arXiv subjects

Yong-Fan Chen

Publications and source records attributed to Yong-Fan Chen.

At least 19 recordsLinked to original sources

arXiv2026

Time-resolved correlation engineering in DLCZ Raman photon sources

Memory-assisted quantum networks require photon sources with controllable temporal and correlation properties. The Duan-Lukin-Cirac-Zoller (DLCZ) protocol provides a platform based on spontaneous Raman scattering in atomic ensembles, but a unified predictive theory connecting control parameters to correlations under realistic propagation and noise conditions remains lacking. Here we present a propagation-inclusive open-system quantum theory that retains write-induced population redistribution while combining Heisenberg-Langevin dynamics with Maxwell-Schr\"odinger propagation. We experimentally validate its key predictions. The theory predicts time-dependent Stokes generation, spin-wave evolution, retrieved anti-Stokes wavepackets, and time-resolved cross-correlations. Experiments confirm robust correlations under retrieval tuning and enhanced correlations for shorter write pulses, consistent with the different scaling of correlated coincidences and accidental backgrounds with the mean spin-wave excitation number. Classically controlled retrieval enables temporal gating and slicing of the anti-Stokes wavepacket, establishing a quantitative framework for correlation engineering in memory-compatible DLCZ photon sources.

quant-ph↗

arXiv2026

High-efficiency telecom conversion of heralded atomic biphoton wavepackets

We demonstrate high-efficiency telecom frequency conversion of heralded atomic biphoton wavepackets using a diamond-type atomic ensemble. By placing a 2.5 MHz heralded-photon spectrum within the high-efficiency region of the converter response, we achieve a conversion efficiency of 79.4(2.6)% while maintaining strong time-resolved correlations and well-defined temporal wavepackets. For a broader 17.4 MHz input bandwidth, the conversion efficiency is reduced to about 55%, whereas the temporal waveform remains largely preserved. This behavior reflects the nearly flat central response of the converter, which mainly causes spectral-edge loss rather than temporal-mode distortion. These results identify spectral matching as an effective route to efficient and low-distortion telecom conversion of narrowband quantum light from atomic systems.

quant-ph↗

arXiv2026

Microscopic Origin of Superradiant Biphoton Emission in Atomic Ensembles

Superradiant biphoton emission from atomic ensembles provides a powerful route to generating highly correlated quantum light, yet its microscopic physical origin has remained incompletely understood. In particular, it is often unclear how collective enhancement, spontaneous emission, and vacuum fluctuations jointly give rise to both paired biphoton generation and unavoidable unpaired background within a single, self-consistent framework. Here we present a fully quantum microscopic theory within a unified Heisenberg--Langevin--Maxwell framework that explicitly incorporates dissipation and quantum noise, thereby revealing the microscopic origin of superradiant biphoton emission in atomic ensembles. The theory provides a consistent description of parametric gain and unpaired noise within the same open-quantum-system framework and applies to both Doppler-free cold atomic ensembles and Doppler-broadened warm vapors. In the high-optical-depth regime, the coupled propagation equations admit analytical solutions, under which the biphoton dynamics rigorously reduce to an effective collective two-level emission process. Within this limit, the biphoton correlation time and spectral properties are shown to obey closed-form scaling relations governed by optical depth and excited-state decoherence. Our results establish a unified microscopic picture of superradiant biphoton generation and clarify the fundamental role of vacuum fluctuations and dissipation in setting the brightness, pairing efficiency, and temporal structure of atomic biphoton sources, with direct relevance to quantum networking and atomic quantum interfaces.

