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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High spectral purity silicon ring resonator photon-pair source

Jeffrey A. Steidle, Michael L. Fanto, Christopher C. Tison, Zihao Wang, Stefan F. Preble, Paul M. Alsing

Published May 21, 2015• 13 citations
Publisher

Abstract

Here we present the experimental demonstration of a Silicon ring resonator photon-pair source. The crystalline Silicon ring resonator (radius of 18.5μm) was designed to realize low dispersion across multiple resonances, which allows for operation with a high quality factor of Q~50k. In turn, the source exhibits very high brightness of >3x105 photons/s/mW2/GHz since the produced photon pairs have a very narrow bandwidth. Furthermore, the waveguidefiber coupling loss was minimized to <1.5dB using an inverse tapered waveguide (tip width of ~150nm over a 300μm length) that is butt-coupled to a high-NA fiber (Nufern UHNA-7). This ensured minimal loss of photon pairs to the detectors, which enabled very high purity photon pairs with minimal noise, as exhibited by a very high Coincidental-Accidental Ratio of >1900. The low coupling loss (3dB fiber-fiber) also allowed for operation with very low off-chip pump power of <200μW. In addition, the zero dispersion of the ring resonator resulted in the production of a photon-pair comb across multiple resonances symmetric about the pump resonance (every ~5nm spanning >20nm), which could be used in future wavelength division multiplexed quantum networks.

DOI
10.1117/12.2177179

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Here we present the experimental demonstration of a Silicon ring resonator photon-pair source. The crystalline Silicon ring resonator (radius of 18.5μm) was designed to realize low dispersion across multiple resonances, which allows for operation with a high quality factor of Q~50k. In turn, the source exhibits very high brightness of >3x105 photons/s/mW2/GHz since the produced photon pairs have a very narrow bandwidth. Furthermore, the waveguidefiber coupling loss was minimized to <1.5dB using an inverse tapered waveguide (tip width of ~150nm over a 300μm length) that is butt-coupled to a high-NA fiber (Nufern UHNA-7). This ensured minimal loss of photon pairs to the detectors, which enabled very high purity photon pairs with minimal noise, as exhibited by a very high Coincidental-Accidental Ratio of >1900. The low coupling loss (3dB fiber-fiber) also allowed for operation with very low off-chip pump power of <200μW. In addition, the zero dispersion of the ring resonator resulted in the production of a photon-pair comb across multiple resonances symmetric about the pump resonance (every ~5nm spanning >20nm), which could be used in future wavelength division multiplexed quantum networks.
We experimentally demonstrate a high-spectral-purity photon source by designing a dual-Mach–Zehnder-interferometer-coupled silicon ring resonator, wherein the linewidths of pump and signal (idler) resonances can be engineered independently. A high spectral purity of 95 % ± 1.5 % is obtained via a time-integrated g ( 2 ) correlation measurement, which exceeds the theoretical 93% bound of a traditional ring’s spontaneous four-wave-mixing photon source. This source also possesses high performance in other metrics including a measured coincidence of 9599 pairs/s and a preparation heralding efficiency of 52.4% at a relatively low pump power of 61 µW as well as high drop-to-through suppression of 20.2 dB. By overcoming the trade-off between spectral purity and brightness in the post-filtering way, such a method guarantees bright pure photons and will pave the way for development of on-chip quantum information processing with improved operation fidelity and efficiency.
Silicon ring resonators are 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%50\%50% loss. By slightly decoupling the input waveguide from the ring, the drop port coincidence ratio can be significantly increased with the trade-off being that the pump is less efficiently coupled into the ring. Ring resonators with this design have been demonstrated having coincidence ratios of ∼96%\sim 96\%∼96% but requiring a factor of ∼10\sim 10∼10 increase in the pump power. Through the modification of the coupling design that relies on additional spectral dependence, it is possible to achieve similar coincidence ratios without the increased pumping requirement. This can be achieved by coupling the input waveguide to the ring multiple times, thus creating a Mach-Zehnder interferometer. 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 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. Recently, this device has been realized with preliminary results exhibiting the desired spectral dependence and with a coincidence ratio as high as ∼97%\sim 97\%∼97% while allowing the pump to be nearly critically coupled to the ring. The demonstrated near unity coincidence ratio infers a near maximal heralding efficiency from the fabricated device. This device has the potential to greatly improve the scalability and performance of quantum computing and communication systems.
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 an efficient energy-time entangled photon-pair source based on four-wave mixing in a CMOS-compatible silicon photonics ring resonator. Thanks to suitable optimization, the source shows a large spectral brightness of 400\,pairs of entangled photons /s/MHz for 500 μW\rm 500\,\mu W500μW pump power. Additionally, the resonator has been engineered so as to generate a frequency comb structure compatible with standard telecom dense wavelength division multiplexers. We demonstrate high-purity energy-time entanglement, i.e., free of photonic noise, with near perfect raw visibilities (>>>~98\%) between various channel pairs in the telecom C-band. Such a compact source stands as a path towards more complex quantum photonic circuits dedicated to quantum communication systems.
Presented here are results on a silicon ring resonator photon pair source with a high heralding efficiency. Previous ring resonator sources suffered from an effective 50% loss because, in order to generate the photons, the pump must be able to couple into the resonator which is an effective loss channel. However, in practice the optical loss of the pump can be traded off for a dramatic increase in heralding efficiency. This research found theoretically that the heralding efficiency should increase by a factor of ∼ 3:75 with a factor of 10 increase in the required pump power. This was demonstrated experimentally by varying the separation (gap) between the input waveguide and the ring while maintaining a constant drop port gap. The ring (R = 18:5μm, W = 500nm, and H = 220nm) was pumped by a tunable laser (λ ≈ 1550nm). The non-degenerate photons, produced via spontaneous four wave mixing, exited the ring and were coupled to fiber upon which they were filtered symmetrically about the pump. Coincidence counts were collected for all possible photon path combinations (through and drop port) and the ratio of the drop port coincidences to the sum of the drop port and cross term coincidences (one photon from the drop port and one from the through port) was calculated. With a 350nm pump waveguide gap (2:33 times larger than the drop port gap) we confirmed our theoretical predictions, with an observed improvement in heralding efficiency by a factor of ∼ 2:61 (96:7% of correlated photons coupled out of the drop port). These results will enable increased photon flux integrated photon sources which can be utilized for high performance quantum computing and communication systems.