Active offset-frequency control of optical frequency comb via sum-frequency mixing of passively mode-locked laser and continuous-wave laser.

We propose a method for actively controlling the frequency of an optical frequency comb (OFC) using sum-frequency generation (SFG) with a nonlinear crystal. For the first time, OFC generation was experimentally demonstrated via sum-frequency mixing of a narrowband continuous wave (CW) laser and a passively mode-locked fiber laser. By adjusting the optical frequency of the CW laser, we successfully controlled the offset-frequency of the SFG-OFC, which was mapped from the OFC of the pulse pump laser.

Quantum interference of multidimensional quantum states via space-division multiplexing of a long-coherent single photon from a warm Rb atomic ensemble.

The high-dimensional encoding of single photons can offer various possibilities for enhancing quantum information processing. This work experimentally demonstrates the quantum interference of an engineered multidimensional quantum state through the space-division multiplexing of a heralded single-photon state with a spatial light modulator (SLM) and spatial-mode mixing of a single photon through a long multimode fiber (MMF). In our experiment, the heralded single photon generated from a warm Rb atomic ensemble was bright, robust, and long-coherent.

Highly efficient diode-pumped alkali-vapor amplification with near-extreme-limit gain.

We report high-efficiency optical amplification with near-extreme-limit gain from a diode-pumped Cs vapor cell. We used wavelength-division multiplexing to couple 852 nm pump and 895 nm seed lasers to achieve nearly overlapping spatial modes in the Cs vapor cell. We investigated the amplification factor as a function of the focal length of the lens focusing on the combined pump and seed signals and determined the optimal focal length under our experimental conditions.

Photon-pair generation from a chip-scale Cs atomic vapor cell.

The realization of a narrowband photonic quantum source based on an atomic device is considered essential in the practical development of photonic quantum information science and technology. In this study, we present the first step toward the development of a photon-pair source based on a microfabricated Cs atomic vapor cell. Time-correlated photon pairs from the millimeter-scale Cs vapor cell are emitted via the spontaneous four-wave mixing process of the cascade-type 6S-6P-8S transition of Cs.

Indistinguishability of temporally separated pairwise two-photon state of thermal photons in Franson-type interferometry.

The phenomenon of Franson interference with time-energy entangled photon pairs beyond the single-photon coherence length observed upon nonlocal measurement at two space-like separated locations is of particular research interest. Herein, we determine the coherence length of temporally separated pairwise two-photon (TSPT) states of thermal photons emitted from a warm atomic ensemble in Franson-type interferometry, with the setup consisting of two spatially separated unbalanced Michelson interferometers beyond the coherence length of a thermal photon. Using a novel method of square-modulated thermal photons, we show that the sinusoidal Franson-type interference fringe of thermal photons is determined by the presence or absence of TSPT states (corresponding to the time delay between the long and short paths in Franson-type interferometry).

Two-photon interferences of weak coherent lights.

Multiphoton interference is an important phenomenon in modern quantum mechanics and experimental quantum optics, and it is fundamental for the development of quantum information science and technologies. Over the last three decades, several theoretical and experimental studies have been performed to understand the essential principles underlying such interference and to explore potential applications. Recently, the two-photon interference (TPI) of phase-randomized weak coherent states has played a key role in the realization of long-distance quantum communication based on the use of classical light sources.

Four-wave mixing in a ladder configuration of warm Rb atoms: a theoretical study.

We present a theoretical study of the four-wave mixing (FWM) spectra of 5S - 5P - 5D ladder-type transitions of Rb atoms. The density matrix equations are solved by considering all the magnetic sublevels to calculate the FWM signals in the atomic vapor cell. These results are subsequently compared with the experimental results.

Light manipulation via spontaneous four-wave mixing in a warm double-Λ-type atomic ensemble.