quant-ph↗

arXiv2025

Polarization Entanglement in Atomic Biphotons via OAM-to-Spin Mapping

We demonstrate polarization-entangled biphotons in a cold-atom double-$Λ$ system, overcoming atomic selection rules that suppress polarization correlations and favor orbital angular momentum (OAM) entanglement. Using spatial light modulators, we coherently map a selected two-dimensional OAM subspace onto the polarization basis and thereby open an otherwise inaccessible polarization channel. Quantum-state tomography confirms that the mapping preserves the biphoton coherence. The four polarization Bell states are generated with fidelities of $92\text{-}94\%$ with few-percent statistical uncertainties, and an average Clauser-Horne-Shimony-Holt parameter of $S=2.44$ verifies the survival of nonlocal correlations. To the best of our knowledge, this work presents the first demonstration of OAM-to-polarization entanglement transfer in a cold-atom spontaneous four-wave mixing platform and establishes a practical interface for integrating atomic OAM resources with polarization-based quantum communication networks.

quant-ph↗

arXiv2025

Fundamental limit on the heralded single photons' spectral brightness

Heralded single photons (HSPs) are the versatile flying qubits in quantum communication and networks due to their ability to remove the randomness of arrival time and enhance the transmission reliability. As the generation rate of HSPs increases or their linewidth narrows, both of which are desirable for quantum information processing, the fundamental limit of spectral brightness (SB), defined as the generation rate per unit linewidth, remains unclear. To examine the existence and value of such a limit, we systematically studied the SB together with the cross-correlation function, or equivalently, the signal-to-background ratio (SBR). We ultimately derive an upper bound on SB that applies universally to all types of HSP sources. A newly defined quantity governs this limit, the quality factor, which is the product of SBR and effective SB. The quality factor indicates how closely an HSP source approaches an ideal noise-free source. Furthermore, by employing an HSP source based on hot atomic vapor, we achieved an SB of $(8.5\pm0.3)$$\times$$10^5$ pairs/s/MHz and a quality factor of $0.73\pm0.02$ under the single-photon criterion. Both values represent the highest reported performance to date among all HSP platforms. These results provide a unified benchmark for evaluating and optimizing HSP sources.

quant-ph↗

arXiv2025

Entanglement preservation and Clauser-Horne nonlocality in electromagnetically induced transparency quantum memories

Entanglement preservation in noisy quantum memories represents a central challenge in quantum information science. While experiments have shown that electromagnetically induced transparency (EIT) memories can store entangled photons, a quantitative theoretical analysis of whether nonlocal quantum correlations can survive storage loss induced by ground-state decoherence remains limited. Here we combine the dark-state polariton formalism with a reduced density-operator treatment to derive an EIT-specific effective pure-loss description for the retrieved photonic state in the ground-state-decoherence-limited regime. The analysis reveals that decoherence transforms an initially pure Bell state into a mixed state with a vacuum component and predicts a protocol-dependent storage-efficiency benchmark of 89.7% for violating the chosen unconditional Clauser-Horne (CH) inequality. Above this benchmark, the retrieved photonic state violates the CH inequality without post-selection, whereas below it, this unconditional CH violation is no longer obtained. This framework provides a quantitative theoretical description of entanglement retention, retrieved photonic density operators, and protocol-dependent Bell-test benchmarks in EIT quantum memories.

quant-ph↗

arXiv2025

High-efficiency telecom frequency conversion via a diamond-type atomic ensemble

Efficient telecom frequency conversion (TFC) in atomic systems is crucial for integrating atom-based quantum nodes into low-loss fiber-optic quantum networks. Here, we demonstrate high-efficiency TFC from 795 nm to 1367 nm in a cold 87Rb ensemble via diamond-type four-wave mixing (FWM), achieving conversion efficiencies of 66% and 80% at optical depths of 75 and 110, respectively, using a weak coherent probe field. These results surpass all previously reported values in atomic systems, enabled by a systematic investigation of the built-in V-type and cascade-type electromagnetically induced transparency spectra that guided the optimization of FWM conditions. Although this work employs coherent fields, our previous theoretical study has shown that quantum states can be preserved with high fidelity during the conversion process, highlighting the promise of diamond-type atomic FWM as a robust interface for long-distance quantum communication.

