Publications
2025
Highly entangled multiphoton graph states are a crucial resource in photonic quantum computation and communication. Yet, the lack of photon-photon interactions makes the construction of such graph states especially challenging. Typically, these states are produced through probabilistic single-photon sources and linear-optics entangling operations that require indistinguishable photons. The resulting inefficiency of these methods necessitates a large overhead in the number of sources and operations, creating a major bottleneck in the photonic approach. Here, we show how harnessing single-atom-based photonic operations can enable deterministic generation of photonic graph states, while also lifting the requirement for photon indistinguishability. To this end, we introduce a multigate quantum node comprising a single atom in a W-type level scheme coupled to an optical resonator. This configuration provides a versatile toolbox for generating graph states, allowing the operation of both the controlled-Z and swap photon-atom gates, as well as the deterministic generation of single photons. Furthermore, the ability to deterministically entangle photonic qubits enables the expansion of the generated state by stitching together graph states produced by different nodes. We investigate the implementation of this gate-based approach using 87Rb atoms and evaluate its performance through numerical simulations.
2024
Of the many applications of whispering-gallery-mode (WGM) micro-resonators, single-atom cavity quantum electrodynamics (cavity-QED) poses the most extreme demands on mode volume, dimensions, and quality factor (Q). Here we present the procedure for the fabrication of chip-based, small mode-volume, ultrahigh-Q silica WGM micro-resonators, designed for single-emitter cavity-QED. We demonstrate micro-resonators at varying geometries, from toroidal to micro-spheres, yielding ultrahigh-qualities as high as 1.7 × 108 at 780nm. We present a comprehensive theoretical model that allows tailoring the fabrication process to attain the desired micro-resonator geometry.
Linear-optics gates, the enabling tool of photonic quantum information processing, depend on indistinguishable photons, as they harness quantum interference to achieve nonlinear operations. Traditionally, meeting this criterion involves generating pure identical photons, a task that remains a significant challenge in the field. Yet, the required indistinguishability is linked to the spatial exchange symmetry of the multiphoton wave function and does not strictly necessitate identical photons. Here, we show that the ideal operation of two-photon gates, particularly fusion gates, can be recovered from distinguishable photons by ensuring the exchange symmetry of the input photonic state. To this end, we introduce a temporal quantum eraser between a pair of modally impure single-photon sources, which heralds the symmetry of the generated two-photon state. We demonstrate this mechanism in two relevant platforms: parametric photon pair generation and single-photon extraction by a single quantum emitter. The ability to lift the requirement for identical photons bears considerable potential in linear-optics quantum information processing.
Single-photon nonlinearity, namely, the change in the response of the system as the result of the interaction with a single photon, is generally considered an inherent property of a single quantum emitter. Although the dependence on the number of emitters is well understood for the case of two-level systems, deterministic operations such as single-photon switching or photon-atom gates inherently require more complex level structures. Here, we theoretically consider single-photon switching in ensembles of emitters with a Λ-level scheme and show that the switching efficiency vanishes with the number of emitters. Interestingly, the mechanism behind this behavior is the quantum Zeno effect, manifested in a slowdown of the photon-controlled dynamics of the atomic ground states.
We present rapid and robust protocols for stimulated rapid adiabatic passage and quantum logic gates. Our gates are based on geometric phases acquired by instantaneous eigenstates of a slowly accelerating "inertial"Hamiltonian. To begin, we establish the criteria for inertial evolution and subsequently engineer pulse shapes that fulfill these conditions. These tailored pulses are then used to optimize geometric logic gates. We analyze a realization of our protocols with 87Rb atoms, resulting in gate fidelity that approaches the current state of the art, with marked improvements in robustness.
Photon-number-splitting (PNS) is a well-known theoretical attack on quantum key distribution (QKD) protocols that employ weak coherent states produced by attenuated laser pulses. However, beyond the fact that it has not yet been demonstrated experimentally, its plausibility and effect on quantum bit error rate are questioned. In this work, an experimental scheme is presented for PNS attack employing demonstrated technological capabilities, specifically a single-photon Raman interaction (SPRINT) in a cavity-enhanced three-level atomic system. Several aspects of the proposed implementation are addressed, analytically and simulatively, and the eavesdropper's information gain by the attack is calculated. Furthermore, it is analytically shown that the scheme results in a small (yet non-zero) quantum bit error rate, and a comparison to purely theoretical analyses in the literature is presented. It is believed that the inherent nonlinearity of the PNS attack unavoidably affects the optical modes sent to the receiver, and accordingly will always result in some error rate.
