the protection of the entangled information from decoherence in such schemes. It is based on selectively modulating the photon polarizations in each channel. We show that by applying selective modulation, one can independently control the symmetry and spatial memory attributes of the. channel. We then explore the effects of these attributes on the channel capacity. Remarkably, we show that there is a nontrivial interplay between the effects of asymmetry and memory on the channel capacity.

We theoretically investigate the free dispersion, scattering and disintegration of an N-particle quantum soliton by Bragg light pulses, comparing the exact quantum result with its quasiclassical (mean-field) limit. Remarkably, we find that the correlation properties of a quantum soliton approach the (mean-field) limit very rapidly as the particle number N grows. A modest discrepancy between the classical and quantum results is observed for N = 3.

An entangled multipartite system coupled to a zero-temperature bath undergoes rapid disentanglement in many realistic scenarios due to local, symmetry-breaking differences in the particle-bath couplings. We show that locally controlled perturbations, addressing each particle individually, can impose a symmetry allowing the existence of decoherence-free multipartite entangled systems.

Here we aim at setting the principles of and quantifying translational entanglement by collisions and half-collisions. In collisions, the resonance width s and the initial phase-space distributions axe shown to determine the degree of post-collisional momentum entanglement. Half-collisions (dissociation) axe shown to yield different types of approximate EPR states. We analyse a feasible realization of translational EPR entanglement and teleportation via cold-molecule Raman dissociation and subsequent collisions, resolving both practical and conceptual difficulties it has faced so far.

We show that the giant Kerr nonlinearity in the regime of electromagnetically induced transparency in vapor can give rise to the formation of Thirring-type spatial solitons, which are supported solely by cross-phase modulation.

Periodic structures with resonant dopants allow giantly enhanced cross-coupling between ultraweak pulses within photonic band gaps, subject to self-induced or electromagnetically-induced transparency. These features may lead to novel quantum optical applications.

A unified theory is given of dynamically modified decay and decoherence of field-driven multilevel multipartite entangled states that are weakly coupled to zero-temperature baths. The theory allows for arbitrary local differences in their coupling to the environment. Due to such differences, the optimal driving-field modulation to ensure maximal fidelity is found to substantially differ from conventional pi-phase flips of the single-qubit evolution.

A unified theory is presented of dynamically modified decay and decoherence in driven multilevel quantum systems that are weakly coupled to arbitrary zero-temperature reservoirs. Examples of different phase and amplitude modulations are given for two-level systems (qubits). Analysis of modulations on multilevel systems is detailed with a numerical example using quasiperiodic impulsive phase jumps. The merits and disadvantages of the different modulation types are discussed.

We show that the multidimensional Zeno effect combined with non-holonomic control allows one to efficiently protect quantum systems from decoherence by a method similar to classical random coding. The method is applicable to arbitrary error-inducing Hamiltonians and general quantum systems. The quantum encoding approaches the Hamming upper bound for large dimension increases. Applicability of the method is demonstrated with a seven-qubit toy computer.

A unified theory is given of dynamically modified decay and decoherence in driven two-level and multilevel quantum systems that are weakly coupled to arbitrary finitetemperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of decoherence suppression and the limitations of this approach are obtained. For a driven qubit that is strongly coupled to the continuum edge of reservoir's spectrum, we demonstrate that only an appropriately ordered sequence of abrupt changes of the resonance frequency, near the continuum edge, can effectively protect the qubit state from decoherence.

In this paper, we show how the non-holonomic control technique can be employed to build completely controlled quantum devices. Examples of such controlled structures are provided.

In this paper, we present a realistic application of the coherence protection method proposed in the previous article. A qubit of information encoded on the two spin states of a Rubidium isotope is protected from the action of electric and magnetic fields.

The protection of the coherence of open quantum systems against the influence of their environment is a very topical issue. A scheme is proposed here which protects a general quantum system from the action of a set of arbitrary uncontrolled unitary evolutions. This method draws its inspiration from ideas of standard error correction (ancilla adding, coding and decoding) and the quantum Zeno effect. A demonstration of our method on a simple atomic system-namely, a rubidium isotope-is proposed.

