Shaping quantum pulses of light via coherent atomic memory

Deterministic Shaping and Reshaping of Single-Photon Temporal Wave Functions

Thorough control of the optical mode of a single photon is essential for quantum information applications. We present a comprehensive experimental and theoretical study of a light-matter interface based on cavity quantum electrodynamics. We identify key parameters like the phases of the involved light fields and demonstrate absolute, flexible, and accurate control of the time-dependent complex-valued wave function of a single photon over several orders of magnitude. This capability will be an important tool for the development of distributed quantum systems with multiple components that interact via photons.

Optical quantum memory based on electromagnetically induced transparency

Electromagnetically induced transparency (EIT) is a promising approach to implement quantum memory in quantum communication and quantum computing applications. In this paper, following a brief overview of the main approaches to quantum memory, we provide details of the physical principle and theory of quantum memory based specifically on EIT. We discuss the key technologies for implementing quantum memory based on EIT and review important milestones, from the first experimental demonstration to current applications in quantum information systems.

Generation of single photons with highly tunable wave shape from a cold atomic ensemble

The generation of ultra-narrowband, pure and storable single photons with widely tunable wave shape is an enabling step toward hybrid quantum networks requiring interconnection of remote disparate quantum systems. It allows interaction of quantum light with several material systems, including photonic quantum memories, single trapped ions and opto-mechanical systems. Previous approaches have offered a limited tuning range of the photon duration of at most one order of magnitude. Here we report on a heralded single photon source with controllable emission time based on a cold atomic ensemble, which can generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the readout efficiency. We prove the nonclassicality of the emitted photons, show that they are emitted in a pure state, and demonstrate that ultra-long photons with nonstandard wave shape can be generated, which are ideally suited for several quantum information tasks.

Generation of narrowband pure and storable single photons is an enabling step towards hybrid quantum networks interconnecting different systems. Here the authors report on a heralded single photon source based on a cold ensemble of atoms with controllable emission time and high photon shape tunability.

Magnetically tuned, robust and efficient filtering system for spatially multimode quantum memory in warm atomic vapors

Warm atomic vapor quantum memories are simple and robust, yet suffer from a number of parasitic processes which produce excess noise. For operating in a single-photon regime precise filtering of the output light is essential. Here, we report a combination of magnetically tuned absorption and Faraday filters, both light-direction insensitive, which stop the driving lasers and attenuate spurious fluorescence and four-wave mixing while transmitting narrowband Stokes and anti-Stokes photons generated in write-in and readout processes. We characterize both filters with respect to adjustable working parameters. We demonstrate a significant increase in the signal-to-noise ratio upon applying the filters seen qualitatively in measurements of correlation between the Raman scattered photons.

Simple and efficient absorption filter for single photons from a cold atom quantum memory

The ability to filter unwanted light signals is critical to the operation of quantum memories based on neutral atom ensembles. Here we demonstrate an efficient frequency filter which uses a vapor cell filled with (85)Rb and a buffer gas to attenuate both residual laser light and noise photons by nearly two orders of magnitude with little loss to the single photons associated with our cold (87)Rb quantum memory. This simple, passive filter provides an additional 18 dB attenuation of our pump laser and erroneous spontaneous emissions for every 1 dB loss of the single photon signal. We show that the addition of a frequency filter increases the non-classical correlations and the retrieval efficiency of our quantum memory by ≈ 35%.

Quantum memory in warm rubidium vapor with buffer gas

The realization of quantum memory using warm atomic vapor cells is appealing because of their commercial availability and the perceived reduction in experimental complexity. In spite of the ambiguous results reported in the literature, we demonstrate that quantum memory can be implemented in a single cell with buffer gas using the geometry where the write and read beams are nearly copropagating. The emitted Stokes and anti-Stokes photons display cross-correlation values greater than 2, characteristic of quantum states, for delay times up to 4 μs.