2023
We present an inertial protocol for STIRAP-based single- and two-qubit quantum logic gates. Our protocol achieves an order-of-magnitude lower infidelity compared to existing protocols. A similar approach can be used to improve any adiabatic protocol.
2022
Crystals and fibers doped with rare-earth (RE) ions provide the basis for most of today's solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations, the former providing high homogeneity and coherence and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide─a tapered silica optical fiber─with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb3+[Zn(II)MC (QXA)] complexes, which isolate the RE ion from the environment and suppress nonradiative decay channels. We demonstrate that the beneficial optical transitions of the Yb3+ are retained after deposition on the tapered fiber and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb-doped inorganic crystals.
We propose a deterministic yet fully passive scheme to transfer the quantum state from a frequency-encoded photon to the spin of a quantum dot mediated by a nanophotonic waveguide. We assess the quality of the state transfer by studying the effects of all relevant experimental imperfections on the state-transfer fidelity. We show that a transfer fidelity exceeding 95% is achievable for experimentally realistic parameters. Our work sets the stage for deterministic solid-state quantum networks tailored to frequency-encoded photonic qubits.
2021
We present a new pathway towards fault-tolerant photonic quantum computing by using moderate nonlinearity to improve elementary computation operations. This improvement can lead to a three orders-of-magnitude reduction of the resource overhead in large-scale computations.
Photonic measurement-based quantum computation (MBQC) is a promising route towards fault-tolerant universal quantum computing. A central challenge in this effort is the huge overhead in the resources required for the construction of large photonic clusters using probabilistic linear-optics gates. Although strong single-photon nonlinearity ideally enables deterministic construction of such clusters, it is challenging to realise in a scalable way. Here we explore the prospects of using moderate nonlinearity (with conditional phase shifts smaller than π ) to boost photonic quantum computing and significantly reduce its resources overhead. The key element in our scheme is a nonlinear router that preferentially directs photonic wavepackets to different output ports depending on their intensity. As a relevant example, we analyze the nonlinearity provided by Rydberg blockade in atomic ensembles, in which the trade-off between the nonlinearity and the accompanying loss is well understood. We present protocols for efficient Bell measurement and GHZ-state preparationboth key elements in the construction of cluster states, as well as for the cnot gate and quantum factorization. Given the large number of entangling operations involved in fault-tolerant MBQC, the increase in success probability provided by our protocols already at moderate nonlinearities can result in a significant reduction in the required resources.
We fabricate an extremely thin optical fiber that supports a super-extended mode with a diameter as large as 13 times the optical wavelength, residing almost entirely outside the fiber and guided over thousands of wavelengths (5 mm), in order to couple guided light to warm atomic vapor. This unique configuration balances between strong confinement, as evident by saturation powers as low as tens of nW, and long interaction times with the thermal atoms, thereby enabling fast and coherent interactions. We demonstrate narrow coherent resonances (tens of MHz) of electromagnetically induced transparency for signals at the single-photon level and long relaxation times (10 ns) of atoms excited by the guided mode. The dimensions of the guided mode's evanescent field are compatible with the Rydberg blockade mechanism, making this platform particularly suitable for observing quantum non-linear optics phenomena.
2020
Whispering-gallery-mode (WGM) microresonators are a promising platform for highly sensitive, label-free detection and probing of individual nano-objects. Our work expands these capabilities by providing the analysis tools required for three-dimensional (3D) characterization of arbitrarily shaped nanoparticles. Specifically, we introduce a theoretical model that describes interactions between nanoparticles and WGM resonators, taking into account effects that were often not considered, such as the elliptical polarization of the transverse-magnetic (TM) mode, the possible non-spherical shape of the nanoparticle, its finite size, and the open-system nature of the modes. We also introduce a self referencing measurement method that allows the extraction of information from measurements done at arbitrary positions of the nanoparticles within the WGM. We verify our model by experimentally probing a single Tungsten-disulfide (WS2) nanotube with a silica microtoroid resonator inside a scanning electron-microscope (SEM) and perform 3D characterization of the nanotube.
