Review of Biphoton Sources Based on the Double‐Λ$\Lambda$ Spontaneous Four‐Wave Mixing Process

Utilizing the double-Λ spontaneous four-wave mixing (SFWM) process, the biphoton source generates narrow-linewidth pairs of signal and probe photons. In a medium, the signal photon propagates at nearly the speed of light in a vacuum, while the probe photon propagates as slow light. Typically, signal photons arrive at the detector first and are used as the heralding photons in conventional biphoton sources. In this work, we propose using probe photons as the heralding photons to enhance the heralding probability, an approach that has been overlooked previously. We also investigate a time-reversed double-Λ SFWM biphoton source using heated atomic vapor. Compared with the conventional biphoton source under the same experimental conditions, the time-reversed one exhibits a time-reversed temporal profile with a similar full-width-at-half-maximum linewidth of 3.4 MHz, increased the heralding efficiency by a factor of 5.3, and enhanced the detection rate by 1.3 times. With the time-reversed source, we achieved a heralding probability of 82±6% and a generation rate of (1.8 ± 0.2)× 106 pairs/s, referring to biphotons collected within polarization-maintained single-mode optical fibers. Furthermore, the time-reversed temporal profile is more suitable for quantum memory. Simulation results show that, at an optical depth of 150 (or 50), the storage efficiency of a quantum memory using the time-reversed source can reach 91% (or 81%), compared with 81% (or 67%) using the conventional source. This study demonstrates the significance of using the slow-light photon in biphoton pairs as the heralding photon for quantum operations. We have achieved a biphoton source with high heralding probability, high generation rate, and narrow linewidth in a room-temperature or hot medium.

We demonstrate deterministic storage and retrieval of light from a semiconductor quantum dot single-photon source in an atomic ensemble quantum memory at telecommunications wavelengths, which is a long sought-after goal in photonic quantum technologies.

In this work, we experimentally demonstrate a photon-pair source with correlations in the frequency and polarization degrees of freedom. We base our source on the spontaneous four-wave mixing (SFWM) process in a photonic crystal fiber. We show theoretically that the two-photon state is the coherent superposition of up to six distinct SFWM processes, each corresponding to a distinct combination of polarizations for the four waves involved and giving rise to an energy-conserving pair of peaks. Our experimental measurements, both in terms of single and coincidence counts, confirm the presence of these pairs of peaks, while we also present related numerical simulations with excellent experiment-theory agreement. We explicitly show how the pump frequency and polarization may be used to effectively control the signal-idler photon-pair properties, defining which of the six processes can participate in the overall two-photon state and at which optical frequencies. We analyze the signal-idler correlations in frequency and polarization, and in terms of fiber characterization, we input the SFWM-peak experimental data into a genetic algorithm which successfully predicts the values of the parameters that characterize the fiber cross section, as well as predict the particular SFWM process associated with a given pair of peaks. We believe our work will help advance the exploitation of photon-pair correlations in the frequency and polarization degrees of freedom.

Narrow linewidth fiber lasers are intensively studied during these years, for their wide use in coherent optical communication, optical fiber sensing, high-precision spectroscopy and many other industrial and military applications. A usual technique to suppress linewidth is narrow band filtering in the resonating structure of the laser. However, it is hard in fabrication for a tradition optical filter to achieve an ultra narrow pass band such as kilo-Hertz. In this paper, the parallel feedback structure is proposed and experimentally studied. A fiber laser with ultra narrow linewidth (15Hz detected) is achieved in laboratory, basing on the parallel feedback mechanism in a fiber ring cavity. In multimode fiber, each transverse mode has a different propagation constant. Equivalently, when a light beam propagates form single mode fiber to multimode fiber, it will split into a few parallel light paths with different propagation constants. The parallel feedback structure of a fiber ring laser is carried out by introducing one or more pieces of multi-mode fiber into its resonant cavity. Lasing light in the cavity must fit the restrictions of all light paths, thus linewidth of the laser output is suppressed. Ultra narrow linewidth can be achieved by carefully adjusting the amount and length of multi-mode fiber pieces. The narrowest linewidth achieved in laboratory is 15Hz detected by a delayed self-heterodyne interferometer with a 100km fiber delay line. In this work, we focus on pressuring linewidth, and mechanism of selecting and stabilizing mode isn't employed. Therefore, the narrow linewidth fiber laser isn't single-longitudinal-mode (SLM). A similar ring laser structure with bidirectional output is also experimental implemented, achieving a bandwidth of the same order. The bidirectional ring laser is the basic component of a laser gyroscope, and its linewidth is one key limiting factor of the gyroscope accuracy. This narrow linewidth bidirectional ring laser is a good candidate for accurate fiber laser gyroscope. Theory analysis and experiment shows the fiber ring laser based on parallel feedback structure is robust against environmental disturbance, achieving high stability and ultra narrow linewidth. The parallel feedback mechanism in optical circuits can also used in other applications demanding narrow linewidth.

We propose and investigate a protocol for Gaussification of quantum states of traveling light beams in an atomic quantum memory that couples to light via quantum non-demolition interaction. The protocol relies on a periodic switching between two different QND couplings and the total coupling strength scales only logarithmically with number of Gaussified light modes. The present scheme can be used to prepare entangled states of two distant atomic ensembles and to purify and Gaussify noisy non-Gaussian entangled states of light while simultaneously storing the purified state in atomic memories.