We propose a method for controlling the decay and decoherence of a driven qubit that is strongly coupled to a reservoir, when the qubit resonance frequency is close to a continuum edge of the reservoir spectrum. This strong-coupling regime is outside the scope of existing methods of decay or decoherence control. We demonstrate that an appropriate sequence of nearly abrupt changes of the resonance frequency can protect the qubit state from decay and decoherence more effectively than the intuitively obvious alternative, which is to fix the resonance well within a forbidden band gap of the reservoir spectrum, as far as possible from the continuum edge. The " counterintuitive" nonadiabatic method outlined here can outperform its adiabatic counterparts in maintaining a high fidelity of quantum logic operations. The remarkable effectiveness of the proposed method, which requires much lower rates of frequency changes than previously proposed control methods, is due to the ability of appropriately alternating detunings from the continuum edge to augment the interference of the emitted and backscattered quanta, thereby helping to stabilize the qubit state against decay. Applications to the control of decay and decoherence near the edge of radiative, vibrational, and photoionization continua are discussed.

We present an example of a situation where Fermi's golden rule does not apply, even if an unstable quantum system decays exponentially. The work is inspired by the recent experiments by Chatzidimitriou-Dreismann and co-workers on reduction of the neutron Compton scattering cross-section in hydrogenated compounds. Although full quantitative explanation of these experimental results is still to be accomplished, our preliminary results show a possibility of strong modification of the scattering cross-section.

We derive and investigate an expression for the dynamically modified decay of states coupled to an arbitrary continuum. This expression is universally valid for weak temporal perturbations. The resulting insights can serve as useful recipes for optimized control of decay and decoherence.

Keywords: decoherence; quantum information processing; errors correction codes; quantum Zeno and anti-Zeno effects; Josephson junction qubits; degenerate ground-state systems; optically-manipulated atoms; photonic band gap; nonadiabatic periodic dynamics; quantum memory; collective multiatom states; equivalent storage classes; decoherence free subspaces

We investigate the scattering threshold and cavity-enhanced gain in nonlinear spheres with second- or third-order permeability. Pairs of pump-driven idler and signal modes are considered, satisfying morphology-dependent resonance conditions. The thresholds and gain coefficients of amplified and stimulated Raman scattering, parametric downconversion, and analogous parametric processes in microspheres are derived and evaluated under typical conditions. Applications may include the measurement of chemical impurity concentrations or the creation of low-threshold optical parametric amplifiers using microspheres.

We develop a unified approach to dynamical suppression, by control fields, of decay and decoherence in qubits coupled to finite-temperature baths and dephasing sources. Considerations for optimizing such suppression are outlined.

We study a gaseous atomic Bose-Einstein condensate with laser-induced dipole-dipole interactions using the Hartree-Fock-Bogoliubov theory within the Popov approximation. The dipolar interactions introduce long-range atom-atom correlations which manifest themselves as increased depletion at momenta similar to that of the laser wavelength, as well as a 'roton' dip in the excitation spectrum. Surprisingly, the roton dip and the corresponding peak in the depletion are enhanced by raising the temperature above absolute zero.

The inversionless free electron laser having a drift region consisting of two magnets is analyzed. Performing numerical simulations of electron motion inside wigglers and drift region, we have shown that this system has a positive mean gain over the entire energy distribution of the electron beam. We study the influence of emittance and the spread of electron energies on the gain in small- and high-gain regimes.

The inversionless free-electron laser having a drift region consisting of two magnets is analyzed. Performing numerical simulations of electron motion inside wigglers and the drift region, we have shown that this system has a positive mean gain over the entire energy distribution of the electron beam. We study the influence of emittance and the spread of electron energies on the gain.

We consider a gaseous atomic BoseEinstein condensate with dipoledipole interactions induced by a far off-resonance laser. The long-range dipolar interactions introduce atomatom correlations on the new scale of the laser wavelength, giving a controllable method of squeezing low energy modes. At high enough laser intensities the correlations lead to a roton minimum in the excitation spectrum.

We propose two alternative scheme for highly efficient nonlinear interaction between weak optical fields. The first scheme is based on the attainment of electromagnetically induced transparency simultaneously for two fields via transitions between magnetically split F = 1 atomic sublevels, in the presence of two driving fields. The second scheme relies on simultaneous electromagnetically- and self-induced transparencies of the two fields. Thereby, equal slow group velocities and giant cross-phase modulation of the weak fields over long distances are achieved.

