We model the generation of multiple photon pairs (biphotons) through engineering of two coexisting nearly resonant spontaneous four-wave-mixing (SFWM) processes in five-level atoms driven to a quasi-dark state trapping regime. This is achieved through a suitable choice of two pump beams and one coupling beam whose spectral profiles are taken into account both to provide realistic estimates for the effects of their linewidths on atomic populations, coherences, and photon correlations and to engineer the joint spectral amplitude (JSA) distributions of generated biphotons directly by tuning the beams' parameters. We specifically leverage this tunability to control the degree of spectral indistinguishability of two biphotons with ultranarrow linewidths. The large degree of indistinguishability achievable in our model can be harnessed for advanced applications in quantum communication and information processing and potentially adapted to compatible solid-state interfaces.
We model the generation of multiple photon pairs (biphotons) through engineering of two coexisting nearly resonant spontaneous four-wave-mixing (SFWM) processes in five-level atoms driven to a quasi-dark state trapping regime. This is achieved through a suitable choice of two pump beams and one coupling beam whose spectral profiles are taken into account both to provide realistic estimates for the effects of their linewidths on atomic populations, coherences, and photon correlations and to engineer the joint spectral amplitude (JSA) distributions of generated biphotons directly by tuning the beams' parameters. We specifically leverage this tunability to control the degree of spectral indistinguishability of two biphotons with ultranarrow linewidths. The large degree of indistinguishability achievable in our model can be harnessed for advanced applications in quantum communication and information processing and potentially adapted to compatible solid-state interfaces.
We utilized the all-copropagating scheme, which maintains the phase-match condition, in the spontaneous four-wave mixing (SFWM) process to generate biphotons from a hot atomic vapor. The linewidth and spectral brightness of our biphotons surpass those of the biphotons produced with the hot-atom SFWM in the previous works. Moreover, the generation rate of the sub-MHz biphoton source in this work can also compete with those of the sub-MHz biphoton sources of the cold-atom SFWM or cavity-assisted spontaneous parametric down conversion. Here, the biphoton linewidth is tunable for an order of magnitude. As we tuned the linewidth to 610 kHz, the generation rate per linewidth is 1,500 pairs/(s·MHz) and the maximum two-photon correlation function, g s , a s ( 2 ) , of the biphotons is 42. This g s , a s ( 2 ) violates the Cauchy-Schwarz inequality for classical light by 440 folds, and demonstrates that the biphotons have a high purity. By increasing the pump power by 16 folds, we further enhanced the generation rate per linewidth to 2.3×10 4 pairs/(s·MHz), while the maximum g s , a s ( 2 ) became 6.7. In addition, we are able to tune the linewidth down to 290±20 kHz. This is the narrowest linewidth to date among all single-mode biphoton sources of room-temperature and hot media.
Review of Biphoton Sources Based on the Double‐ΛΛ Spontaneous Four‐Wave Mixing Process
Abstract This review article focuses on biphoton sources based on the double‐ spontaneous four‐wave mixing (SFWM) process in laser‐cooled as well as room‐temperature or hot atomic ensembles. These biphoton sources have the advantage of providing stable frequencies, ultranarrow linewidths, and a tunability of the temporal biphoton width of more than one order of magnitude for high‐bandwidth applications. Therefore, the generated photons can be efficiently interfaced to, e.g., atomic quantum memories. In contrast, solid‐state biphoton sources typically require assistance by an optical cavity to operate at narrow linewidth that limits the tunability of the temporal width of the biphotons. Present state‐of‐the‐art double‐ SFWM biphoton sources can achieve one of the following results: a spectral linewidth of 50 kHz (290 kHz) or a temporal width of 13 (580 ns) with cold (hot) atoms, a detection rate of about 7 cps, and a generation rate of cps at a duty cycle of 0.4% or of cps in the steady state. The theoretical background of these biphoton sources, experimental implementations with cold and hot atoms, and progress over the years, will be illustrated.
We theoretically investigate the coherent manipulation of biphoton generation via spontaneous four-wave mixing in a cavity-QED system with a single atom. The atom is driven by pumping, coupling, and driving fields, and the generation of the Stokes and anti-Stokes photons are enhanced by two cavities. By solving the master equation in the steady state, we analyze the spectral brightness, as well as the degree of the auto-correlation and cross-correlation. Our results show that when the pumping and driving fields are in two-photon resonance, the dark state established between the ground and Rydberg states. efficiently enhances the controllability of the driving field over the biphoton generation and the quantum statistics. In contrast, under large two-photon detuning, the control capability of the driving field is significantly reduced. The coupling field, which directly relates to the electromagnetically induced transparency, modifies the linewidth of the biphoton, while the atom-cavity coupling strength only changes the brightness without affecting the linewidth.
We present a scheme to generate photon-pair states through resonant spontaneous four-wave mixing (SFWM) in a five-level double-bowtie $(\ensuremath{\bowtie})$ configuration subject to two pumping fields and one coupling field when a quasidark state is created between two of three ground levels. The quasidark state is used here to largely suppress linear resonant absorption (gain) of the generated photon pairs while supporting strong SFWM nonlinearities and nonclassical cross-correlation. Numerical results for cold atomic samples show that the generation efficiency may be comparable to or even larger than that obtained in the off-resonance four-level single-bowtie configuration with only one pumping field. This scheme is robust to fluctuations of atomic populations and enables one to easily control the ratio of two rates with which distinct SFWM processes generate photon-pair states.
We present experimental results on tuning biphoton frequency by introducing a detuned coupling field in spontaneous four-wave mixing (SFWM), and examine its impact on the pairing ratio. This tunability is achieved by manipulating the inherent electromagnetically induced transparency (EIT) effect in the double-$\mathrm{\ensuremath{\Lambda}}$ scheme. Introducing a detuned coupling field degrades the efficiency of EIT-based stimulated four-wave mixing, which in turn reduces the biphoton pairing ratio. However, this reduction can be mitigated by increasing the optical power of the coupling field. Additionally, we observe that blue- and red-detuning the biphoton frequency results in distinct temporal profiles of biphoton wave packets due to phase mismatch. These findings provide insights into the mechanisms of frequency-tunable biphoton generation via SFWM, and suggest potential optimizations for applications in quantum communication and information processing.
We study theoretically the generation of photon pairs by spontaneous four-wave mixing (SFWM) in photonic crystal optical fiber. We show that it is possible to engineer two-photon states with specific spectral correlation ("entanglement") properties suitable for quantum information processing applications. We focus on the case exhibiting no spectral correlations in the two-photon component of the state, which we call factorability, and which allows heralding of single-photon pure-state wave packets without the need for spectral post filtering. We show that spontaneous four wave mixing exhibits a remarkable flexibility, permitting a wider class of two-photon states, including ultra-broadband, highly-anticorrelated states.