We report on the dynamic manipulation of light in a warm 87Rb atomic ensemble using light storage based on the atomic spin coherence arising from the electromagnetically induced transparency (EIT) and spontaneous four-wave mixing (FWM) processes. We demonstrate that, subsequent to the generation of atomic spin coherence between two hyperfine ground states via the EIT storage process, it is possible to control the delay time, direction, and optical frequency of the retrieved light according to the timing sequence and powers of the coupling, probe, and driving lasers used for atomic-spin-coherence generation and the spontaneous FWM process. We believe that our results provide useful ideas in photon frequency conversion and photon control in connection with the quantum memories that is essential in the quantum communications technology.

Stimulated four-wave mixing in the cascade-type atomic system of a warm Cs atomic ensemble.

We investigate stimulated four-wave mixing (FWM) in the 6S-6P-8S open transition of a warm Cs atomic ensemble. Despite the absence of the two-photon cycling transition, we measure high-contrast FWM signals in the 6P-8S transition between the upper excited states according to the frequency detuning and powers of the coupling and driving lasers. The FWM light generation in the upper excited states is interpreted as the FWM phenomena induced by the driving laser of the 6S-6P transition from the cascade-type two-photon coherent atomic ensemble with the coupling and pump lasers.

Spectral-temporal biphoton waveform of photon pairs from cascade-type warm atoms.

We investigate the spectral-temporal biphoton waveforms of the photon pairs emitted from cascade-type two-photon-coherent warm Rb atoms via the spontaneous four-wave mixing process in the 5S-5P-5D transition, under the condition of the different detuning frequencies (symmetric detuning conditions of ± 1 GHz) of the pump and coupling lasers relative to the 5P state. In both detuning cases corresponding to ± 1 GHz, the biphoton temporal waveforms and biphoton spectral waveforms of the photon pairs are measured by means of time-resolved coincidence photon counting and stimulated measurements, respectively. Although photon-pairs were generated using opposite detunings, we confirm that the spectral-temporal biphoton waveforms of the photon pairs are very similar.

Joint spectral intensity of entangled photon pairs from a warm atomic ensemble via stimulated emission beat interferometry.

Photonic quantum states generated from atomic systems play prominent roles in long-distance quantum networks and scalable quantum communication, because entangled photon pairs from atomic ensembles possess a universal identity and narrow spectral bandwidth for quantum repeaters. In this study, we propose and demonstrate a novel, to the best of our knowledge, method for the joint spectral intensity measurement of narrowband continuous wave (CW)-mode photon pairs from a warm atomic ensemble using stimulated emission and beat interferometry for the first time. Our approach offers the advantage of sub-megahertz resolution, absolute optical frequency measurements with megahertz-level accuracy, fast collection time, and high signal-to-noise ratio; thus, our method can find important applications in the characterization of narrowband photon pairs generated from sources including atoms and artificially structured material.

Entanglement swapping with autonomous polarization-entangled photon pairs from a warm atomic ensemble.

Entanglement swapping forms a key concept in the realization of scalable quantum networks and large-scale quantum communication. For the practical implementation of entanglement swapping, completely autonomous entanglement sources and a joint Bell-state measurement (BSM) between two independent photons are essential. Here, we experimentally demonstrate entanglement swapping between two independent polarization-entangled photon-pair sources obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of ${^{87}}{\rm Rb}$Rb atoms.

Temporal- and spectral-property measurements of narrowband photon pairs from warm double-Λ-type atomic ensemble.

We investigate the temporal and spectral properties of narrowband photon pairs from a double-Λ-type atomic system of a warm Rb atomic ensemble. The temporal properties of the narrowband photons are investigated by measuring their auto-correlation and cross-correlation functions. The spectral measurement of the photon pair is obtained by applying the stimulated emission method.

Hyperfine-state dependence of highly efficient amplification from diode-pumped cesium vapor.

We report optical amplification with an optical-to-optical conversion efficiency of 70 ± 1% from a diode-pumped Cs vapor cell. When pump (852 nm; D-line) and signal (895 nm; D-line) lasers with a narrow spectral width of ∼2 MHz are resonant on the hyperfine states (F = 3 or 4) of the 6S state, we observe that the amplification factors are significantly changed according to the hyperfine-state combination of the pump and signal lasers. We find that the optical frequencies of the pumping and signal lasers need to be controlled near the hyperfine state of 6S (F = 4) to obtain an efficient diode-pumped alkali amplifier (DPAA).

