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 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-\ensuremathΛ 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.
Efficient frequency conversion of photons has important applications in optical quantum technology because the frequency range suitable for photon manipulation and communication usually varies widely. Recently, an efficient frequency conversion system using a double- Λ four-wave mixing (FWM) process based on electromagnetically induced transparency (EIT) has attracted considerable attention because of its potential to achieve a nearly 100% conversion efficiency (CE). To obtain such a high CE, the spontaneous emission loss in this resonant-type FWM system must be suppressed considerably. A simple solution is to arrange the applied laser fields in a backward configuration. However, the phase mismatch due to this configuration can cause a significant decrease in CE. Here, we demonstrate that the phase mismatch can be effectively compensated by introducing the phase shift obtained by two-photon detuning. Under optimal conditions, we observe a wavelength conversion from 780 to 795 nm with a maximum CE of 91.2 % ± 0.6 % by using this backward FWM system at an optical depth of 130 in cold 87 Rb atoms. The current work represents an important step toward achieving low-loss, high-fidelity quantum frequency conversion based on EIT.
Biphotons are entangled photon pairs that cannot be described as a product state of two individual photons. In this talk we summarize our recent progress in generating narrowband biphotons from spontaneous four-wave mixing (SFWM) in cold atoms with electromagnetically induced transparency (EIT). Here we are particularly interested in the group delay regime, where the biphoton time-frequency entanglement and joint wave function can be manipulated by controlling the group-delay-induced phase matching condition of the paired photons. The biphoton coherence time, thus its bandwidth of the joint spectrum, is determined by the relative group delay between the paired photons. The longest biphoton coherence time on record is more than 3 µs. We find that for a fixed optical depth, the twophoton waveform is independent of the atomic density distribution. However, the spatial profiles of the two driving laser beams have significant effects on the biphoton temporal wavefunction. We demonstrate a technique of shaping the biphoton temporal wavefunction by directly mapping the spatial profile of the pump laser beam onto the biphoton temporal waveform. This spatial-to-temporal mapping, or series-toparallel conversion, allows for coding the spatial information of the pump spatial profile to the sequential temporal modes of the biphtoon quantum state. Using a computer-controlled spatial light modulator, we produced biphotons with temporal waveforms of Gaussian, exponential, and double-peak shapes. As compared to temporally modulating the driving laser fields that reduces the frequency entanglement, this spatial-to-temporal biphoton shaping does not scarify the time-frequency entanglement and can be used to produce heralded narrowband single photons with controllable waveforms.
We proposed and experimentally demonstrated a new idea of using a far-detuned coupling field to enhance the generation rate of the hot-atom biphoton source based on the double-Λ spontaneous four-wave mixing (SFWM) process. Previous theories indicated that the biphoton generation rate decreases with the coupling detuning. Thus, the new idea seems counterintuitive, and no literature has explored it, to the best of our knowledge. Here, we showed that the far-detuned coupling field not only increased the biphoton generation rate but also narrowed down the biphoton linewidth, and consequently, it enhanced the spectral brightness (SB) or generation rate per linewidth by 10 folds, compared to the resonant coupling field. This surprising result led us to develop a new theoretical framework, considering that the far-detuned frequency protects the heralded single photons from a previously unexplored physical mechanism. The predictions from the new theory agree with the experimental data and indicate that the optical depth (OD) of atomic media can further amplify the enhancements of the generation rate and SB induced by the coupling detuning. Moreover, in the past, a higher SB always resulted in a lower signal-to-background ratio (SBR) by varying the OD, pump power, pump detuning, and/or coupling power in the hot-atom biphoton source. The new tuning parameter of coupling detuning significantly enhanced the heralding probability by efficiently suppressing noise-generating mechanisms and increased the SB and SBR together. Hence, the SB (or SBR) under the same SBR (or SB) of the atom-based biphoton sources with the detuned coupling field is now on a par with that of crystal- or chip-based biphoton sources. This work introduces a new parameter for tuning the double-Λ SFWM biphoton source, advances our understanding of biphoton generation, and opens new avenues for enhancing the performance of these sources.
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.
Abstract An experimental investigation of how ground‐state decoherence and phase mismatch influence biphoton generation in double‐Λ spontaneous four‐wave mixing (SFWM) within a cold atomic ensemble is presented. The results reveal significant asymmetry in the Stokes and anti‐Stokes photon generation rates, arising from the distinct effects of phase mismatch and ground‐state decoherence. While phase mismatch primarily drives this asymmetry under minimal decoherence, larger decoherence further amplifies it, underscoring the complex interplay between these factors. Using the coincidence count rate representation, insights are provided into pairing ratios, demonstrating that the stimulated four‐wave mixing process inherent in SFWM explains the observed phenomena. Interestingly, although ground‐state decoherence reduces the generation of temporally correlated photons, it paradoxically enhances biphoton purity, as confirmed through conditional autocorrelation measurements. This counterintuitive phenomenon is reported here for the first time. Furthermore, unconditional autocorrelation measurements show that the generated photons follow a thermal‐state distribution, consistent with theoretical predictions. This study advances the understanding of biphoton generation dynamics and temporal photon correlations in SFWM, offering valuable insights for optimizing SFWM‐based biphoton sources and their applications in quantum technologies.
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.