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Compact sub-GHz bandwidth single-mode time-energy entangled photon source for high-speed quantum networks

Yin‐Hai Li, Zhi‐Yuan Zhou, Shi-Long Liu, Yan Li, Shi-Kai Liu, Chen Yang, Shuang Wang, Zhi‐Han Zhu, Wei Gao, Guang‐Can Guo, Bao-Sen Shi

Published Nov 26, 2020• 5 citations
Publisher

Abstract

The potential data rate of a quantum network is limited by both the entangled photon source (EPS) and quantum memories. While an EPS whose bandwidth matches with broadband quantum memories (BBQMs) can take full advantage of the data rate limit. The EPS with GHz bandwidth is usually obtained by filtering a much broader EPS signal that is generated through spontaneous parametric down-conversion (SPDC), but this method has obvious drawbacks, e.g., large space requirements, high losses, and relatively low spectral brightness. Here we present a simple and compact method to generate a single-longitudinal-mode sub-GHz-bandwidth time-energy EPS using a type-II SPDC in a submillimeter-length Fabry-Pérot cavity. The proposed photon source offers superior figures of merit: the maximum coincidence to accidental coincidence ratio is approximately 1800, the detected pair flux ranges up to 42500 pairs per second, and the source has a high Klyshko efficiency of 25%. This source offers a very potential way to boost the performance of broadband quantum memories and high-speed quantum networks.