We present a scheme for deterministic ion-photon qubit exchange, namely a SWAP gate, based on realistic cavity-QED systems with
171Yb
+
40Ca
+ and
138Ba
+ ions. The gate can also serve as a single-photon quantum memory, in which an outgoing photon heralds the successful arrival of the incoming photonic qubit. Although strong coupling, namely having the single-photon Rabi frequency be the fastest rate in the system, is often assumed essential, this gate (similarly to the Duan-Kimble C-phase gate) requires only Purcell enhancement, i.e. high single-atom cooperativity. Accordingly, it does not require small mode volume cavities, which are challenging to incorporate with ions due to the difficulty of trapping them close to dielectric surfaces. Instead, larger cavities, potentially more compatible with the trap apparatus, are sufficient, as long as their numerical aperture is high enough to maintain small mode area at the ion's position. We define the optimal parameters for the gate's operation and simulate the expected fidelities and efficiencies, demonstrating that efficient photon-ion qubit exchange, a valuable building block for scalable quantum computation, is practically attainable with current experimental capabilities.
2019
We introduce a multi-step protocol for optical quantum state engineering that performs as \u201cbright quantum scissors,\u201d namely truncating an arbitrary input quantum state to have at least a certain number of photons. The protocol exploits single-photon pulses and is based on the effect of single-photon Raman interaction, which is implemented with a single three-level Λ system (e.g., a single atom) Purcell-enhanced by a single-sided cavity. A single step of the protocol realizes the inverse of the bosonic annihilation operator. Multiple iterations of the protocol can be used to deterministically generate a chain of single photons in a W state. Alternatively, upon appropriate heralding, the protocol can be used to generate Fock-state optical pulses. This protocol could serve as a useful and versatile building block for the generation of advanced optical quantum states that are vital for quantum communication, distributed quantum information processing, and all-optical quantum computing.
2018
Deterministic quantum interactions between single photons and single quantum emitters are a vital building block towards the distribution of quantum information between remote systems(1-4). Deterministic photon-atom state transfer has previously been demonstrated with protocols that include active feedback or synchronized control pulses(5-10). Here we demonstrate a passive swap operation between the states of a single photon and a single atom. The underlying mechanism is single-photon Raman interaction(11-15)-an interference-based scheme that leads to deterministic interaction between two photonic modes and the two ground states of a Lambda-system. Using a nanofibre-coupled microsphere resonator coupled to single Rb atoms, we swap a photonic qubit into the atom and back, demonstrating fidelities exceeding the classical threshold of 2/3 in both directions. In this simultaneous write and read process, the returning photon, which carries the readout of the atomic qubit, also heralds the successful arrival of the write photon. Requiring no control fields, this single-step gate takes place automatically at the timescale of the atom's cavity-enhanced spontaneous emission. Applicable to any waveguide-coupled Lambda-system, this mechanism, which can also be harnessed to construct universal gates(16,17), provides a versatile building block for the modular scaling up of quantum information systems.
2017
The long-standing goal of deterministic quantum interactions between single photons and single atoms was recently realized in various experiments. Among these, an appealing demonstration relied on single-photon Raman interaction (SPRINT) in a three-level atom coupled to a single-mode waveguide. In essence, the interference-based process of SPRINT deterministically swaps the qubits encoded in a single photon and a single atom, without the need for additional control pulses. It can also be harnessed to construct passive entangling quantum gates, and can therefore form the basis for scalable quantum networks in which communication between the nodes is carried out only by single-photon pulses. Here we present an analytical and numerical study of SPRINT, characterizing its limitations and defining parameters for its optimal operation. Specifically, we study the effect of losses, imperfect polarization, and the presence of multiple excited states. In all cases we discuss strategies for restoring the operation of SPRINT.
Using single-photon Raman interaction, we demonstrate a deterministic and passive SWAP gate and quantum memory between the states of a single photon and a single 87Rb atom coupled to high-Q microresonator.
2016
Removing a single photon from a pulse is one of the most elementary operations that can be performed on light, having both fundamental significance and practical applications in quantum communication and computation. So far, photon subtraction, in which the removed photon is detected and therefore irreversibly lost, has been implemented in a probabilistic manner with inherently low success rates using low-reflectivity beam splitters. Here we demonstrate a scheme for the deterministic extraction of a single photon from an incoming pulse. The removed photon is diverted to a different mode, enabling its use for other purposes, such as a photon number-splitting attack on quantum key distribution protocols. Our implementation makes use of single-photon Raman interaction (SPRINT) with a single atom near a nanofibre-coupled microresonator. The single-photon extraction probability in our current realization is limited mostly by linear loss, yet probabilities close to unity should be attainable with realistic experimental parameters.