Photonic crystals doped with resonant atoms allow for uniquely advantageous nonlinear modes of optical propagation. The first type of mode is self-induced transparency (SIT) solitons and multidimensional localized "bullets" propagating at photonic-bandgap frequencies. Such modes can exist even at ultraweak intensities (few photons) and therefore differ substantially either from solitons in Kerr-nonlinear photonic crystals or from SIT solitons in uniform media. The second type of mode is cross coupling between pulses exhibiting electromagnetically induced transparency and SIT gap solitons. We show that extremely strong correlations (giant cross-phase modulation) can be formed between the two pulses. These features may find applications in high-fidelity classical and quantum optical communications.

We predict the existence of a new type of spatiotemporal soliton (so-called light bullets) in twodimensional self-induced-transparency media with refractive-index modulation in the direction transverse to that of pulse propagation. These self-localized guided modes are found in an approximate analytical form. Their existence and stability are confirmed by numerical simulations, and they may have advantageous properties for signal transmission.

The modification of the properties of a Bose-Einstein or a Fermi-Dirac atomic gas due to laser-induced dipole-dipole interactions between the atoms are considered. Nearly-isotropic illumination of the sample by spectrally-fluctuating laser beams averages out the static r(-3) dipole-dipole interaction, leaving the retarded r(-1) "self-gravitating" attraction in the near zone. The analogies of ultracold many-atom systems, self-bound by such laser-induced "gravity", with compact stars ("Bose stars" or "White Dwarfs") are emphasized. Even a single plane-wave laser induces dipole-dipole interactions capable of causing a cigar-shaped Bose condensate to exhibit self binding and density modulations.

The inhibition or acceleration of the decay of an unstable state by its projective measurements was recently analyzed for a broad class of processes. As such, the arsenal of decal control, whether measurement-like or fully coherent was purported to substantially expand. A universal form of the decay rate of unstable states into any reservoir, modified by weak perturbations with arbitrary time dependence was derived.

We derive and investigate an expression for the dynamically modified decay of states coupled to an arbitrary continuum. This expression is universally valid for weak temporal perturbations. The resulting insights can serve as useful recipes for optimized control of decay and decoherence.

The possibility of realizing photon-photon nonlinear phase shifters and switches with a potentially very high efficiency and without external laser fields in photonic crystals (PCs) doped at a low concentration with four-level atoms is illustrated. These atoms have two transitions tuned to two incident photons and an intermediate transition tuned to a singular feature of the structured density of modes (DOM) spectrum of the PC. These features have unique advantages for quantum information applications.

Novel regimes of probe scattering by atoms trapped in optical lattices in the random-density and multiple-scattering regimes were identified. These regimes were characterized by a universal feature of large density fluctuations.

Cold diatomic molecules are analyzed as a source of atom pairs. The dissociation of these molecules are investigated. The atoms pairs have highly correlated positions and momenta. The collision with one of the EPR correlated atoms and manipulation of the other atom in the pair can cause wave packet teleportation.

We consider the influence of refractive index fluctuations (due to randomly distributed inclusions inside a dielectric spherical droplet) on the line shapes of scattering Mie resonances. The significant difference in the spatial distributions of the mode functions participating in the process does not allow one to employ the standard statistical ensembles used in random matrix theory. We propose to model the system by a simplified random ensemble which gives a very good agreement with the available experimental data, and we predict a type of line shape for narrow scattering resonances.

Summary form only given. The quantum Zeno effect (QZE) is the striking possibility of inhibiting the decay of an unstable quantum state by sufficiently frequent measurements. This effect is currently believed to be universal, even though it has only been observed for oscillations between discrete states. Recently we showed that both the QZE and the inverse (anti-) Zeno effect (AZE) can be observed on spontaneous decay in cavities. We develop a general theory of evolution of quantum systems in the presence of frequent measurements. We consider both the ideal case of instantaneous projections of the wave function to the initial state and some possible measurement schemes. The considerations imply the impossibility of the continuous-observation limit necessary to stop quantum evolution (the quantum Zeno paradox). Indeed, this limit leads to an unbounded increase of the energy fluctuations of the system, which would cause the system to disintegrate. The present theory describes different measurement effects, including the QZE and AZE, and allows us to obtain the general criteria of their validity. In particular, we show that the AZE, rather than the QZE, is achievable for spontaneous decay of atoms in free space.

