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arXiv:2609.04171

Twin-photon generation in a silicon nitride microresonator

Franz Pacher, Haochen Yan, Alekhya Ghosh, Arghadeep Pal, Toby Bi, Hao Zhang, Lixing You, Hao Li, Daniela Salvoni, Shuangyou Zhang, Pascal Del'Haye

physics.opticsquant-phSubmitted Sep 3, 2026• Announced Sep 4, 2026• 0 citations
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Abstract

Photonic chips with silicon nitride ($\mathrm{Si_3N_4}$) microring resonators are well established as heralded single-photon sources, but their operation as frequency-degenerate twin-photon sources has not previously been demonstrated. Here, we realise a twin-photon source at telecommunication wavelengths in a $\mathrm{Si_3N_4}$ ring microresonator via an inverse four-wave mixing (FWM) process, in which two photons from spectrally distinct pumps are converted into a pair of identical twin photons. The measurements show a maximum coincidence-to-accidental ratio (CAR) of $5.4\pm0.6$. In addition, the microresonator functions as a heralded single-photon source through pump-degenerate spontaneous four-wave mixing (SFWM), exhibiting a spectral purity of $P=0.67\pm0.05$ and a heralded anti-bunching of $g^{(2)}_h(0)=0.0042\pm0.0015$. Together, these results demonstrate both photon-generation schemes on a single integrated $\mathrm{Si_3N_4}$ platform, highlighting its potential for scalable, tailored quantum light generation.

