Doppler-broadened Atomic Ensemble Research Articles

Collective biphoton temporal waveform of photon-pair generated from Doppler-broadened atomic ensemble

Photonic quantum states generated from atomic ensembles will play important roles in future quantum networks and long-distance quantum communication because their advantages, such as universal identity and narrow spectral bandwidth, are essential for quantum nodes and quantum repeaters based on atomic ensembles. In this study, we report the collectively coherent superposition of biphoton wavefunction emitted from different velocity classes in a Doppler-broadened cascade-type atomic ensemble. We experimentally demonstrate that the three times difference of temporal width of both biphoton temporal waveforms varies dependent on the wavelengths of the signal and idler photons from both 6S1/2–6P3/2–6D5/2 and –8S1/2 transitions of 133Cs, corresponding to the idler and signal wavelengths of 852 nm–917 nm and 852 nm–795 nm, respectively. Our results help understand the characteristics of biphoton sources from a warm atomic ensemble and can be applied to long-distance quantum networks and practical quantum repeaters based on atom–photon interactions.

Spectrally entangled biphoton state of cascade emissions from a Doppler-broadened atomic ensemble

We theoretically investigate the spectral property of biphoton state from the cascade emissions from a Doppler-broadened atomic ensemble. This biphoton state is spontaneously created in the four-wave-mixing process. The upper transition of the emissions lies in telecom bandwidth, which prevails in fiber-based quantum communication for low-loss transmission. We obtain the spectral function of the biphoton state in terms of superradiant decay rates of the lower transition, excitation pulse durations, and temperature of the medium. We quantify their frequency entanglement by Schmidt decomposition and find that more entangled source can be generated with longer excitation pulses, enhanced decay rates, and significant Doppler broadening. A minimally entangled biphoton source can also be located at some optimal temperature of the atoms. By tuning these parameters, this biphoton source allows spectral shaping of continuous frequency entanglement, which is useful in multimode long-distance quantum communication.

Polarization-Entangled Photons from a Warm Atomic Ensemble Using a Sagnac Interferometer.

We report a polarization-entangled photon-pair source obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of ^{87}Rb atoms using a Sagnac interferometer. Collective two-photon coherence occurs in the Doppler-broadened ladder-type atomic system with bidirectional counterpropagating two-photon resonant pump and coupling fields; hence, polarization-entangled photon pairs are collectively radiated in the phase-matched direction. Without phase stabilization of the interferometry for polarization entanglement, we robustly produce all four Bell states via a polarization Sagnac configuration. The brightness, stability, and temporal purity advantages provided by our polarization-entangled SFWM photon-pair source have very important applications in the context of a practical scalable quantum network.

Time-Energy Entangled Photon Pairs from Doppler-Broadened Atomic Ensemble via Collective Two-Photon Coherence.

We experimentally demonstrate two-photon interference of a time-energy entangled photon pair generated via collective two-photon coherence in Doppler-broadened cascade-type ^{87}Rb atoms. The two photons originally proposed by J. D. Franson are realized as a photon pair due to collective effects, which are generated from the cascade atomic system with a relatively long lifetime of the initial state and a considerably shorter lifetime of the intermediate state. The achievement of two-photon interference with photon-pair sources generated from inhomogeneous atomic ensembles constitutes an important result for time-energy entanglement based on an atom-photon interaction.

Temporal intensity correlation of bunched light from a warm atomic vapor with a ladder-type two-photon transition

We report the temporal intensity correlation (TIC) of scattered photons (SPs) generated via a two-photon transition in a Doppler-broadened warm atomic vapor of the 5S1/2 − 5P3/2 − 5D5/2 transition of 87Rb atoms. Through the investigation of the TICs of the SPs obtained via both one- and two-photon transitions, the second-order correlation values g(2)(0) (i.e., at zero time delay) of both SPs were measured as approximately 1.75, respectively. The widths of the g(2)(τ) spectra were measured as 26 ns (corresponding to the natural lifetime of the 5P3/2 state) for the one-photon transition and 1.8 ns (corresponding to the Doppler width of the warm atomic vapor) for the two-photon transition. We confirmed that the coherence time of the SPs can vary in accordance with the photons emitted from the one- or two-photon transitions in the ladder-type atomic system. The correlated SPs obtained via the two-photon transition contributed to almost all the velocity classes of the atoms in the Doppler-broadened atomic ensemble.

Single-photon superradiant beating from a Doppler-broadened ladder-type atomic ensemble

We report on heralded-single-photon superradiant beating in the spontaneous four-wave mixing process of Doppler-broadened ladder-type $^{87}\mathrm{Rb}$ atoms. When Doppler-broadened atoms contribute to two-photon coherence, the detection probability amplitudes of the heralded single photons are coherently superposed despite inhomogeneous broadened atomic media. Single-photon superradiant beating is observed, which constitutes evidence for the coherent superposition of two-photon amplitudes from different velocity classes in the Doppler-broadened atomic ensemble. We present a theoretical model in which the single-photon superradiant beating originates from the interference between wavelength-separated two-photon amplitudes via the reabsorption filtering effect.

Two-photon interferences of nondegenerate photon pairs from Doppler-broadened atomic ensemble.

We report two-photon interference experiments performed with correlated photon pairs generated via spontaneous four-wave mixing in a Doppler-broadened atomic ensemble involving the 5S1/2-5P3/2-5D5/2 transition of 87Rb atoms. When two photons with different wavelengths are incident on a polarization-based Michelson interferometer, two kinds of two-photon superposition states, the frequency-entangled state and dichromatic path-entangled state depending on whether the two photons are in different paths or in the same path, are probabilistically generated within the interferometer arms. Hong-Ou-Mandel-type interference fringes resulting from the frequency-entangled state are observed over the range of the single-photon coherence length, following introduction of a coarse path-length difference between the two interferometer arms and employing phase randomization. When the interferometer is highly phase-sensitive without phase randomization, a phase super-resolved fringe arising from the dichromatic path-entangled state is observed, both with and without the accompanying one-photon interference fringes.

Laser frequency stabilization by bichromatic saturation absorption spectroscopy

We present and analyze a bichromatic version of the well-known saturation absorption spectroscopy technique used to stabilize lasers to atomic resonances without Doppler broadening. We show that the spectroscopic properties can be transferred between two transitions at very different wavelengths by spectral hole burning in a Doppler-broadened atomic ensemble. Specifically, we demonstrate that this technique can be used to stabilize a laser to the D1 line of Rb87 by obtaining a saturation absorption spectrum with a strong pump laser stabilized to the D2 line. This allows for more freedom in the choice of the probe laser frequency offsets compared to conventional saturation absorption spectroscopy. We compare our experimental results to an analytical model and numerical results based on rate equations.

Highly bright photon-pair generation in Doppler-broadened ladder-type atomic system.

We report a bright photon-pair source with a coincidence counting rate per input power (cps/mW) of tens of thousands, obtained via spontaneous four-wave mixing from a Doppler-broadened atomic ensemble of the 5S1/2-5P3/2-5D5/2 transition of 87Rb. The photon-pair generation rate is enhanced by the two-photon coherence contributions from almost all the atomic velocity groups in the Doppler-broadened ladder-type atomic system. We obtained the violation of the Cauchy-Schwarz inequality by a factor of 2370 ± 150. We believe that our scheme for highly bright paired photons is important as a useful quantum light source for quantum entanglement swapping between completely autonomous sources.