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Chinese Physics B

Reconfigurable microring resonators for multipurpose quantum light sources

Yuxing Du, Yingwen Liu, Chao Wu, Pingyu Zhu, Changhui Zhao, Miaomiao 苗苗 Yu 余, Yan 焱 Wang 王, Kaikai 凯凯 Zhang 张, Ping 平 Xu 徐

Published Apr 14, 2025• 0 citations
Publisher

Abstract

Abstract Microring resonators (MRRs) are extensively utilized in photonic chips for generating quantum light sources and enabling high-efficiency nonlinear frequency conversion. However, conventional microrings are typically optimized for a single specific function, limiting their versatility in multifunctional applications. In this work, we propose a reconfigurable microring resonator architecture designed to accommodate diverse application requirements. By integrating a cascaded Mach–Zehnder interferometer (MZI) as the microring coupler, the design enables independent control of the quality factors for pump, signal and idler photons through two tunable phase shifters. This capability allows for dynamic tuning and optimization of critical performance parameters, including photon-pair generation rate (PGR), spectral purity and single photon heralding efficiency (HE). The proposed structure is implemented on a silicon photonic chip, and experimental results exhibit a wide range of tunability for these parameters, with excellent agreement with theoretical predictions. This flexible and multi-functional design offers a promising pathway for high-performance, highly integrated on-chip quantum information processing systems.

DOI
10.1088/1674-1056/adcc88

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Abstract Microring resonators (MRRs) are extensively utilized in photonic chips for generating quantum light sources and enabling high-efficiency nonlinear frequency conversion. However, conventional microrings are typically optimized for a single specific function, limiting their versatility in multifunctional applications. In this work, we propose a reconfigurable microring resonator architecture designed to accommodate diverse application requirements. By integrating a cascaded Mach–Zehnder interferometer (MZI) as the microring coupler, the design enables independent control of the quality factors for pump, signal and idler photons through two tunable phase shifters. This capability allows for dynamic tuning and optimization of critical performance parameters, including photon-pair generation rate (PGR), spectral purity and single photon heralding efficiency (HE). The proposed structure is implemented on a silicon photonic chip, and experimental results exhibit a wide range of tunability for these parameters, with excellent agreement with theoretical predictions. This flexible and multi-functional design offers a promising pathway for high-performance, highly integrated on-chip quantum information processing systems.
Being a key component on a photonic chip, the microring usually specializes in a certain nonlinear optical process and can not simultaneously meet different working conditions for different processes. Here, we theoretically and experimentally investigate a reconfigurable silicon microring resonator to act as a optimization strategy for both classical four-wave mixing and quantum light sources. Experimental results show that the four-wave mixing efficiency with continuous wave and pulsed pump can be both optimized to a high value well matching numerical analysis. A variety of quantum light sources - including the heralded single-photon source, two-photon source and multi-photon source - are demonstrated to present a high performance and their key parameters including the pair generation rates (PGR), the heralding efficiency (HE) and the coincidence-to-accidental ratio (CAR) are controllable and optimizable. Such tunable nonlinear converter is immune to fabrication variations and can be popularized to other nonlinear optical materials, providing a simple and compact post-fabrication trimming strategy for on-chip all-optical signal processing and photonic quantum technologies.
ABSTRACT High‐brightness–integrated quantum light sources represent a fundamental element for large‐scale photonic quantum technologies. Integrated microring resonator (MRR) in highly nonlinear and ultralow‐loss material platforms enable entangled photon‐pair generation, but device performance is affected by fabrication tolerances and can thus not be judiciously controlled. Redundant designs based on parameter variation are impractical for chip‐scale circuits, as the sheer number of parameters would be too complex. To address this, passive photonic circuits must be combined with active control elements to optimize photon‐pair emission and to match emission characteristics among sources. Here, we present a comprehensive study of an actively tunable, interferometrically coupled integrated MRR implemented in silicon nitride (SiN). By manipulating two thermally controlled phase shifters, the ring–bus coupling and the resonator phase can be dynamically adjusted, enabling post‐fabrication control of its quality factor (Q‐factor) between 10 5 and 10 6 to optimize the photon emission bandwidth and the pair generation rate. We directly demonstrate for the first time that the optimal coupling and emission characteristics do not occur at critical coupling and vary across frequency modes due to dispersion effects, in good agreement with our theoretical model. These findings highlight the role of adaptive integrated photon sources for single‐batch fabrication of reconfigurable devices with application‐specific spectral properties.
We demonstrate a reconfigurable photon pair source based on spontaneous parametric downconversion in a periodically poled aluminum nitride doubly resonant system. The device features two linearly uncoupled microring resonators integrated in a Mach-Zehnder interferometer, enabling independent tuning of pump and signal/idler resonances without complex dispersion engineering. Using a silicon nitride strip-loaded waveguide on poled aluminum nitride, we achieve quasi-phase-matching and resonant enhancement over a broad spectral range. Simulations predict efficient generation across a bandwidth of over 200 nm, with rates exceeding 4 MHz/mW per resonance. This platform offers high performance, spectral tunability, and fabrication tolerance for integrated quantum photonic applications.
Integrated photonics is emerging as a key technology for use in quantum information and communication. Driven by classical communication, which already has a swathe of scalable, efficient, and robust components, integrated photonics promises to transfer quantum science from a mere lab curiosity into a commercial reality. Chip-based quantum light sources are increasingly being recognized as a practical and high-performance alternative to bulky and power-hungry, table-top photon-pair sources. In particular, photon-pair sources based on spontaneous four-wave mixing in microring resonators (MRR) have emerged as a viable integrated photonic solution for quantum information and communication. This thesis relies on silicon nitride (Si3N4) MRR to realize narrowband, telecom photonpair sources. High quality factor MRRs are enabled by the low propagation losses in the Si3N4 platform; in this thesis, they are exploited to realize bright photon-pair sources with heralded spectral purity up to 0.98. Low quality factor MRRs are exploited for a proof-of-concept high-rate sequential time-bin entanglement with net visibilities up to 99.96 ± 0.03 %. For the goal of practical, real-world deployment of integrated photon-pair sources, it is not enough to demonstrate the performance of a single source. One also needs to demonstrate their high-performance operation in complex quantum networking protocols, such as entanglement swapping between genuinely independent sources. Here, for the first time, we demonstrate entanglement swapping between two independent, spatially separated, and asynchronously-pumped MRR photon-pair sources. Our sources operate in the continuous-wave regime, and time-resolved detection results in high Hong-Ou-Mandel (93.2 ±1.6 %) and entanglement swapping (91.2 ± 3.4 %) visibilities. Finally, to further improve our rates while maintaining high visibilities in multi-source interference scenarios, the origin of an on-chip-generated noise must be identified and mitigated. Here, we present our first efforts in the quest for tackling this notorious noise problem.
Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.
Heralded single photon sources generated through spontaneous four wave mixing in silicon microring resonators are highly demanding for large-scale integrated quantum photonics. However, the spectral purity of conventional microring resonators is limited to 93%, which impacts the quality of quantum interference within photonic circuits. The wavelength dependent coupling between ring and bus waveguides plays a significant role in the generation/heralding of single photons from the resonator. In this letter, we have presented a detailed investigation of the dispersive nature of relatively long directional couplers in order to design a microring resonator in all-pass configuration for chip-scale generation of single photons with maximum purity. In our studies, we have shown that a photon purity level close to 99% (λ ∼ 1550 nm) can be achieved by the use of a dispersive directional coupler in a race-track microring resonator in all-pass configuration, without affecting the photon heralding rate. The design parameters of the device are optimized such that it can be fabricated in commercial CMOS foundries.