quant-ph↗

arXiv2025

Asymmetric Biphoton Generation under Ground-State Decoherence and Phase Mismatch in a Cold Atomic Ensemble

We present 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. Our 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, we provide insights into pairing ratios and demonstrate 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.

quant-ph↗

arXiv2025

Protecting Heralded Single Photons Generated from Double-$Λ$ Biphoton Sources with Doppler-Broadened Atomic Media

Biphoton sources that use room-temperature or hot atoms are valuable for real-world applications in long-distance quantum communication and photonic quantum computation. The heralded single photons produced by biphoton sources using the double-$Λ$ spontaneous four-wave mixing (SFWM) process offer advantages of narrow linewidth, stable frequency, and tunable linewidth -- qualities not found in other types of biphoton sources. In this study, we investigated a hot-atom SFWM double-$Λ$ biphoton source. We discovered that, under the condition counterintuitive to the present theory, heralded single photons of the source enhanced their generation rate by a factor of 3.6, heralding probability by a factor of 3.0, temporal width by 2.1, and spectral brightness by a factor of 10. These unexpected findings led us to propose a new theoretical framework for a previously unexplored physical mechanism. Our proposed theory effectively explains the observed results. Traditionally, similar spectral brightness (SB) from atom-based sources resulted in a lower signal-to-background ratio (SBR) than crystal- or chip-based biphoton sources, mainly due to poorer heralding probabilities. In our work, we experimentally demonstrated that the SBR improved by a factor of 4.8 while maintaining a comparable SB. As a result, the SBR performance of the atom-based biphoton source is now on par with that of crystal- or chip-based sources. This research introduces a new tuning parameter for double-$Λ$ SFWM biphoton sources, enhances our understanding of biphoton generation, and opens new avenues for improving the performance of these sources.

quant-ph↗

arXiv2024

Frequency-tunable biphoton generation via spontaneous four-wave mixing

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-$Λ$ 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 wavepackets 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.

quant-ph↗

arXiv2024

Observation of Highly Correlated Ultrabright Biphotons Through Increased Atomic Ensemble Density in Spontaneous Four-Wave Mixing

The pairing ratio, a crucial metric assessing a biphoton source's ability to generate correlated photon pairs, remains underexplored despite theoretical predictions. This study presents experimental findings on the pairing ratio, utilizing a double-$Λ$ spontaneous four-wave mixing biphoton source in cold atoms. At an optical depth (OD) of 20, we achieved an ultrahigh biphoton generation rate of up to $1.3\times10^7$ per second, with a successful pairing ratio of $61\%$. Increasing the OD to 120 significantly improved the pairing ratio to $89\%$, while maintaining a consistent biphoton generation rate. This achievement, marked by high generation rates and robust biphoton pairing, holds great promise for advancing efficiency in quantum communication and information processing. Additionally, in a scenario with a lower biphoton generation rate of $5.0 \times 10^4$ per second, we attained an impressive signal-to-background ratio of 241 for the biphoton wavepacket, surpassing the Cauchy-Schwarz criterion by approximately $1.5\times10^4$ times.

quant-ph↗

arXiv2024

Quantum interface for telecom frequency conversion based on diamond-type atomic ensembles

