Figure 1. Single photons are coupled into the ring resonator on top of which the SNSPD has been deposited. The enhance in the interaction time between the single photon and the nanowire increases the detection efficiency.
In the last couple of decades, photonic quantum computing has become a leading contender as a platform for quantum information processing . Recently, CMOS-fabrication technology has been used for quantum optics applications using compact silicon-on-insulator (SOI) photonic circuits. Integrated photonic components allow us to have more complex, stable and scalable quantum photonics devices.
Single photon detectors (SPD) are one of the fundamental building blocks for quantum information processing and therefore highly efficient and fast SPDs with potential for integration are crucial.
Among currently available technologies for single photon detection, superconducting nanowire SPDs (SNSPDs) are the most promising candidates since they have proven to reach detection efficiencies above 90%, with low dark count rates (as low as a few Hz), short dead time (a few ns) and low timing jitter (~20ps) . In addition, SNSPDs have been successfully integrated with photonic waveguides. An example of integrated SNSPDs on SOI is shown in figure 2, where a U-bend nanowire is fabricated on a single mode silicon waveguide. As photon travels along the waveguide, its mode overlaps with that of the nanowires and is absorbed by them. If the system is operated below critical temperature and biased with just below critical current, then the dissipated energy of the photon leads to normal conducting areas in the nanowire. The change in resistance serves as detection signal.
Detection efficiency increases with the length of SNSPDs since the interaction time between photon and nanowire increases. However, long nanowires have higher probability of suffering from imperfections, which limits their critical current and hence their efficiency. Therefore, superconducting nanowires made of a crystalline material such as NbN show a low fabrication yield which is an obstacle when aiming for large scale circuits .
Figure 2. Integrated SNSPD on Silicon-On-Insulator photonic waveguide.
In order to enhance the interaction between the single photon and the nanowire, SNSPDs can be integrated in waveguide cavities . In our work, we study a new design for the integration of SNSPDs in a ring resonator, see figure 1. We assumed the nanowires are made of niobium nitride (NbN) and deposited on top of silicon-on-insulator waveguides. We have developed an analytical model as well as numerical simulations and compared the results.
Our simulations show that the cavity can be designed in a way, that photons of the wavelength of interest can be detected with more than 99 % probability while using superconducting nanowires as short as 1 um. We therefore expect the decrease in length of the normally few tens of um-long nanowires will increase the fabrication yield of SNSPDs. Furthermore we show that due to the presence of a cavity, the detector will be sensitive to wavelength which can be used for building single photon spectrometers.
More details will be presented at the NUSOD 2016 conference in Sydney (MP01).
 E. Knill, R, Laflamme, and G.J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46-52 (2001).
 M. D. Eisaman, J. Fan, A. Migdall, and S. V. Polyakov, “Invited review article: Single-photon sources and detectors,” Rev Sci Instrum. 82, 071101 (2011).
 R. Gaudio, K. P. M. op ‘t Hoog, Z. Zhou, D. Sahin, and A. Fiore, “Inhomogeneous critical current in nanowire superconducting single-photon detectors,” Appl. Phys. Lett. 105, 222602 (2014).
 M. K. Akhlaghi, E. Schelew, and J. F. Young, “Waveguide integrated superconducting single-photon detectors implemented as near-perfect absorbers of coherent radiation ,” Nat. Commun. 6, 8233 (2015).