We consider a scheme of two noncollinear wigglers with an intermediate magnetic drift region, constituting a free-electron laser without inversion (FELWI). Two mechanisms of phase shifts in the drift region between the wigglers owing to a series of magnetic lenses can give rise to FELWI: velocity- and angle-dependent shifts. An appropriate combination of these shifts is shown to provide the conditions for amplification without inversion. The phase shifts optimizing the gain are found. A specific scheme for the drift region is suggested.

We show that the fidelity of teleportation of continuous quantum variables can be improved by conditional photon-number measurement of the entangled state. Further, we propose a teleportation scheme based on photon counting on the output fields of a squeezer that combines the mode whose quantum state is desired to be teleported and one mode of the two-mode squeezed vacuum playing the role of the entangled state.

We show that the recently proposed scheme of teleportation of continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)] can be improved by a conditional measurement of the entangled state shared by the sender and the recipient. The conditional measurement subtracts photons from the original entangled two-mode squeezed vacuum, by transmitting each mode through a low-reflectivity beam splitter and performing a joint photon-number measurement on the reflected beams. In this way the degree of entanglement of the shared state is increased and so is the fidelity of the teleported state.

We show that the recently proposed scheme of teleportation of continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)] can be improved by a conditional measurement of the entangled state shared by the sender and the recipient. The conditional measurement subtracts photons from the original entangled two-mode squeezed vacuum, by transmitting each mode through a low-reflectivity beam splitter and performing a joint photon-number measurement on the reflected beams. In this way the degree of entanglement of the shared state is increased and so is the fidelity of the teleported state.

We show that particular configurations of intense off-resonant laser beams can give rise to an attractive 1/r interatomic potential between atoms located well within the laser wavelength. Such a \u201cgravitational-like\u201d interaction is shown to give stable Bose-Einstein condensates that are self-bound (without an additional trap) with unique scaling properties and measurably distinct signatures.

We point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable only in spectrally finite reservoirs, i.e., in cavities and waveguides, using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. By contrast, such measurements can only accelerate decay in free space.

We consider an optical medium consisting of a periodic refractive-index grating and a periodic set of thin layers of two-level systems resonantly interacting with the electromagnetic field. Recently, it has been shown that such a system gives rise to a vast variety of stable bright solitons. In this work, we demonstrate that the system has another very unusual property: stable bright solitons can coexist with stable continuous-wave (cw) states and stable dark solitons (DSs). Depending on the parameters values, a DS frequency band coexists (without overlap) with one or two bright-soliton bands. Quiescent (standing) DSs are found in an analytical form, and moving ones are obtained numerically. Simulations show that a considerable part of the DS solutions are completely stable against arbitrary small perturbations. The fact that this system supports both stable bright and dark solitons for the same parameters values may find interesting applications in photonics.

The injection of spectrally narrow probe wavepackets into quantized parametrically amplifying media can give rise to transient tachyonic wavepackets. Remarkably, their quantum information, albeit causal, is confined to times corresponding to superluminal velocities, which is advantageous for communications.

Spontaneous decay of excited cold atoms into a cavity can drastically affect their translational dynamics, namely, atomic reflection and transmission through a potential barrier.

With the help of some remarkable examples, it is shown that conditional measurements performed on two-level atoms just after they have interacted with a resonant cavity field mode are able to recover the coherence of number-state superpositions, which is lost due to dissipation.

We demonstrate the possibility of simply and effectively controlling the inversionless gain between two excited states in a ladder configuration, by injecting atoms (molecules) into a cavity. The control is effected by the cavity-mode decay rate and vacuum Rabi frequency, chosen according to the decay and dephasing rates of the upper and lower states. The analogy with the control of inversionless gain by a classical strong field is discussed. This scheme is expected to be robust under conditions of thermal equilibrium or non-selective pumping of excited states.