We report the direct characterization of energy-time entanglement of narrow-band biphotons produced from spontaneous four-wave mixing in cold atoms. The Stokes and anti-Stokes two-photon temporal correlation is measured by single-photon counters with nanosecond temporal resolution, and their joint spectrum is determined by using a narrow linewidth optical cavity. The energy-time entanglement is verified by the joint frequency-time uncertainty product of 0.063±0.0044, which does not only violate the separability criterion but also satisfies the continuous variable Einstein-Podolsky-Rosen steering inequality.

We report on a signal-to-noise ratio characterizing the generation of identical photon pairs of more than 4 orders of magnitude in a ring resonator system. Parasitic noise, associated with single-pump spontaneous four-wave mixing, is essentially eliminated by employing a novel system design involving two resonators that are linearly uncoupled but nonlinearly coupled. This opens the way to a new class of integrated devices exploiting the unique properties of identical photon pairs in the same optical mode.

We theoretically investigate the influence of a dressing field on parametric amplification multi-wave mixing (PA-MWM) processes in an atomic ensemble for the first time. The quantum spatial properties of PA-MWM signals are demonstrated by studying the cone emissions. Meanwhile, we investigate the intensities, the intensity difference squeezing, and the intensity noise correlation of bright twin PA-MWM beams, which are generated by injecting the MWM signals into the input ports of a spontaneous parametric four-wave mixing process. Both the spatial properties and the quantum correlation can be enhanced or suppressed via a bright state or a dark state, which is induced by the single or double dressing effect. This study has potential applications in quantum security, quantum imaging and long distance quantum communications.

Quantum information has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.

Orbital angular momentum (OAM) entangled photon pairs with narrow bandwidths play a crucial role in the interaction of light and quantum states of matter. In this article, we demonstrate an approach for generating OAM entangled photon pairs with a narrow bandwidth by using a single driving beam in a 85Rb atomic vapor cell. This single driving beam is able to simultaneously couple two atomic transitions and directly generate OAM entangled biphotons by leveraging the OAM conservation law through the spontaneous four-wave mixing (SFWM) process. The photon pairs exhibit a maximum cross-correlation function value of 27.7 and a linewidth of 4 MHz. The OAM entanglement is confirmed through quantum state tomography, revealing a fidelity of 95.7% and a concurrence of 0.926 when compared to the maximally entangled state. Our scheme is notably simpler than previously proposed schemes and represents the first demonstration of generating subnatural-linewidth entangled photon pairs in hot atomic systems.

In this Letter, telecom-band degenerate-frequency photon pairs are generated in a specific mode of a silicon microring cavity by the nondegenerate spontaneous four-wave mixing (SFWM) process, under two continuous-wave pumps at resonance wavelength of two different cavity modes. The ratio of coincidence to accidental coincidence is up to 100 under a time bin width of 5ns, showing their characteristics of quantum correlation. Their quantum interference in balanced and unbalanced Mach-Zehnder interferometers is investigated theoretically and experimentally, and the results show potential in quantum metrology and quantum information.

Polarization-encoded photons at telecommunication wavelengths provide a compelling platform for practical realizations of photonic quantum information technologies due to the ease of performing single qubit manipulations, the availability of polarization-entangled photon-pair sources, and the possibility of leveraging existing fiber-optic links for distributing qubits over long distances. An optical quantum memory compatible with this platform could serve as a building block for these technologies. Here we present the first experimental demonstration of an atomic quantum memory that directly allows for reversible mapping of quantum states encoded in the polarization degree of freedom of a telecom-wavelength photon. We show that heralded polarization qubits at a telecom wavelength are stored and retrieved with near-unity fidelity by implementing the atomic frequency comb protocol in an ensemble of erbium atoms doped into an optical fiber. Despite remaining limitations in our proof-of-principle demonstration such as small storage efficiency and storage time, our broadband light-matter interface reveals the potential for use in future quantum information processing.

Strongly correlated Stokes and anti-Stokes photon pairs (biphotons) exhibiting very large generation rates and spectral brightnesses could be attained at extremely low pump powers and optical depths. This is realized via spontaneous four-wave mixing in cold atoms with enhanced nonlocal (Rydberg) optical nonlinearities and prepared into a dark state with a large population imbalance. The scheme works with all light fields on resonance yet with negligible linear absorption and Raman gain.

We have numerically investigated the biphoton generation rate as a function of several parameters in the spontaneous four-wave mixing in cold atoms. It has been found that the biphoton generation rate can easily reach saturation with the intensity of the coupling laser increasing. The saturation intensity is mainly dependent on the dephasing rate of the ground states, unrelated to the pumping laser. It implies that though the biphoton waveform can be manipulated by the coupling laser, the generation rate of the biphoton cannot increase markedly after the saturation. The saturation effect also suggests that there is an optimal coupling laser for obtaining the largest biphoton generation rate with a sufficiently long coherence time.

We experimentally investigate the transverse spatial coherence of an optical pulse stored in atomic vapor quantum memory. Using Young-type spatial interference, it is demonstrated that the atomic vapor quantum memory preserves transverse spatial coherence.