DOI
10.1364/osac.411053

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The potential data rate of a quantum network is limited by both the entangled photon source (EPS) and quantum memories. While an EPS whose bandwidth matches with broadband quantum memories (BBQMs) can take full advantage of the data rate limit. The EPS with GHz bandwidth is usually obtained by filtering a much broader EPS signal that is generated through spontaneous parametric down-conversion (SPDC), but this method has obvious drawbacks, e.g., large space requirements, high losses, and relatively low spectral brightness. Here we present a simple and compact method to generate a single-longitudinal-mode sub-GHz-bandwidth time-energy EPS using a type-II SPDC in a submillimeter-length Fabry-Pérot cavity. The proposed photon source offers superior figures of merit: the maximum coincidence to accidental coincidence ratio is approximately 1800, the detected pair flux ranges up to 42500 pairs per second, and the source has a high Klyshko efficiency of 25%. This source offers a very potential way to boost the performance of broadband quantum memories and high-speed quantum networks.
A high-quality, compact, and narrow-bandwidth entangled photon source (EPS) is indispensable for realization of many quantum communication protocols. Usually, a free space cavity containing a nonlinear crystal is used to generate a narrow bandwidth EPS through spontaneous parametric down-conversion (SPDC). One major drawback is that this occupies a large space and requires complex optical and electrical control systems. Here we present a simple and compact method to generate a single-longitudinal-mode time-energy EPS via type II SPDC in a submillimeter Fabry-Perot cavity. We characterize the quality of the EPS by measuring the coincidence to accidental coincidence ratio, the two-photon time cross-correlation function and the two-photon interference fringes. All measured results clearly demonstrate that the developed source is of high quality when compared with EPSs generated using other configurations. We believe this source is very promising for applications in the quantum communication field.
The time-energy entangled photon source is a core resource in quantum key distribution, quantum teleportation, and quantum metrology. The output of orthogonally polarized photon pairs can enhance the effective coincidence counts of the entangled source in these applications. Additionally, an all-fiber entangled source offers small size and high mobility advantages. However, most current orthogonally polarized all-fiber time-energy entangled sources are limited to a bandwidth of around 1 nm, which is detrimental to the effective utilization of channel resources in fiber networks and restricts precision in quantum metrology. To address this issue, we employ a fiber-coupled 775 nm narrow linewidth pump laser in a periodically poled lithium niobate (PPLN) waveguide to achieve type-II spontaneous parametric down-conversion (SPDC). This results in an all-fiber orthogonally polarized time-energy entangled photon source with a bandwidth greater than 7 nm. We measured the single counts, coincidence counts, coincidence to accidental coincidence ratio (CAR), single photon spectra, and joint spectral intensity of the entangled source, calculating the Schmidt number S = 7.8. Furthermore, we measured Hong-Ou-Mandel (HOM) interference by inserting two 2 nm bandpass filters, and the interference visibility remained around 95.8% after continuous operation for 3 hours. The experimental results indicate that while maintaining an all-fiber structure, this source possesses a broadband range, low noise, and high stability, providing efficient and reliable biphoton resources for applications in quantum communication, quantum networks, and precision metrology.
Efficiently generating photon pairs with high heralding efficiency and high single photon purity that are bandwidth matched to quantum emitters, quantum memories, and other matter-based qubits is critical for quantum networking applications. However, nonlinear optics-based sources require substantial spectral engineering to overcome the orders of magnitude bandwidth mismatch between those sources and qubit systems. A popular solution is cavity-enhanced spontaneous parametric down conversion (SPDC) where the cavity sets the photon bandwidth and simultaneously enhances the spectral brightness of the SPDC. Bulk, free-space configurations are generally required to achieve the MHz-scale bandwidths required to interface with most qubit systems. Replicating these in scalable integrated photonic architectures is an ongoing challenge due to the much higher propagation losses that limit the size and linewidth of chip-based resonators. We show here how an intra-cavity slow light medium, acting as an ultra-narrow filter, would enable narrowband photon pair generation in broadband cavities with high single photon purity and without compromising the heralding efficiency. We show that such metrics can be readily realized in erbium doped thin-film lithium niobate microrings using realistic design parameters.
Entangled photon sources (EPSs) are essential building blocks for scalable quantum communication and quantum key distribution (QKD). We present a stable, highly nondegenerate EPS based on type-0 spontaneous parametric down-conversion (SPDC) in a crossed-crystal configuration, generating photon pairs at 680~nm and 1550~nm when pumped by a 473~nm laser. This wavelength combination, reported here for the first time, simultaneously benefits from the peak detection efficiency of the most of the Si-SPADs in the visible/near-infrared spectral range and the low-loss fiber transmission of the telecom C-band. This configuration provides the most favorable balance between performance and cost for detection using Si-SPADs and InGaAs detectors. The source exhibits a measured spectral bandwidth of 300~GHz, corresponding to a spectral brightness of up to 1.9×1031.9\times 10^31.9×103~pairs~s−1^{-1}−1~mW−1^{-1}−1~GHz−1^{-1}−1. Heralding efficiencies reach 18~\% (signal) and 34~\% (idler) with Si-SPAD and superconducting nanowire single-photon detectors (SNSPD) detection. The entangled state achieves visibilities of (97.3±1.0) %(97.3\pm 1.0)\,\%(97.3±1.0)% in the H/V basis and (94.9±1.6) %(94.9\pm1.6)\,\%(94.9±1.6)% in the D/A basis, yielding a fidelity of ≥(96.1±1.3) %\geq(96.1\pm1.3)\,\%≥(96.1±1.3)%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.
Summary form only given. Hybrid quantum technologies seek to combine the advantages of two individual quantum architectures by transferring the information between the two systems. We want to benefit from the high mobility and ease of transmission of photons for quantum communication and exploit the excellent readout and storage capabilities of atomic qubits as a quantum memory, which is essential to build up quantum repeater networks for quantum data processing. Efficient interaction of photons with atoms requires a match of the photons spectral properties to those of the resonances of the atomic species. The atomic transition usually has a much narrower bandwidth than single photons generated by spontaneous parametric down-conversion (SPDC), the current gold standard of producing high-purity heralded single photons at flexible wavelengths. To develop hybrid quantum technologies therefore requires significantly reducing the single photon emission spectra to fulfill these requirements. The way we try to achieve this is by using an optical cavity to enhance the probability of creating the photons in the spectral and spatial resonator mode.We report 100% duty cycle generation of sub-MHz single photon pairs at the Rubidium D1 line using cavityenhanced spontaneous parametric down-conversion [3]. The double exponential decay of the temporal intensity cross-correlation function exhibits a bandwidth of 666 ± 16 kHz for the single photons, an order of magnitude below the natural linewidth of the target transition. This is, to our knowledge, the narrowest bandwidth of single photons from SPDC to date. A new method of placing a half-wave plate inside the cavity helps to achieve triple resonance between pump, signal and idler photon, reducing the bandwidth and simplifying the locking scheme. Additionally, stabilisation of the cavity to the pump frequency enables the 100% duty cycle. The quantum nature of the source is confirmed by the idler-triggered second-order autocorrelation function at τ = 0 to be g(2) s,s (0) = 0.016 ± 0.002 for a heralding rate of 5 kHz and antibunching below 0.5 is observed up to heralding rates of 250 kHz [3]. The high purity of the source is matched by high indistinguishability of the photons demonstrated in a Hong-Ou-Mandel (HOM) interference experiment with a visibility V = 96.5 ± 0.5 %. Additionally, the modelocked two-photon state of the generated pairs leads to revivals of the HOM dip. We observe these revivals up to 100 m path difference between signal and idler photon, independently proofing the long coherence length of our photons.
Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.