Simultaneous wavelength translation and amplitude modulation of single photons from a quantum dot

Hybrid quantum information devices that combine disparate physical systems interacting through photons offer the promise of combining low-loss telecommunications wavelength transmission with high fidelity visible wavelength storage and manipulation. The realization of such systems requires control over the waveform of single photons to achieve spectral and temporal matching. Here, we experimentally demonstrate the simultaneous wavelength translation and amplitude modulation of single photons generated by a quantum dot emitting near 1300 nm with an exponentially decaying waveform (lifetime ≈1.5 ns). Quasi-phase-matched sum-frequency generation with a pulsed 1550 nm laser creates single photons at 710 nm with a controlled amplitude modulation at 350 ps time scales.

Phase-controlled switching by interference between incoherent fields in a double-Λ system

We showed experimentally interference could be occurred between incoherent lights in a double-Λ lambda transition implemented with rubidium atomic vapor. Switching of probe transmission was controlled by the phases of two` independent probe lasers with low light intensity. More than 70% of the probe transmission could be switched by ultra-weak incoherent field. We suggested optically cryptic information could be delivered by the phase-controlled switching with incoherent fields in a double-Λ system.

Observation of the Rabi oscillation of light driven by an atomic spin wave

Coherent conversion between a Raman pump field and its Stokes field is observed in a Raman process with a strong atomic spin wave initially prepared by another Raman process operated in the stimulated emission regime. The oscillatory behavior resembles the Rabi oscillation in atomic population in a two-level atomic system driven by a strong light field. The Rabi-like oscillation frequency is found to be related to the strength of the prebuilt atomic spin wave. High conversion efficiency of 40% from the Raman pump field to the Stokes field is recorded and it is independent of the input Raman pump field. This process can act as a photon frequency multiplexer and may find wide applications in quantum information science.

Single-molecule photon emission statistics for systems with explicit time dependence: Generating function approach

Generating function calculations are extended to allow for laser pulse envelopes of arbitrary shape in numerical applications. We investigate photon emission statistics for two-level and V- and Lambda-type three-level systems under time-dependent excitation. Applications relevant to electromagnetically induced transparency and photon emission from single quantum dots are presented.

Non-degenerated nonclassical photon pairs in a hot atomic ensemble

We experimentally demonstrate the generation of non-degenerate nonclassical paired photons in a hot atomic ensemble using off-axis four-wave mixing. The time-resolved second-order correlated function between the Stokes photon and the anti-Stokes photon is given. The two-photon correlation between the photons obtained in this experiment is 1.87+/- 0.04, which leads to the violation of Cauchy-Schwarz inequality by a factor of 1.69+/- 0.14.

Electro-optic modulation of single photons

We use the Stokes photon of a biphoton pair to set the time origin for electro-optic modulation of the wave function of the anti-Stokes photon thereby allowing arbitrary phase and amplitude modulation. We demonstrate conditional single-photon wave functions composed of several pulses, or instead, having Gaussian or exponential shapes.

Storing images in warm atomic vapor

Reversible and coherent storage of light in an atomic medium is a promising method with possible applications in many fields. In this work, arbitrary two-dimensional images are slowed and stored in warm atomic vapor for up to 30 micros, utilizing electromagnetically induced transparency. Both the intensity and the phase patterns of the optical field are maintained. The main limitation on the storage resolution and duration is found to be the diffusion of atoms. A technique analogous to phase-shift lithography is employed to diminish the effect of diffusion on the visibility of the reconstructed image.

Low-frequency vacuum squeezing via polarization self-rotation in Rb vapor

We observed squeezed vacuum light at 795 nm in (87)Rb vapor via resonant polarization self-rotation and report noise sidebands suppression of approximately 1 dB below shot-noise level spanning from 30 kHz to 1.2 MHz frequencies. To our knowledge, this is the first demonstration of submegahertz quadrature vacuum squeezing in atomic systems. The spectral range of observed squeezing matches well typical bandwidths of electromagnetically induced transparency (EIT) resonances, making this simple technique for generation of optical fields with nonclassical statistics at atomic transitions wavelengths attractive for EIT-based quantum information protocols applications.

Optimal control of light pulse storage and retrieval

We demonstrate experimentally a procedure to obtain the maximum efficiency for the storage and retrieval of light pulses in atomic media. The procedure uses time-reversal to obtain optimal input signal pulse shapes. Experimental results in warm Rb vapor are in good agreement with theoretical predictions and demonstrate a substantial improvement of efficiency. This optimization procedure is applicable to a wide range of systems.