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Photonic chips with silicon nitride (Si3N4\mathrm{Si_3N_4}Si3​N4​) microring resonators are well established as heralded single-photon sources, but their operation as frequency-degenerate twin-photon sources has not previously been demonstrated. Here, we realise a twin-photon source at telecommunication wavelengths in a Si3N4\mathrm{Si_3N_4}Si3​N4​ ring microresonator via an inverse four-wave mixing (FWM) process, in which two photons from spectrally distinct pumps are converted into a pair of identical twin photons. The measurements show a maximum coincidence-to-accidental ratio (CAR) of 5.4±0.65.4\pm0.65.4±0.6. In addition, the microresonator functions as a heralded single-photon source through pump-degenerate spontaneous four-wave mixing (SFWM), exhibiting a spectral purity of P=0.67±0.05P=0.67\pm0.05P=0.67±0.05 and a heralded anti-bunching of gh(2)(0)=0.0042±0.0015g^{(2)}_h(0)=0.0042\pm0.0015gh(2)​(0)=0.0042±0.0015. Together, these results demonstrate both photon-generation schemes on a single integrated Si3N4\mathrm{Si_3N_4}Si3​N4​ platform, highlighting its potential for scalable, tailored quantum light generation.
We report the generation of high-purity twin photon pairs through cavity-enhanced non-degenerate four-wave mixing (FWM) in a high-Q silicon microdisk resonator. Twin photon pairs are created within the same cavity mode and are consequently expected to be identical in all degrees of freedom. The device is able to produce twin photons at telecommunication wavelengths with a pair generation rate as large as (3.96 ± 0.03) × 105 pairs/s, within a narrow bandwidth of 0.72 GHz. A coincidence-to-accidental ratio of 660 ± 62 was measured, the highest value reported to date for twin photon pairs, at a pair generation rate of (2.47 ± 0.04) × 104 pairs/s. Through careful engineering of the dispersion matching window, we have reduced the ratio of photons resulting from degenerate FWM to non-degenerate FWM to less than 0.15.
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.
We report the design of an innovative visible-telecom photon pair source based on counter-propagating spontaneous four wave mixing (CP-SFWM) in a silicon nitride microring resonator. Unlike previous designs, the proposed integrated source achieves automatic phase matching, eliminating the need for dispersion engineering. By employing two lasers at wavelengths of 800 nm and 1550 nm as pumps on opposite ends of the bus waveguides, the resonator generates signal and idler photons at the same wavelengths as the pumps, but propagating in opposite directions. The photon pairs are produced in high-quality factor resonant modes, exhibiting a purity of 1, a brightness of 118.70pairs⋅s−1⋅mW−2, and bandwidths of 157.9 MHz and 79.7 MHz for signal and idler photons, respectively. Our proposal outperforms previous CP-SFWM designs in terms of spectral properties of the photon pairs, emission rate, and scalability, making it an interesting alternative for the implementation of integrated photon pair sources for photonic networks.
With the rapid development of quantum information technology, fully connected multi-user quantum entanglement distribution networks have received increasing attention. Among these, multi-wavelength quantum light sources are key devices for establishing connections between multiple users. Despite recent impressive advances, there are still challenges in increasing the wavelength number of photon pairs due to limitations in the design and fabrication of nonlinear optical devices. The potentials of silicon nitride (Si3N4) microring resonators (MRRs), as scalable platforms for multi-wavelength quantum light sources, are explored in this work. The key design parameters of the Si3N4 MRRs, including waveguide dimension, resonator dispersion, and coupling condition, are comprehensively analyzed to optimize photon-pair generation. Based on these parameters, a Si3N4 MRR with a free spectral range of 20 GHz and an average quality factor of 1.6 million is designed and fabricated. This small free spectral range can generate more channels of correlated photon pairs by using the same wavelength resources. The high-quality resonator contributes to the enhancement of the rate of generating high photon pairs , which are critical for quantum entanglement distribution. With a continuous-wave pump laser, correlated photon pairs across a wide spectral range are generated through the spontaneous four-wave mixing (SFWM). The coincidence-to-accidental ratio (CAR) measurements verify the strong quantum correlation between photon pairs, highlighting the reliability of the system for entanglement distribution. Furthermore, the generation and output characteristics of quantum-correlated photon pairs are experimentally investigated with a tunable bandpass filter. The results demonstrate that 71 wavelength-correlated photon pairs within a 25.6 nm spectral range are successively generated as shown in the Fig. A. Our results pave the way for developing the multi-wavelength quantum light sources with Si3N4 platform, thereby advancing the multi-user quantum networks.
Squeezed light constitutes a key resource for quantum optical technologies including quantum sensing, computing, communication and metrology. For many applications the generation of squeezed light typically requires at least two nonlinear optical stages involving careful phase and frequency matching to achieve the required mixing of squeezed and coherent light. In our work, we introduce an on-chip system that simplifies the generation of coherent-squeezed light, utilizing only a single squeezing stage. We achieve this by pumping a silicon nitride (Si3N4\mathrm{Si_3N_4}Si3​N4​) microring resonator to produce single-mode squeezed light through four-wave mixing at the same frequency as the pump mode, leveraging the inherent χ(3)χ^{(3)}χ(3)-nonlinearity of the Si3N4\mathrm{Si_3N_4}Si3​N4​ resonator. Our on-chip system demonstrates a squeezing of -4.7 dB with a clear perspective towards -10 dB squeezing. We also provide a theoretical model that describes the straightforward yet robust generation of single-mode squeezing at the injection locking point of the ring resonator. In fact, we show that a design with a normal dispersion can be used for robust generation of bright squeezed light without the need for careful suppression of unwanted nonlinear processes. Overall, our findings highlight an approach which drastically simplifies the generation of coherent-squeezed light in photonic integrated circuits.
On-chip heralded single photon sources are of key importance in the development of chip-scale devices exploiting the quantum properties of light. Single photon states can be produced as single photons heralded from correlated photon pairs generated through spontaneous four wave mixing. On-chip heralded single photon sources based on spontaneous four wave mixing have been already demonstrated. However, the heralded and herald photons are usually generated with wavelengths very close to the pump one, limiting the pump rejection efficiency and the application to the mid infrared. Moreover, the common sources of heralded photons based on spontaneous four wave mixing require spectral post filtering to achieve high purity, limiting the brightness and the integration of these sources. A solution to these problems can be provided by intermodal four wave mixing. In this work, we demonstrate the generation of photon pairs through intermodal four wave mixing in silicon waveguides, measuring the coincidences between the idler at 1.281 μm and the signal at 1.952 μm. We then discuss the application of intermodal four wave mixing to on-chip heralded single photon sources.