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New Journal of Physics

Asymmetric photon bunching recurring interferences and distributions via quantum frequency combs

Kai-Chi Chang, Xiang Cheng, Yujie Chen, Chee Wei Wong

Published May 18, 2026• 0 citations
Publisher

Abstract

Abstract Quantum frequency combs (QFCs) exhibit periodic temporal recurrences arising from the Fourier transform of their mode-locked frequency spectra. While QFCs have been experimentally demonstrated, asymmetric Hong–Ou–Mandel (HOM) revival interference in QFCs has not been systematically investigated, and the efficient distribution of photon-bunching revivals has remained unreported. Here, we present the first experimental observation and distribution of asymmetric HOM recurrence interference using QFCs. First, we verified HOM revivals spanning 10 time-bins in a 10.21 GHz QFC, obtaining a 72.95 ± 0.8% visibility for its central dip. Then, we measured up to 20 time-bins from a 50 GHz QFC, with an accidental-subtracted central HOM dip visibility of 70.82 ± 0.7%. Finally, we distributed the QFC after 21 km optical fibers without dispersion compensation, observing photon bunching measurements across 12 time-bins, with a central dip visibility of 70.5 ± 0.9%. The deviation from ideal HOM interference is attributed to residual frequency distinguishability between the signal and idler photons, as confirmed in our theoretical calculations. These results establish QFCs as a viable platform for advanced quantum information processing tasks, such as high-dimensional entanglement swapping and quantum communication testbeds.