High-contrast coherent population trapping resonances using four-wave mixing in 87Rb

We demonstrate very high-contrast coherent population trapping(1) (CPT) resonances by using four-wave mixing in (87)Rb atoms. In the experiment, we take advantage of the spectral overlap between F=2-->F(?) and F=3-->F(?) optical resonances on the D1 line of (87)Rb and (85)Rb atoms, respectively, to eliminate the DC-light background from the CPT resonance signal. We observe a CPT resonance with a contrast in the range of 90%, compared with a few percent achieved by alternative methods.

Interfacing collective atomic excitations and single photons

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of chi=0.84(11) and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.

Deterministic and storable single-photon source based on a quantum memory

A single-photon source is realized with a cold atomic ensemble (87Rb atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.

Direct measurement of decoherence for entanglement between a photon and stored atomic excitation

Violations of a Bell inequality are reported for an experiment where one of two entangled qubits is stored in a collective atomic memory for a user-defined time delay. The atomic qubit is found to preserve the violation of a Bell inequality for storage times up to 21 micros, 700 times longer than the duration of the excitation pulse that creates the entanglement. To address the question of the security of entanglement-based cryptography implemented with this system, an investigation of the Bell violation as a function of the cross correlation between the generated nonclassical fields is reported, with saturation of the violation close to the maximum value allowed by quantum mechanics.

A High-Brightness Source of Narrowband, Identical-Photon Pairs

We generated narrowband pairs of nearly identical photons at a rate of 5 x 10(4) pairs per second from a laser-cooled atomic ensemble inside an optical cavity. A two-photon interference experiment demonstrated that the photons could be made 90% indistinguishable, a key requirement for quantum information-processing protocols. Used as a conditional single-photon source, the system operated near the fundamental limits on recovery efficiency (57%), Fourier transform-limited bandwidth, and pair-generation-rate-limited suppression of two-photon events (factor of 33 below the Poisson limit). Each photon had a spectral width of 1.1 megahertz, ideal for interacting with atomic ensembles that form the basis of proposed quantum memories and logic.

Deterministic single photons via conditional quantum evolution

A source of deterministic single photons is proposed and demonstrated by the application of a measurement-based feedback protocol to a heralded single-photon source consisting of an ensemble of cold rubidium atoms. Our source is stationary and produces a photoelectric detection record with sub-Poissonian statistics.

Unconditional two-mode squeezing of separated atomic ensembles

We propose schemes for the unconditional preparation of a two-mode squeezed state of effective bosonic modes realized in a pair of atomic ensembles interacting collectively with optical cavity and laser fields. The scheme uses Raman transitions between stable atomic ground states and under ideal conditions produces pure entangled states in the steady state. The scheme works both for ensembles confined within a single cavity and for ensembles confined in separate, cascaded cavities.

Low-light-level cross-phase-modulation based on stored light pulses

We experimentally demonstrate a low-light-level cross-phase-modulation (XPM) scheme based on the light-storage technique in laser-cooled 87Rb atoms. The proposed scheme can achieve a similar phase shift and has the same figure of merit as one using static electromagnetically induced transparency under the constant coupling field. Nevertheless, the phase shift and the energy loss of a probe pulse induced by a signal pulse are neither influenced by the coupling intensity nor by the atomic optical density in the light-storage XPM scheme. This scheme enhances the flexibility of the experiment and makes possible conditional phase shifts on the order of pi with single photons.

Observation of dark state polariton collapses and revivals

By time-dependent variation of a control field, both coherent and single-photon states of light are stored in, and retrieved from, a cold atomic gas. The efficiency of retrieval is studied as a function of the storage time in an applied magnetic field. A series of collapses and revivals is observed, in very good agreement with theoretical predictions. The observations are interpreted in terms of the time evolution of the collective excitation of atomic spin wave and light wave, known as the dark-state polariton.