Novel methods are discussed for the state control of atoms coupled to multi-mode reservoirs with non-Markovian spectra: 1) Excitation decay control: we point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable in open cavities and waveguides using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. 2) Location-dependent interference of decay channels - nonadiabatic (resonant) control: We show that the control of populations and coherences of two metastable states is feasible via resonant single-photon absorption to an intermediate state, by controlled spontaneous emission in a cavity. 3) Decoherence control by conditionally interfering parallel evolutions: We demonstrate that an arbitrary internal atomic state can be completely protected from decoherence by interference of its interactions with the reservoir over many different time intervals in parallel. Such interference is conditional upon the detection of appropriate atomic-momentum observables. Realization in cavities is suggested. The rich arsenal of control methods described above can improve the performance of single-atom devices. It can also advance the state-of-the-art of quantum information encoding and processing.

We theoretically analyze wave packet transmission through a phase-conjugating mirror and show that the transmission of a suitably chosen input pulse is superluminal, i.e. the peak of the pulse emerges from the mirror before the time it takes to travel the same distance in vacuum. This pulse reshaping effect can be attributed directly to the dispersion relation in the nonlinear medium constituting the mirror. Thus, for the first time a connection is laid between optical phase conjugation and superluminal behavior. In view of its additional amplifying ability, a phase-conjugating mirror is a most promising candidate for an experimental observation of tachyonic signatures.

We theoretically investigate the response of optical phase conjugators to incident probe pulses. In the stable (subthreshold) operating regime of an optical phase conjugator it is possible to transmit probe pulses with a superluminally advanced peak, whereas conjugate reflection is always subluminal. In the unstable (above-threshold) regime, superluminal response occurs both in reflection and in transmission, at times preceding the onset of exponential growth due to the instability.

Remarkable quantum effects are predicted for an initially excited two-level wave packet incident on a broadband vacuum-field reservoir with sharp spatial boundaries: (a) reflection of the excited wave packet from the boundary (skin effect); (b) transient three-dimensional binding of the decayed wave packet to the boundary by photons carrying more energy than the incident wave packet. These effects are realizable for cold atoms in cavities where excitation decay is nearly exponential but strongly enhanced.

A mechanism for controlling arbitrary quantum-field states in a cavity is proposed. It is based on a single conditional measurement of the state of a two-level atom, following a periodic sequence of field-atom couplings with alternating near-resonant and off-resonant detunings. The resulting field state is controlled by the arbitrarily large number of parameters that characterize the sequence of detunings.

We show that by combining nonselective and conditional measurements on a sequence of excited two-level atoms traversing a resonant cavity, one can construct a practical strategy for compressing an arbitrary photon-number distribution (thermal distributions included) into a desired Fock state with high probability of success and low number of measurements.

A system of two identical atoms sharing a photon with a high-[Formula Presented] resonator mode is studied by a nonperturbative formalism. As opposed to previously considered models, here the interatom coupling is shown to result from competing effects of vacuum Rabi splitting and photon exchange via off-resonant modes. Strong suppression of interatom excitation transfer is predicted at both near- and far-zone separations.

We investigate the resonant interaction of a dipolar j=0⇆j=1 angular-momentum transition with the quantized field in a dielectric sphere. New features arise on account of the degeneracy of atomic levels and field modes with low azimuthal angular momentum, in slightly deformed spheres (oblate spheroids). For TE-mode excitation we obtain the dynamics of a degenerate Λ or V configuration with the usual coherent-state collapse and revivals. For TM-mode excitation new behavior is found: due to interference between σ- and π-polarized transitions, which can be controlled by the atomic position and/or dipole orientation, coherent-state revivals and the corresponding atom-field energy exchange may be suppressed or delayed.

Superluminal peak-traversal times of evanescent photonic wavepackets in dielectric structures are shown to result from destructive interference between waves which traverse multiple paths corresponding to successive causal time-delays. Information is shown to be transmitted always causally and only by non-analytic disturbances. EPR correlations between two photons are diminished if the photon traversing a structure is split between distinguishable paths on which the polarization vector is rotated proportionally to the path length.

We survey our recent results on the modifications of optical processes by field confining structures.

Quantum theory is applied to interference of emission and absorption in a two-section Cerenkov free-electron laser (FEL). A scheme for absorption cancellation with enhanced gain, which resembles ''lasing without inversion'' in atomic systems, is proposed. This scheme is based on the adjustment of the phases of electrons and light by appropriate dispersion between the two sections. It enables FEL operation even with a broad electron momentum spread. Such a scheme is interesting in the context of EEL operation in the ultraviolet and X-ray regimes.