DOI
10.1088/1367-2630/ae6f45

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Abstract Quantum frequency combs (QFCs) exhibit periodic temporal recurrences arising from the Fourier transform of their mode-locked frequency spectra. While QFCs have been experimentally demonstrated, asymmetric Hong–Ou–Mandel (HOM) revival interference in QFCs has not been systematically investigated, and the efficient distribution of photon-bunching revivals has remained unreported. Here, we present the first experimental observation and distribution of asymmetric HOM recurrence interference using QFCs. First, we verified HOM revivals spanning 10 time-bins in a 10.21 GHz QFC, obtaining a 72.95 ± 0.8% visibility for its central dip. Then, we measured up to 20 time-bins from a 50 GHz QFC, with an accidental-subtracted central HOM dip visibility of 70.82 ± 0.7%. Finally, we distributed the QFC after 21 km optical fibers without dispersion compensation, observing photon bunching measurements across 12 time-bins, with a central dip visibility of 70.5 ± 0.9%. The deviation from ideal HOM interference is attributed to residual frequency distinguishability between the signal and idler photons, as confirmed in our theoretical calculations. These results establish QFCs as a viable platform for advanced quantum information processing tasks, such as high-dimensional entanglement swapping and quantum communication testbeds.
Mode-locked biphoton frequency combs exhibit multiple discrete comblike temporal correlations from the Fourier transform of its phase-coherent frequency spectrum. Both temporal correlation and Franson interferometry are valuable tools for analyzing the joint properties of biphoton frequency combs, and the latter has proven to be essential for testing the fundamental quantum nature, the time-energy entanglement distribution, and the large-alphabet quantum key distributions. However, the Franson recurrence interference visibility in biphoton frequency combs unavoidably experiences a falloff that deteriorates the quality of time-energy entanglement and channel capacity for longer cavity round trips. In this paper, we provide a new method to address this problem towards optimum Franson interference recurrence. We first observe mode-locked temporal oscillations in a 5.03 GHz free-spectral range singly filtered biphoton frequency comb using only commercial detectors. Then, we observe similar falloff trend of time-energy entanglement in 15.15 GHz and 5.03 GHz free-spectral range singly filtered biphoton frequency combs, whereas, the optimum central time-bin accidental-subtracted visibility over 97% for both cavities. Here, we find that by increasing the cavity finesse F , we can enhance the detection probability in temporal correlations and towards optimum Franson interference recurrence in our singly filtered biphoton frequency combs. For the first time, via a higher cavity finesse F of 45.92 with a 15.11 GHz free-spectral range singly filtered biphoton frequency comb, we present an experimental ≈3.13-fold improvement of the Franson visibility compared to the Franson visibility with a cavity finesse F of 11.14 at the sixth time bin. Near optimum Franson interference recurrence and a time-bin Schmidt number near 16 effective modes in similar free-spectral range cavity are predicted with a finesse F of 200. Our configuration is versatile and robust against changes in cavity parameters that can be designed for various quantum applications, such as high-dimensional time-energy entanglement distributions, high-dimensional quantum key distributions, and wavelength-multiplexed quantum networks.
Quantum entanglement is a fundamental resource for secure information processing and communications. The canonical optical quantum information processing encodes a single qubit per photon, with remarkable demonstrations of secure quantum communications, linear optical quantum computing amongst others. Often the polarization qubit, a discrete variable with entangled Bell states, is used. Continuous variables such as energy-time modes and spatial modes, however, present a dramatically larger Hilbert space for quantum information processing. The high-dimensional entanglement capture more qubits per photon, enabling dramatically higher secure key rates over longer distances and with better error resilience. Here I will describe our results on high-dimensional entanglement in quantum frequency combs for dense quantum communications. We first demonstrate revival of the Hong-Ou-Mandel interferences, long postulated by theorists more than a decade ago, up to 19-dimensions and with visibilities up to 96.5%. The mode-locked two-photon state in high-dimensions is further witnessed through a stabilized Franson interferometer, as a generalized Bell’s inequality test and hyperentangled through multiple degrees-of-freedom. Entanglement revivals of the non-local interference at discrete time-bins are uncovered, up to 97.8% visibility, as a fundamental resource for dense secure information processing.
Abstract Qudit entanglement is an indispensable resource for quantum information processing since increasing dimensionality provides a pathway to higher capacity and increased noise resilience in quantum communications, and cluster-state quantum computations. In continuous-variable time–frequency entanglement, encoding multiple qubits per photon is only limited by the frequency correlation bandwidth and detection timing jitter. Here, we focus on the discrete-variable time–frequency entanglement in a biphoton frequency comb (BFC), generating by filtering the signal and idler outputs with a fiber Fabry–Pérot cavity with 45.32 GHz free-spectral range (FSR) and 1.56 GHz full-width-at-half-maximum (FWHM) from a continuous-wave (cw)-pumped type-II spontaneous parametric downconverter (SPDC). We generate a BFC whose time-binned/frequency-binned Hilbert space dimensionality is at least 324, based on the assumption of a pure state. Such BFC’s dimensionality doubles up to 648, after combining with its post-selected polarization entanglement, indicating a potential 6.28 bits/photon classical-information capacity. The BFC exhibits recurring Hong–Ou–Mandel (HOM) dips over 61 time bins with a maximum visibility of 98.4% without correction for accidental coincidences. In a post-selected measurement, it violates the Clauser–Horne–Shimony–Holt (CHSH) inequality for polarization entanglement by up to 18.5 standard deviations with an S-parameter of up to 2.771. It has Franson interference recurrences in 16 time bins with a maximum visibility of 96.1% without correction for accidental coincidences. From the zeroth- to the third-order Franson interference, we infer an entanglement of formation (Eof) up to 1.89 ± 0.03 ebits—where 2 ebits is the maximal entanglement for a 4 × 4 dimensional biphoton—as a lower bound on the 61 time-bin BFC’s high-dimensional entanglement. To further characterize time-binned/frequency-binned BFCs we obtain Schmidt mode decompositions of BFCs generated using cavities with 45.32, 15.15, and 5.03 GHz FSRs. These decompositions confirm the time–frequency scaling from Fourier-transform duality. Moreover, we present the theory of conjugate Franson interferometry—because it is characterized by the state’s joint-temporal intensity (JTI)—which can further help to distinguish between pure-state BFC and mixed state entangled frequency pairs, although the experimental implementation is challenging and not yet available. In summary, our BFC serves as a platform for high-dimensional quantum information processing and high-dimensional quantum key distribution (QKD).
Abstract The frequency degree of freedom of optical photons has been recently explored for efficient quantum information processing. Significant reduction in hardware resources and enhancement of quantum functions can be expected by leveraging the large number of frequency modes. Here, we develope an integrated photonic platform for the generation and parallel processing of quantum frequency combs (QFCs). Cavity-enhanced parametric down-conversion with Sagnac configuration is implemented to generate QFCs with identical spectral distributions. On-chip quantum interference of different frequency modes is simultaneously realized with the same photonic circuit. High interference visibility is maintained across all frequency modes with the identical circuit setting. This enables the on-chip reconfiguration of QFCs. By deterministically separating QFCs without spectral filtering, we further demonstrate high-dimensional Hong-Ou-Mandel effect. Our work provides the critical step for the efficient implementation of quantum information processing with integrated photonics using the frequency degree of freedom.