Electromagnetically induced transparency with tunable single-photon pulses

Techniques to facilitate controlled interactions between single photons and atoms are now being actively explored. These techniques are important for the practical realization of quantum networks, in which multiple memory nodes that utilize atoms for generation, storage and processing of quantum states are connected by single-photon transmission in optical fibres. One promising avenue for the realization of quantum networks involves the manipulation of quantum pulses of light in optically dense atomic ensembles using electromagnetically induced transparency (EIT, refs 8, 9). EIT is a coherent control technique that is widely used for controlling the propagation of classical, multi-photon light pulses in applications such as efficient nonlinear optics. Here we demonstrate the use of EIT for the controllable generation, transmission and storage of single photons with tunable frequency, timing and bandwidth. We study the interaction of single photons produced in a 'source' ensemble of 87Rb atoms at room temperature with another 'target' ensemble. This allows us to simultaneously probe the spectral and quantum statistical properties of narrow-bandwidth single-photon pulses, revealing that their quantum nature is preserved under EIT propagation and storage. We measure the time delay associated with the reduced group velocity of the single-photon pulses and report observations of their storage and retrieval.

Storage and retrieval of single photons transmitted between remote quantum memories

An elementary quantum network operation involves storing a qubit state in an atomic quantum memory node, and then retrieving and transporting the information through a single photon excitation to a remote quantum memory node for further storage or analysis. Implementations of quantum network operations are thus conditioned on the ability to realize matter-to-light and/or light-to-matter quantum state mappings. Here we report the generation, transmission, storage and retrieval of single quanta using two remote atomic ensembles. A single photon is generated from a cold atomic ensemble at one site , and is directed to another site through 100 metres of optical fibre. The photon is then converted into a single collective atomic excitation using a dark-state polariton approach. After a programmable storage time, the atomic excitation is converted back into a single photon. This is demonstrated experimentally, for a storage time of 0.5 microseconds, by measurement of an anti-correlation parameter. Storage times exceeding ten microseconds are observed by intensity cross-correlation measurements. This storage period is two orders of magnitude longer than the time required to achieve conversion between photonic and atomic quanta. The controlled transfer of single quanta between remote quantum memories constitutes an important step towards distributed quantum networks.

Measurement-induced entanglement for excitation stored in remote atomic ensembles

A critical requirement for diverse applications in quantum information science is the capability to disseminate quantum resources over complex quantum networks. For example, the coherent distribution of entangled quantum states together with quantum memory (for storing the states) can enable scalable architectures for quantum computation, communication and metrology. Here we report observations of entanglement between two atomic ensembles located in distinct, spatially separated set-ups. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10(5) atoms at each site. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations represent significant progress in the ability to distribute and store entangled quantum states.

On-demand superradiant conversion of atomic spin gratings into single photons with high efficiency

We create quantized spin gratings by single-photon detection and convert them on demand into photons with retrieval efficiencies exceeding 40% (80%) for single (a few) quanta. We show that the collective conversion process, proceeding via superradiant emission into a moderate-finesse optical resonator, requires phase matching. The storage time of 3 micros in the cold-atom sample, as well as the peak retrieval efficiency, are likely limited by Doppler decoherence of the entangled state.

Stopped light with storage times greater than one second using electromagnetically induced transparency in a solid

We report on the demonstration of light storage for times greater than a second in praseodymium doped Y2SiO5 using electromagnetically induced transparency. The long storage times were enabled by the long coherence times possible for the hyperfine transitions in this material. The use of a solid-state system also enabled operation with the probe and coupling beam counter-propagating, allowing easy separation of the two beams. The efficiency of the storage was low because of the low optical thickness of the sample; as is discussed, this deficiency should be easy to rectify.

Entanglement of a photon and a collective atomic excitation

We describe a new experimental approach to probabilistic atom-photon (signal) entanglement. Two qubit states are encoded as orthogonal collective spin excitations of an unpolarized atomic ensemble. After a programmable delay, the atomic excitation is converted into a photon (idler). Polarization states of both the signal and the idler are recorded and are found to be in violation of the Bell inequality. Atomic coherence times exceeding several microseconds are achieved by switching off all the trapping fields--including the quadrupole magnetic field of the magneto-optical trap--and zeroing out the residual ambient magnetic field.

Generation of paired photons with controllable waveforms

We describe experiments and theory showing the generation of counterpropagating paired photons with coherence times of about 50 ns and waveforms that are controllable at a rudimentary level. Using cw lasers, electromagnetically induced transparency and cold 87Rb atoms we generate paired photons into opposing single-mode optical fibers at a rate of approximately 12 000 pairs per second.