SNSPDs performances are strongly dependent on the readout electronics. Recovery time of the efficiency, latching current, or crosstalk between multiple detectors are all issues that can be dramatically worsen or improved using an appropriate readout scheme.
More generally, interfacing superconductors with semiconductor electronics is challenging.
SNSPDs are a great solution as their resistive state impedance allow them to be directly read-out by conventional electronics. However, superconducting devices proposed initially to interface superconductors to semiconductors might be of interest to further push the performances of SNSPDs. In particular, a thermally activated switch has been proposed for use in superconducting neuromorphic circuits [85]. This device can produce a direct gain to the output signal of SNSPDs. Moreover, this device is not sensitive to latching when connected to a high impedance load, as is the case for SNSPDs. Finally the device terminations are not galvanically connected, which could allow to readout multiple SNSPDs through a single line with no electronic crosstalk at all.
Work on those solutions has been started in our group and I had the opportunity to participate in the development of preliminary tests. This work goes beyond the scope of the thesis, nevertheless, a brief presentation of the concepts as well as first results obtained in our group are available in appendix A.
5 SNSPDs in quantum experiments
The main activities of the Group of Applied Physics are Quantum Key Distribution and Quantum Communication. Pushing further the limitation of these fields by providing high-efficiency and high-speed single-photon detection has been one of the main motivations to the research presented in this work. The devices fabricated have been implemented by colleagues in many experiments of the group as well as in international collaborations. A technology transfer to the industry has also been done with the company ID Quantique SA.
The present chapter briefly describes quantum information experiments carried out by colleagues, which have particularly benefited from using the detectors created during this thesis. I had the chance to contribute to their work by designing and fabricating SNSPDs tailored to their needs and helping them implement the detectors in their experiments.
5.1 Long distance quantum key distribution
A time-bin QKD platform with a repetition rate of 2.5 GHz has been implemented by our colleagues. The qubits are encoded in photons, hence single photon detectors are required for reading the states. This particular QKD protocol uses the time-bins of the laser source as the degree of freedom for the state preparation, and requires the use of two bases. A photon can be in two time bins : |earlyi or |latei, or in a superposition of the two : |earlyi+|latei√
2 or |earlyi−|latei√
2 .
Figure 5.1: Optical setup for the quantum key distribution experiment.
The experiment is based on time-bins of 100 ps and therefore relies on the ability of the detector to detect single-photon with high timing accuracy. Figure 5.2b shows the dependency of the Quantum Bit Error Rate (QBER) on the detector jitter. Figure 5.2a shows the secret key rate obtained vs the attenuation due to the optical fiber losses. In a long-distance experiment, losses due to the fiber attenuation will be high. Moreover the secret key rate will approach that of the detector DCR. It is necessary to reduce the detector DCR below the expected SKR, while keeping a high detection efficiency.
(a) Qualitative influence of the single-photon detector on the secret key rate of the QKD protocol developed in our lab.
(b) Quantum Bit Error Rate (QBER) induced as a function of the detector jitter for the QKD experimental setup.
Figure 5.2: Limitations of the QKD protocol which depends on the performances on the detectors.
5.1. Long distance quantum key distribution During this thesis, I participated in the group effort to improve the fabrication process and testing of our high efficiency SNSPDs. This allowed us to provide a detector with
>80% efficiencies and jitter <40 ps. Coiling the fiber close to the detector created an effective filtering of wavelengths above 1550 nm [86] without impacting the efficiency at 1550 nm. Further reducing the DCR required filtering the black-body radiation present in the optical fiber. We achieved this by adding a fibered standard 200 GHz dense wavelength division multiplexer (DWDM) bandpass filter on the optical fiber inside the cryostat, at a temperature of 40 Kelvin. We performed SDE and DCR measurements before and after placing the filter (figure 5.3).
37 38 39 40 41 42
Figure 5.3: SDE and DCR measurement on the single-meander SNSPD used for the long-distance QKD experiment. Adding the DWDM filter drastically reduces the DCR with little impact on the SDE.
Losses of the DWDM filter were ∼2 dB at 40 Kelvin, which impacts the SDE by attenuating the signal photons at 1550 nm. However the efficiency loss is small compared to the DCR reduction achieved. Choosing a bias currentIb of 40μA maximizes the DCR reduction while still providing almost saturated efficiency. In this regime, a reduction of SDE from 78.4% to 59.9% is measured, while the DCR is reduced by almost three orders of magnitude, from 192 cps to 0.23 cps.
This detector was integrated in a long-distance experiment, which ultimately demonstrated QKD over 421 km of optical fiber [43]. More details about the experiment can be found in reference [87].