In a fiber-based quantum network, utilizing the telecom band is crucial for long-distance quantum information (QI) transmission between quantum nodes. However, the near-infrared wavelength is identified as optimal for processing and storing QI through alkaline atoms. Efficiently bridging the frequency gap between atomic quantum devices and telecom fibers while maintaining QI carried by photons is a challenge addressed by quantum frequency conversion (QFC) as a pivotal quantum interface. This study explores a telecom-band QFC mechanism using diamond-type four-wave mixing (FWM) with rubidium energy levels. The mechanism converts photons between the near-infrared wavelength of 795 nm and the telecom band of 1367 or 1529 nm. Applying the Heisenberg-Langevin approach, we optimize conversion efficiency (CE) across varying optical depths while considering quantum noises and present corresponding experimental parameters. Unlike previous works neglecting the applied field absorption loss, our results are more relevant to practical scenarios. Moreover, by employing the reduced-density-operator theory, we demonstrate that this diamond-type FWM scheme maintains quantum characteristics with high fidelity, unaffected by vacuum field noise, enabling high-purity QFC. Another significant contribution lies in examining how this scheme impacts QI encoded in photon-number, path, and polarization degrees of freedom. These encoded qubits exhibit remarkable entanglement retention under sufficiently high CE. In the case of perfect CE, the scheme can achieve unity fidelity. This comprehensive exploration provides theoretical support for the application of the diamond-type QFC scheme based on atomic ensembles in quantum networks, laying the essential groundwork for advancing the scheme in distributed quantum computing and long-distance quantum communication.

quant-ph↗

arXiv2023

Controlling Frequency-Domain Hong-Ou-Mandel Interference via Electromagnetically Induced Transparency

Hong-Ou-Mandel (HOM) interference is a compelling quantum phenomenon that demonstrates the nonclassical nature of single photons. In this study, we investigate an electromagnetically induced transparency-based double-$Λ$ four-wave mixing system from the perspective of quantized light fields. The system can be used to realize efficient HOM interference in the frequency domain. By using the reduced density operator theory, we demonstrate that, although the double-$Λ$ medium does not exhibit phase-dependent properties for the closed-loop case of two incident single photons, frequency-domain HOM two-photon interference occurs. For experimentally achievable optical depth conditions, our theory indicates that this double-$Λ$ scheme can perform high-fidelity Hadamard gate operations on frequency-encoded single-photon qubits, and thereby generate HOM two-photon NOON states with a fidelity greater than 0.99. Furthermore, we demonstrate that this scheme can be used to realize arbitrary single-qubit gates and two-qubit SWAP gates by simply controlling the laser detuning and phase, exhibiting its multifunctional properties and providing a new route to scalable optical quantum computing.

quant-ph↗

arXiv2022

Temporally-ultralong biphotons with a linewidth of 50 kHz

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4$\pm$0.3 $μ$s and a spectral FWHM of 50$\pm$1 kHz, via the process of spontaneous four-wave mixing. The temporal width is the longest, and the spectral linewidth is the narrowest up to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy-Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly-tunable-linewidth biphoton source and showed that while the biphoton source's temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15\%. A spectral brightness or generation rate per pump power per linewidth of 1.2$\times$10$^6$ pairs/(s$\cdot$mW$\cdot$MHz) was achieved at the temporal width of 13.4 $μ$s. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as the nearly phase-mismatch-free scheme employed in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source, and has made a substantial advancement in the quantum technology utilizing heralded single photons.

quant-ph↗

arXiv2022

Room-temperature biphoton source with a spectral brightness near the ultimate limit

The biphotons, generated from a hot atomic vapor via the process of spontaneous four-wave mixing (SFWM), have the following merits: stable and tunable frequencies as well as linewidth. Such merits are very useful in the applications of long-distance quantum communication. However, the hot-atom SFWM biphoton sources previously had far lower values of generation rate per linewidth, i.e., spectral brightness, as compared with the sources of biphotons generated by the spontaneous parametric down conversion (SPDC) process. Here, we report a hot-atom SFWM source of biphotons with a linewidth of 960 kHz and a generation rate of 3.7$\times$ $10^5$ pairs/s. The high generation rate, together with the narrow linewidth, results in a spectral brightness of 3.8$\times$ $10^5$ pairs/s/MHz, which is 17 times of the previous best result with atomic vapors and also better than all known results with all kinds of media. The all-copropagating scheme together with a large optical depth (OD) of the atomic vapor is the key improvement, enabling the achieved spectral brightness to be about one quarter of the ultimate limit. Furthermore, this biphoton source had a signal-to-background ratio (SBR) of 2.7, which violated the Cauchy-Schwartz inequality for classical light by about 3.6 folds. Although an increasing spectral brightness usually leads to a decreasing SBR, our systematic study indicates that both of the present spectral brightness and SBR can be enhanced by further increasing the OD. This work demonstrates a significant advancement and provides useful knowledge in the quantum technology using photons.