We demonstrate a passive scheme for deterministic interactions between a single photon and a single atom. Relying on single-photon Raman interaction (SPRINT), this control-fields free scheme swaps a flying qubit, encoded in the two possible input modes of a photon, with a stationary qubit, encoded in the two ground states of the atom, and can be also harnessed to perform universal quantum gates. Using SPRINT we experimentally demonstrated all-optical switching of single photons by single photons, and deterministic extraction of a single photon from an optical pulse. Applicable to any atom-like Lambda system, SPRINT provides a versatile building block for scalable quantum networks based on completely passive nodes interconnected and activated solely by single photons.
2015
Spectroscopy of whispering-gallery mode microresonators has become a powerful scientific tool, enabling the detection of single viruses, nanoparticles and even single molecules. Yet the demonstrated timescale of these schemes has been limited so far to milliseconds or more. Here we introduce a scheme that is orders of magnitude faster, capable of capturing complete spectral snapshots at nanosecond timescales - cavity ring-up spectroscopy. Based on sharply rising detuned probe pulses, cavity ring-up spectroscopy combines the sensitivity of heterodyne measurements with the highest-possible, transform-limited acquisition rate. As a demonstration, we capture spectra of microtoroid resonators at time intervals as short as 16 ns, directly monitoring submicrosecond dynamics of their optomechanical vibrations, thermorefractive response and Kerr nonlinearity. Cavity ring-up spectroscopy holds promise for the study of fast biological processes such as enzyme kinetics, protein folding and light harvesting, with applications in other fields such as cavity quantum electrodynamics and pulsed optomechanics.
We demonstrate deterministic photon-atom and photon-photon interactions with a single atom coupled to a high-Q fiber-coupled microresonator. Based on Deterministic One Photon Raman Interaction (DOPRI), this scheme can form the basis for all-optical quantum information processing.
We demonstrate all-optical deterministic photon-atom and photon-photon interactions with a single Rb atom coupled to high-Q fiber-coupled microresonator. This scheme enables all-optical photon routing, passive quantum memory and quantum gates activated solely by single photons.
2014
The prospect of quantum networks, in which quantum information is carried by single photons in photonic circuits, has long been the driving force behind the effort to achieve all-optical routing of single photons.We realized a single-photon-activated switch capable of routing a photon from any of its two inputs to any of its two outputs. Our device is based on a single atom coupled to a fiber-coupled, chip-based microresonator. A single reflected control photon toggles the switch from high reflection (R ∼ 65%) to high transmission (T ∼ 90%), with an average of ∼1.5 control photons per switching event (∼3, including linear losses). No additional control fields are required.The control and target photons are both in-fiber and practically identical, making this scheme compatible with scalable architectures for quantum information processing.
2013
We experimentally demonstrate first-order (fold) and second-order (cusp) catastrophes in the density of an atomic cloud reflected from an optical barrier in the presence of gravity and show their corresponding universal asymptotic behavior. These catastrophes, arising from classical dynamics, enable robust, field-free refocusing of an expanding atomic cloud with a wide velocity distribution. Specifically, the density attained at the cusp point in our experiment reached 65% of the peak density of the atoms in the trap prior to their release. We thereby add caustics to the various phenomena with parallels in optics that can be harnessed for manipulation of cold atoms. The structural stability of catastrophes provides inherent robustness against variations in the system's dynamics and initial conditions, making them suitable for manipulation of atoms under imperfect conditions and limited controllability.
We demonstrate a new type of weak measurement based on the dynamics of spontaneous emission. The pointer in our scheme is given by the Lorentzian distribution characterizing atomic exponential decay via emission of a single photon. We thus introduce weak measurement, so far demonstrated nearly exclusively with laser beams and Gaussian statistics, into the quantum regime of single emitters and single quanta, enabling the exploitation of a wide class of sources that are abundant in nature. We describe a complete analogy between our scheme and weak measurement with conventional Gaussian pointers. Instead of a shift in the mean of a Gaussian distribution, an imaginary weak value is exhibited in our scheme by a significantly slower-than-natural exponential distribution of emitted photons at the postselected polarization, leading to a large shift in their mean arrival time. The dynamics of spontaneous emission offer a broader view of the measurement process than is usually considered within the weak measurement formalism. Our scheme opens the path for the use of atoms and atomlike systems as sensitive probes in weak measurements, one example being optical magnetometry.