Experimental interference of uncorrelated photons.

The distinguishing of the multiphoton quantum interference effect from the classical one forms one of the most important issues in modern quantum mechanics and experimental quantum optics. For a long time, the two-photon interference (TPI) of correlated photons has been recognized as a pure quantum effect that cannot be simulated with classical lights. In the meantime, experiments have been carried out to investigate the classical analogues of the TPI.

High-visibility Franson interference of time-energy entangled photon pairs from warm atomic ensemble.

We experimentally demonstrate Franson interference of a time-energy entangled photon pair generated via collective two-photon coherence in the 5S-5P-5D transition of warm Rb87 atoms. The two unbalanced Michelson interferometers used in our setup are spatially separated in order to understand entanglement as a nonlocal property of the photon pairs from the warm atomic ensemble. We observe a Franson interference fringe with a high visibility of 99.

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.

Pulsed Sagnac source of polarization-entangled photon pairs in telecommunication band.

We report a source of polarization-entangled photon pairs in the 1550-nm telecommunication band, which is based on non-collinear spontaneous parametric down-conversion in a periodically poled lithium niobate crystal pumped by picosecond pulses. This source is realized utilizing a polarization-based Sagnac interferometer employing a type-0 non-collinear quasi-phase-matching configuration. Polarization entanglement is verified through measurement of the polarization-correlation interference fringes with visibility >96% and by testing the experimental violation of the Clauser-Horne-Shimony-Holt (CHSH) form of Bell's inequality.

In-situ Overhauser-enhanced nuclear magnetic resonance at less than 1 μT using an atomic magnetometer.

The development of atomic magnetometers has led to nuclear magnetic resonance (NMR) in zero and ultralow magnetic fields without using cryogenic sensors. However, in-situ detection, meaning that a sample locates in the detection space beside a vapor cell, has been conducted only with parahydrogen-induced polarization. Other hyperpolarization techniques remain unexplored yet.

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.

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 5S - 5P - 5D transition of Rb atoms. Through the investigation of the TICs of the SPs obtained via both one- and two-photon transitions, the second-order correlation values g(0) (i.e.

Stimulated emission from ladder-type two-photon coherent atomic ensemble.

We investigated the stimulated emission from a ladder-type two-photon coherent atomic ensemble, for the 5S - 5P - 5D transition of Rb atoms. Under the ladder-type two-photon resonance condition obtained using pump and coupling lasers, we observed broad four-wave mixing (FWM) light stimulated from two-photon coherence induced by the seed laser coupled between the ground state of 5S and the first excited state of 5P. A dip in the FWM spectrum was obtained for three-photon resonance due to V-type two-photon coherence using the pump and seed lasers.

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 5S-5P-5D transition of Rb 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.

Determining phase coherence time of stored light in warm atomic vapor.

In quantum memory based on an atomic medium, we may have a question about whether all information on the stored light is preserved. In particular, the phase coherence between the stored and retrieval light pulses is very interesting, because it can indicate the relationship between the coherence time and storage time of the light. In this paper, we investigate the phase coherence time of light stored in a warm atomic vapor, by examining the beat-note interference between the retrieval light pulse and a reference light beam optically delayed using an optical fiber.

Observation of two-photon interference effect with a single non-photon-number resolving detector.

Multiphoton interference effects can be measured with a single detector when two input photons are temporally well separated when compared with the dead time of the single-photon avalanche detector. Here we experimentally demonstrate that the Hong-Ou-Mandel interference effect can be observed with a single non-photon-number resolving detector via a time-delayed coincidence measurement of successive electrical signals from the detector. The two-photon interference experiment is performed by utilizing temporally well-separated pairwise weak coherent pulses, and the interference fringes are successfully measured with high visibility in the range of the limited upper bound for the weak coherent photon source.