A two-stage free-electron laser with a specially designed delay of electrons in the drift region between the two wigglers allows us to obtain essentially positive gain as a function of detuning, i.e., "lasing without inversion" (LWI). The key physical assumption is that it is possible to tell which electrons emit and which absorb laser energy. We here show that straightforward attempts to realize LWI operation in a free-electron laser are frequently (but not inevitably) frustrated by the difficulty of telling the difference between emitting and absorbing electrons. We propose a scheme utilizing both transverse and longitudinal components of the electron velocity to achieve cancellation of absorption. Numerical simulations verifying the validity of the scheme are presented.

We present a universal mechanism which allows us to relinquish momentum population inversion in free-electron lasers, and extract strongly enhanced gain from a broad momentum distribution. This is achievable by appropriately designed magnetic fields between two sequential interaction regions, which cause, purely classically, absorption cancellation (by destructive interference) and emission enhancement (by constructive interference).

We analyze the near-resonant interaction between a two-level atom with resonance frequency in a photonic band gap and a local-defect field mode with initially thermal or Poissonian photon statistics. Whereas spontaneous decay is forbidden in this case, atomic states dressed by the defect field can decay via coupling to the mode continuum outside the band gap. Remarkably, the resulting non-Markovian cascade of dressed states transitions can lead to the accumulation of nearly all of the initial photon-number distribution in a single Fock state, upon measuring the excitation of the atom emerging from the structure. Under certain conditions, such a measurement can yield a correlated combination of two adjacent Fock states, which reflects the initial coherence in either the atom or the field. Another striking coherence effect is the non-decaying oscillation (as a function of interaction time) exhibited by Fock-state populations prepared by measuring the final atomic state.

We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either 'bare' or 'dressed' by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of nondecaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the induced decay of a resonantly driven atom into such a continuum exhibits a cascade of transitions down the ladder of dressed states, which are labelled by decreasing photon numbers of the driving mode. Remarkably, this cascade is terminated at the dressed-state doublet, from which all subsequent transitions to lower doublets are forbidden because they fall within the PBG. This doublet then becomes an attractor state for the populations of higher-lying doublets. As a result, the photon-number distribution of the driving mode becomes strongly sub-Poissonian.

A general analysis of an oscillator with periodic frequency modulation is given. It is shown that squeezing and excitation energy exponentially grow with the number of modulation cycles under the condition of parametric instability. This condition yields a prescription for maximized squeezing when the frequency is periodically swept in an adiabatic fashion, with an abrupt return to the initial frequency at the end of each period. The type of modulation considered is shown to have remarkably broad instability domains near arbitrarily high ratios of the oscillator period to the modulation cycle duration. This property stands in striking contrast to the rapid narrowing of the squeezing domains with the ratio of the pump frequency to that of the signal in existing parametric processes. We discuss a possible realization of the proposed scheme, based on frequency modulation of a cavity mode in the microwave domain by a periodic train of optical pulses, and show that extremely strong squeezing is feasible under rather moderate requirements.

A structural defect in a two-dimensionally periodic dielectric structure may form a local field mode photonic within a band gap in which no other E-polarized field modes exist. We show that such a defect mode can give rise to new quantum-electrodynamic effects in resonant field-atom interaction, owing to the spatially modu- lated standing-wave character of its field. Atomic-beam motion is considered (a) along an interplanar defect and (b) through a spherical defect. The resonant interaction of a moving two-level atom with the quantized field of these defects can yield hitherto unknown featuresoscillatory patterns of the atomic population inversion, fluorescence spectra, and nonclassical field statesthat are essentially different from their counterparts in the standard Jaynes-Cummings model, which holds for atomic-beam motion in a spatially uniform singlemode field.

We put forward a simple, feasible scheme for the preparation and subsequent detection of macroscopic quantum superposition (MQS) states. It is based on the two-photon model which obtains when a cascade of two atomic transitions is resonant with twice the field frequency. The initial conditions amount to a field in a mixed state characteristic of lasers or masers and an excited atom. The MQS is generated by a conditional measurement of the atomic excitation after an interaction time that determines the relative phase of the MQS components. Remarkably, the MQS is subsequently detected and its phase is inferred by measuring the excitation probability of a second, probe, atom, as a function of its interaction time. The realization of the scheme in the optical domain, using dielectric microspheres, is discussed.