In a recent Letter, Brunner and Simon proposed an interferometric scheme using imaginary weak values with a frequency-domain analysis to outperform standard interferometry in longitudinal phase shifts [Phys. Rev. Lett 105, 010405 (2010)]. Here we demonstrate an interferometric scheme combined with a time-domain analysis to measure longitudinal velocities. The technique employs the near-destructive interference of non-Fourier limited pulses, one Doppler shifted due to a moving mirror in a Michelson interferometer. We achieve a velocity measurement of 400 fm/s and show our estimator to be efficient by reaching its Cramér-Rao bound.
2011
The most simple and seemingly straightforward application of the photon blockade effect, in which the transport of one photon prevents the transport of others, would be to separate two incoming indistinguishable photons to different output ports. We show that time-energy uncertainty relations inherently prevent this ideal situation when the blockade is implemented by a two-level system. The fundamental nature of this limit is revealed in the fact that photon blockade in the strong coupling regime of cavity QED, resulting from the nonlinearity of the Jaynes-Cummings energy level structure, exhibits efficiency and temporal behavior identical to those of photon blockade in the bad cavity regime, where the underlying nonlinearity is that of the atom itself. We demonstrate that this limit can be exceeded, yet not avoided, by exploiting time-energy entanglement between the incident photons. Finally, we show how this limit can be circumvented completely by using a three-level atom coupled to a single-sided cavity, enabling an ideal and robust photon routing mechanism.
2009
Single photons from a coherent input are efficiently redirected to a separate output by way of a fiber-coupled microtoroidal cavity interacting with individual cesium atoms. By operating in an overcoupled regime for the input-output to a tapered fiber, our system functions as a quantum router with high efficiency for photon sorting. Single photons are reflected and excess photons transmitted, as confirmed by observations of photon antibunching (bunching) for the reflected (transmitted) light. Our photon router is robust against large variations of atomic position and input power, with the observed photon antibunching persisting for intracavity photon number 0.03 n̄ 0.7.
We demonstrate robust and efficient routing of single photons using a microtoroidal cavity QED system. Single photons from a coherent input are sorted to one output of the fiber with excess photons redirected to the other.
We demonstrate robust and efficient routing of single photons using a microtoroidal cavity QED system. Single photons from a coherent input are sorted to one output of the fiber with excess photons redirected to the other.
2008
Beyond traditional nonlinear optics with large numbers of atoms and photons, qualitatively new phenomena arise in a quantum regime of strong interactions between single atoms and photons. By using a microscopic optical resonator, we achieved such interactions and demonstrated a robust, efficient mechanism for the regulated transport of photons one by one. With critical coupling of the input light, a single atom within the resonator dynamically controls the cavity output conditioned on the photon number at the input, thereby functioning as a photon turnstile. We verified the transformation from a Poissonian to a sub-Poissonian photon stream by photon counting measurements of the input and output fields. The results have applications in quantum information science, including for controlled interactions of single light quanta and for scalable quantum processing on atom chips.
2007
When two-photon interactions are induced by down-converted light with a bandwidth that exceeds the pump bandwidth, they can obtain a behavior that is pulselike temporally, yet spectrally narrow. At low photon fluxes this behavior reflects the time and energy entanglement between the down-converted photons. However, two-photon interactions such as two-photon absorption (TPA) and sum-frequency generation (SFG) can exhibit such a behavior even at high power levels, as long as the final state (i.e., the atomic level in TPA, or the generated light in SFG) is narrow-band enough. This behavior does not depend on the squeezing properties of the light, is insensitive to linear losses, and has potential applications. In this paper we describe analytically this behavior for traveling-wave down conversion with continuous or pulsed pumping, both for high- and low-power regimes. For this we derive a quantum-mechanical expression for the down-converted amplitude generated by an arbitrary pump, and formulate operators that represent various two-photon interactions induced by broadband light. This model is in excellent agreement with experimental results of TPA and SFG with high-power down-converted light and with entangled photons.
We describe a novel non-linear detection method for optical tomography that does not rely on detection of interference fringes and is free of optical background. The method exploits temporally coherent broadband illumination such as ultrashort pulses, and a non-linear two-photon detection process such as sum-frequency generation (SFG). At the detection stage, the reference beam and the sample beam are mixed in a thick non-linear crystal, and only the mixing term, which is free of optical background, is detected. Consequently, the noise limitations posed by the background in standard OCT (excess and shot noise), do not exist here. Due to the non-linearity, the signal to noise ratio scales more favorably with the optical power compared to standard OCT, yielding an inherent improvement for high speed tomographic scans. Careful design of phase matching in the crystal enables non-linear mixing which is both highly efficient and broadband, yielding both high sensitivity and high depth resolution.
