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Journal of Applied Physics

How noise affects quantum detector tomography

Q. Wang, Jelmer J. Renema, Alessandro Gaggero, F. Mattioli, Roberto Leoni, Martin P. van Exter, M. J. A. de Dood

Published Oct 1, 2015• 5 citations
Publisher

Abstract

We determine the full photon number response of a NbN superconducting nanowire single photon detector via quantum detector tomography, and the results show the separation of linear, effective absorption efficiency from the internal detection efficiencies. In addition, we demonstrate an error budget for the complete quantum characterization of the detector. We find that for short times, the dominant noise source is shot noise, while laser power fluctuations limit the accuracy for longer timescales. The combined standard uncertainty of the internal detection efficiency derived from our measurements is about 2%.

DOI
10.1063/1.4932021

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We determine the full photon number response of a NbN superconducting nanowire single photon detector via quantum detector tomography, and the results show the separation of linear, effective absorption efficiency from the internal detection efficiencies. In addition, we demonstrate an error budget for the complete quantum characterization of the detector. We find that for short times, the dominant noise source is shot noise, while laser power fluctuations limit the accuracy for longer timescales. The combined standard uncertainty of the internal detection efficiency derived from our measurements is about 2%.
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We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from λ\lambdaλ = 900 nm to λ\lambdaλ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for energies Et > 0.8eV the current energy relation is linear, similar to what was observed in NbN nanowires, whereas the current-energy relation deviates from linear behaviour for total energies below 0.8eV.
We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors. To this purpose, we fabricated a 250 nm wide and 250 nm long WSi nanowire and measured its response to impinging photons with wavelengths ranging from λ = 900 nm to λ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for total absorbed energies >0.8 eV the current–energy relation is linear, similar to what was observed in NbN nanowires, whereas the current–energy relation deviates from linear behavior for total energies below 0.8 eV.
Superconducting single photon detectors consist of a narrow and thin wire of superconductor and can detect single photons with high efficiency and high speed. Simultaneous optimization of the detection efficiency and detector response time is difficult because the response time is set by the dimensions of the wire (cross section and length) while the detection efficiency is determined by both the internal detection mechanism and the dimensions of the wire. Wider and shorter wires are easier to fabricate, are more robust and lead to shorter detector reset time, but are generally less efficient. Experiments employing detector tomography provide important insights into the photon detection mechanism and indicate a detection mechanism where the edges of the wire are more efficient. This leads to a position dependent local detection efficiency that can be explained in the context of a photon-assisted-vortex-entry model and predicts an optimum wire width ~70 nm. A 50 nm wide silicon nanowire deposited on top of a 150 nm wide NbN nanowire directs light at 1550 nm wavelength to the edges and improves both the total absorption efficiency and the internal detection efficiency of the wire. The total absorption efficiency can be enhanced by 30% while the internal detection efficiency is increased by 70%. Assuming that the wire covers a similar area the detector response time is reduced 4-fold compared to the standard design using a 70 nm wide wire.
Nanophotonic circuitry and superconducting nanowires have been successfully combined for detecting single photons, propagating in an integrated photonic circuit, with high efficiency and low noise and timing uncertainty. Waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs) can nowadays be engineered to achieve subnanosecond recovery times and can potentially be adopted for applications requiring Gcps count rates. However, particular attention shall be paid to such an extreme count rate regime since artifacts in the detector functionality emerge. In particular, a count-rate dependent detection efficiency has been encountered that can compromise the accuracy of quantum detector tomography experiments. Here, we investigate the response of waveguide-integrated SNSPDs at high photon flux and identify the presence of parasitic currents due to the accumulation of charge in the readout electronics to cause the above-mentioned artifact in the detection efficiency. Our approach allows us to determine the maximum photon count rate at which the detector can be operated without adverse effects. Our findings are particularly important to avoid artifacts when applying SNSPDs for quantum tomography.