Keywords: PARTICLES

A system of two colliding neutral or ionized atoms with excited states that exhibit fine-structure splitting is shown to emit resonance fluorescence, the quantum features of which can reveal the collisional dynamical correlations between the atoms. Time-resolved measurements of quantum noise (squeezing properties) of the emitted field are shown to yield detailed information on the dynamical correlations. Notably, they disclose the phase between the dynamically correlated two-atom adiabatic states, which is not available from conventional spectroscopy. Such phase information can be a sensitive probe of collision dynamics. It is also important for the achievement of enhanced selectivity in laser-induced reactive collisions or dissociation, based on controllable coherent superpositions of adiabatic states.

Radiative coupling between identical atoms sharing an excitation is studied in media where dispersion creates photonic band gaps, i.e., forbidden bands of light propagation for all directions and polarizations. Photonic band gaps can exist in media whose dielectric index exhibits strong three-dimensionally periodic modulations (photonic crystals) or in bands of polaritonic media. It is shown that the resonant dipole-dipole interaction as well as cooperative fluorescence of two-atom systems can be strongly enhanced or suppressed, to the extent of being essentially eliminated, by one of the following mechanisms: (a) dependence on location of the atoms within a unit cell of a photonic crystal, i.e., sensitivity to the spatial variation of the field normal modes; (b) dependence on the density of normal modes in allowed bands and on the density of virtual (evanescent) modes in band gaps. The ability to suppress the dipole-dipole interactions at interatomic separations characterizing quasimolecules would have far-reaching implications on their dissociative or collisional dynamics, spectroscopy, and rates of energy transfer.

We predict a new type of time-resolved cooperative fluorescence (CF) obtainable when a homonuclear alkali-metal diatom dissociating via a *1 state evolves (by nonadiabatic radial coupling) into a superposition of two excited adiabatic states correlated to different doublet levels. CF intensity then combines ringing oscillations, caused by time variation of the interference phase between the receding emitting atoms, with quantum beats associated with fine-structure splitting. The calculated CF patterns for Li2 are sensitive to the amplitudes and relative phase of the two superposed adiabatic states.

A semiclassical theory of stimulated processes in dielectric spherical particles is formulated. The theory applies to the small-signal regime and to isotropic (but radially nonuniform) pumping. Iterative treatment of the pumped-medium susceptibility by scattering theory demonstrates the basic features observed experimentally by Chang and co-workers [H. M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, Opt. Lett. 9, 499 (1984); S. X. Qian and R. K. Chang, Phys. Rev. Lett. 56, 926 (1986)], namely, the drastic reduction of the threshold for lasing and multiorder stimulated Raman processes, and the frequency pulling from Mie resonances of the inactive medium.

The time-resolved cooperative emission from a system of correlated neutral dissociation fragments, or molecular collision products in beams, is investigated. The investigation is focused on emission at large fragment separations (between 1 nm and a few emission wavelengths), exceeding the domain of short-range interactions within the reactive or collisional molecular complex. A master-equation approach is used to obtain a general expression for the cooperative emission rate, which consists of nonexponential decay factors multiplied by temporal ringing patterns. These features result from the time-dependent radiative coupling between the receding fragments; they depend in an essential manner on the initial electronic state of the parent molecular complex and its symmetry which determine the correlations between the fragments. In the model system of a pair of identical two-level fragments two cases are considered separately. (a) A single photon shared by the fragments, where the emission is initially superradiant or subradiant (radiation trapping), depending on the spin and inversion symmetry of the parent molecular system and of the nascent fragments. The ringing pattern depends on the electronic angular momentum state of the parent molecule and on the polarization of the emitted light. (Such a ringing has been observed recently by Grangier, Aspect, and Vigué [Phys. Rev. Lett. 54, 418 (1985)] in the emission of photodissociated Ca2.) (b) Two initially excited fragments, where the ringing pattern is of smaller amplitude, and is weakly dependent on the electronic angular momentum of the parent molecule. All the aforementioned cooperative features generally last until the fragments recede several radiation wavelengths away from each other. The application of this time-resolved analysis to various diagnostic problems is discussed, especially with regard to the identification of excited electronic states of the parent molecular complex, and the stereospecificity of the fragmental orbitals.