We demonstrate a scheme to spectrally manipulate a collinear, continuous stream of time and energy entangled photons to generate beamlike, bandwidth-limited fluxes of polarization-entangled photons with nearly degenerate wavelengths. Utilizing an ultrashort-pulse shaper to control the spectral phase and polarization of the photon pairs, we tailor the shape of the Hong-Ou-Mandel interference pattern, demonstrating the rules that govern the dependence of this interference pattern on the spectral phases of the photons. We then use the pulse shaper to generate all four polarization Bell states. The singlet state generated by this scheme forms a very robust decoherence-free subspace, extremely suitable for long-distance fiber-optics-based quantum communication.
2006
Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics with diverse physical systems, including single atoms in Fabry-Perot resonators, quantum dots coupled to micropillars and photonic bandgap cavities and Cooper pairs interacting with superconducting resonators. Experiments with single, localized atoms have been at the forefront of these advances with the use of optical resonators in high-finesse Fabry-Perot configurations. As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems. Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks, scalable quantum logic with photons, and quantum information processing on atom chips.
We present the design and experimental proof of principle of a low threshold optical parametric oscillator (OPO) that continuously oscillates over a large bandwidth allowed by phase matching. The large oscillation bandwidth is achieved with a selective two-photon loss that suppresses the inherent mode competition, which tends to narrow the bandwidth in conventional OPOs. Our design performs pairwise mode locking of many frequency pairs, in direct equivalence to passive mode locking of ultrashort pulsed lasers. The ability to obtain high powers of continuous and broadband down-converted light enables the optimal exploitation of the correlations within the down-converted spectrum, thereby strongly affecting two-photon interactions even at classically high power levels, and opening new venues for applications such as two-photon spectroscopy and microscopy and optical spread spectrum communication.
2005
We experimentally demonstrate shaping of the two-photon wave function of entangled-photon pairs, utilizing coherent pulse-shaping techniques. By performing spectral-phase manipulations we tailor the second-order correlation function of the photons exactly like a coherent ultrashort pulse. To observe the shaping we perform sum-frequency generation with an ultrahigh flux of entangled photons. At the appropriate conditions, sum-frequency generation performs as a coincidence detector with an ultrashort response time (∼100 fs), enabling a direct observation of the two-photon wave function. This property also enables us to demonstrate background-free, high-visibility two-photon interference oscillations.
We experimentally demonstrate sum-frequency generation with entangled photon pairs, generating as many as 40000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the down-converted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantum-measurement tasks.
2004
The smallest spot in optical lithography and microscopy is generally limited by diffraction. Quantum lithography, which utilizes interference between groups of N entangled photons, was recently proposed to beat the diffraction limit by a factor N. Here we propose a simple method to obtain N photons interference with classical pulses that excite a narrow multiphoton transition, thus shifting the "quantum weight" from the electromagnetic field to the lithographic material. We show how a practical complete lithographic scheme can be developed and demonstrate the underlying principles experimentally by two-photon interference in atomic Rubidium, to obtain focal spots that beat the diffraction limit by a factor of 2.
The two-photon absorption in rubidium was investigated with broadband down-converted light. The pulse-shaping methods were used to control two-photon absorption, which demonstrated that the spectral and temporal resolutions were 3-5 orders of magnitude below the actual bandwidth and temporal duration of the light. The pulse shaper separates the spectral components of the beam and ultilizes a computer controlled spatial light modulator (SLM) to introduce desired spectral phase filter to the light. It was found that the broadband down-converted light induced two-photon absorption with the same sharp temporal behavior as femtosecond pulses.
We report two control experiments that exploit the coherent correlations within the spectrum of incoherent broadband down-converted light to demonstrate ultrahigh spectral and temporal resolutions at high power and inherent non-classical features at low power.
A novel approach for an optical direct-sequence spread spectrum is presented. It is based on the complementary processes of broad-band parametric down-conversion and up-conversion. With parametric down-conversion, a narrow-band continuous-wave (CW) optical field is transformed into two CW broad-band white-noise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical direct-sequence spread spectrum. The inverse process of parametric up-conversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical code-division multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal white-noise key is automatically generated, leading to high-capacity versatile code-division multiple-access configurations.
2001
Resonant multiphoton transitions were enhanced by exploiting the general spectral response of the interaction around resonance. Large enhancements of resonant two-photon absorption (TPA) were achieved by properly designing the spectral amplitude and phase of the exciting pulse.