quant-ph↗

arXiv2021

Quantum fidelity of electromagnetically induced transparency: The full quantum theory

We present a full quantum model to study the fidelity of single photons with different quantum states propagating in a medium exhibiting electromagnetically induced transparency (EIT). By using the general reservoir theory, we can calculate the quantum state of the transmitted probe photons that reveal the EIT phenomenon predicted by semiclassical theory while reflecting the influence of the quantum fluctuations of the strong coupling field. Our study shows that the coupling field fluctuations not only change the quantum state of the probe photons, but also slightly affect its transmittance. Moreover, we demonstrate that the squeezed coupling field can enhance the influence of its fluctuations on the quantum state of the probe photons, which means that the EIT effect can be manipulated by controlling the quantum state properties of the coupling field. The full quantum theory in this paper is suitable for studying quantum systems related to the EIT mechanism that would allow us to examine various quantum effects in EIT-based systems from a full quantum perspective.

quant-ph↗

arXiv2021

A Weakly-Interacting Many-Body System of Rydberg Polaritons Based on Electromagnetically Induced Transparency

We proposed utilizing a medium with a high optical depth (OD) and a Rydberg state of low principal quantum number, $n$, to create a weakly-interacting many-body system of Rydberg polaritons, based on the effect of electromagnetically induced transparency (EIT). We experimentally verified the mean field approach to weakly-interacting Rydberg polaritons, and observed the phase shift and attenuation induced by the dipole-dipole interaction (DDI). The DDI-induced phase shift or attenuation can be viewed as a consequence of the elastic or inelastic collisions among the Rydberg polaritons. Using a weakly-interacting system, we further observed that a larger DDI strength caused a width of the momentum distribution of Rydberg polaritons at the exit of the system to become notably smaller as compared with that at the entrance. In this study, we took $n =32$ and the atomic (or polariton) density of 5$\times10^{10}$ (or 2$\times10^{9}$) cm$^{-3}$. The observations demonstrate that the elastic collisions are sufficient to drive the thermalization process in this weakly-interacting many-body system. The combination of the $μ$s-long interaction time due to the high-OD EIT medium and the $μ$m$^2$-size collision cross section due to the DDI suggests a new and feasible platform for the Bose-Einstein condensation of the Rydberg polaritons.

physics.atom-ph↗

arXiv2021

Generation of sub-MHz and spectrally-bright biphotons from hot atomic vapors with a phase mismatch-free scheme

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 scheme enables our biphotons not only to surpass those in the previous works of hot-atom SFWM, but also to compete with the biphotons that are generated by either the cold-atom SFWM or the cavity-assisted spontaneous parametric down conversion. The biphoton linewidth in this work is tunable for an order of magnitude. As we tuned the linewidth to 610 kHz, the maximum two-photon correlation function, $g_{s,as}^{(2)}$, of the biphotons is 42. This $g_{s,as}^{(2)}$ violates the Cauchy-Schwartz inequality for classical light by 440 folds, and demonstrates that the biphotons have a high purity. The generation rate per linewidth of the 610-kHz biphoton source is 1,500 pairs/(s$\cdot$MHz), which is the best result of all the sub-MHz biphoton sources in the literature. By increasing the pump power by 16 folds, we further enhanced the generation rate per linewidth to 2.3$\times$10$^4$ pairs/(s$\cdot$MHz), while the maximum $g_{s,as}^{(2)}$ became 6.7. In addition, we are able to tune the linewidth down to 290$\pm$20 kHz. This is the narrowest linewidth to date, among all the various kinds of single-mode biphotons.

quant-ph↗