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arXiv:1703.08368 · Optics Express

Path to increasing the coincidence efficiency of integrated resonant photon sources

C. C. Tison, J. A. Steidle, M. L. Fanto, Z. Wang, N. A. Mogent, A. Rizzo, S. F. Preble, P. M. Alsing

Published Dec 20, 2017• 45 citations
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Abstract

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 $50\%$ 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 $\sim 96\%$ but requiring a factor of $\sim 10$ 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 $\sim 97\%$ 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.

DOI
10.1364/oe.25.033088

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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.
Ring resonators are used as photon pair sources by taking advantage of the materials second or third order non- linearities through the processes of spontaneous parametric downconversion and spontaneous four wave mixing respectively. Two materials of interest for these applications are silicon for the infrared and aluminum nitride for the ultraviolet through the infrared. When fabricated into ring type sources they 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 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% but requiring a factor of ~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% 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.
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.
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.
The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip.
Silicon micro-ring resonators have been demonstrated to be potential “on- chip” efficient sources of time-energy entangled photons. In these devices, photon pairs are generated by spontaneous four-wave-mixing (SFWM), an elastic scattering event involving two pump photons that results in the generation of an idler and a signal photon. The mass production of such integrated devices for quantum optic experiment requires the implementation of fast and reliable techniques to monitor the emitted two-photon state as well as the device performances. In this work we show that the joint spectral density of photon pairs that would be generated by SFWM in a silicon ring resonator can be measured with unprecedented resolution and speed, improving of several orders of magnitudes the results of traditional approaches based on coincidence measurements.