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Quantum Review Letters

Correlated photon-pair source using silicon microring enhanced by reverse–biased free–carrier depletion

Lu Wang, Huajin Chang, Haoyang Wang, Jingmei Zhang, Qiang Zeng, Rongzhen Jiao, Haowen Shu, Xingjun Wang, Zhiliang L. Yuan

Published Jan 1, 2025• 1 citation
Publisher

Abstract

This study systematically investigates the impact of introducing free-carrier depletion to the microring cavity on the correlated photon-pair generation. The free-carrier depletion is realized through embedding a p-i-n diode adjacent to the silicon microring resonator and applying a reverse bias voltage, thus reducing the nonlinear loss caused by free carrier absorption. Experimental results show a significant reduction in free-carrier lifetime, which improves key metrics of photon-pair generation, including coincidence rate, coincidence-to-accidental ratio (CAR), and heralding efficiency. A high pair generation rate of near 5 GHz/mW 2 with an associated CAR of 231 is achieved at 0.038 mW on-cavity power and 30 V bias. However, it is observed that the side-effect such as linear noise and thermal disturbance due to high pump power can override the improvements. Excessively high voltage can also degrade the performance as the extinction ratio is deviated from the optimum operating point. Additionally, the power-line interference existing in the DC supply is considered, which induces significant oscillation in the transmission spectrum and is shown detrimental to the photon-pair generation.

DOI
10.1016/j.qrl.2025.11.001

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This study systematically investigates the impact of introducing free-carrier depletion to the microring cavity on the correlated photon-pair generation. The free-carrier depletion is realized through embedding a p-i-n diode adjacent to the silicon microring resonator and applying a reverse bias voltage, thus reducing the nonlinear loss caused by free carrier absorption. Experimental results show a significant reduction in free-carrier lifetime, which improves key metrics of photon-pair generation, including coincidence rate, coincidence-to-accidental ratio (CAR), and heralding efficiency. A high pair generation rate of near 5 GHz/mW 2 with an associated CAR of 231 is achieved at 0.038 mW on-cavity power and 30 V bias. However, it is observed that the side-effect such as linear noise and thermal disturbance due to high pump power can override the improvements. Excessively high voltage can also degrade the performance as the extinction ratio is deviated from the optimum operating point. Additionally, the power-line interference existing in the DC supply is considered, which induces significant oscillation in the transmission spectrum and is shown detrimental to the photon-pair generation.
Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome free-carrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.
In this paper, 1.5 μm correlated photon pairs are generated under continuous wave (CW) pumping in a silicon micro-ring cavity with a Q factor of 8.1 × 10(4). The ratio of coincidences to accidental coincidences (CAR) is up to 200 under a coincidence time bin width of 5 ns. The experiment result of single side photon count shows that the generation rate does not increase as the square of the pump level due to the nonlinear losses in the cavity which reduce the Q factor and impact the field enhancement effect in the cavity under high pump level. Theoretical analysis shows that the photon pair generation rate in the cavity is proportional to the seventh power of the Q factor, which agrees well with the experiment result. It provides a way to analyze the performance of CW pumping correlated photon pair generation in silicon micro-ring cavities under high pump levels.
One of the key building blocks of quantum photonic systems is a source of indistinguishable photons. Silicon ring resonators can be used as photon pair sources by taking advantage of silicon's large third order nonlinearity with a process known as spontaneous four wave mixing. These sources are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the desired photons generated in the ring can be directed to the drop port. Thus, the ratio between the coincidences from the drop port and the total number of coincidences from all ports (coincidence efficiency) can be significantly increased, with the trade-off being that the pump is less efficiently coupled into the ring. This asymmetric coupling scheme was used to demonstrate a dual-bus microring source with a coincidence efficiency of ~96%. Unfortunately, the penalty to pump power was quite large, with a required ~10x increase in the pump power to maintain the same pair generation rate.\nTo further improve upon this result, a microring source was designed with multipoint evanescent couplers to induce a strong spectral dependence in the coupling coefficients. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a ring-based photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Asymmetric coupling was also used in this configuration but in a different way than in the previous source. In this case, the input side of the ring was configured to be critically coupled for maximal resonance enhancement while the output side was configured to be over coupled to minimize photon-pair loss within the ring. This device design was used to demonstrate a photon-pair source with coincidence efficiencies of 0.9981+-6.425x10^-4 and 0.9979+-1.665x10^-3 for degenerate and non-degenerate pump configuration respectively.\nIn addition to the source development, work was done to move some of the pump rejection filtering onto the photonic chip. While the fabricated device only exhibited an extinction of 50 dB, the root cause of the limited extinction was explored and a new filter design was proposed to overcome these limitations. With the combination of the microring source with its greatly improved efficiency and the movement toward reducing the required off-chip filtering, this work represents a significant advancement in the scalability of integrated quantum photonic systems which can enable high performance quantum computing and communication systems.
Silicon microring resonators are being recently used for high-brightness and efficient photon-pair generation at telecommunication wavelengths. Here, based on detailed theoretical and numerical modeling, we study the impact on pair generation of increasing the optical pump power, which generally causes nonlinear impairments such as free-carrier and two-photon absorption in silicon micro-resonators. Contrary to expectation, the pair generation properties of such devices may seem to be preserved at increasing pump powers, although not better than at a moderate pump power. These results suggest that silicon microrings can be used for pair generation over a wide range of pump powers, which may benefit applications in remotely pumped architectures, where the pump level might not be known a priori.
1.5um correlated photon pair generation under CW pumping in silicon micro-ring cavity is realized. Experiment result shows that the correlated photon pair generation rate is proportional to the product of the square of the pump power and the cube of the cavity Q value, which varies under different pump level due to the nonlinear loss in the cavity when the pump light is on resonance. The maximum ratio between the coincident and accidental coincident counts (CAR) is up to 240 under a coincidence time bin width of 5ns.