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Exploring NbTiN superconducting single photon detectors with quantum detector tomography

Rivera Hernández, Juan Carlos

Published Sep 9, 2019• 0 citations
OpenAlex

Abstract

The optical response of NbTiN superconducting single photon detectors is studied using quantum detector tomography which separates the detector response in single and multiple photon contributions. Three different NbTiN nanodetectors of 70, 100 and 150 nm width are compared based on their detector tomography curves. The energy dependent photon response for the 100 nm nanodetector at two different wavelengths is explored. The detectors present a non-linear behaviour, which complicates the comparison. For the first time, we find that tomography curves saturate above a certain bias current and we compare the current density required to achieve the single photon regime.

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The optical response of NbTiN superconducting single photon detectors is studied using quantum detector tomography which separates the detector response in single and multiple photon contributions. Three different NbTiN nanodetectors of 70, 100 and 150 nm width are compared based on their detector tomography curves. The energy dependent photon response for the 100 nm nanodetector at two different wavelengths is explored. The detectors present a non-linear behaviour, which complicates the comparison. For the first time, we find that tomography curves saturate above a certain bias current and we compare the current density required to achieve the single photon regime.
Using detector tomography, we investigate the detection mechanism in NbN-based superconducting single photon detectors (SSPDs). We demonstrate that the detection probability uniquely depends on a particular linear combination of bias current and energy, for a large variation of bias currents, input energies and detection probabilities, producing a universal detection curve. We obtain this result by studying multiphoton excitations in a nanodetector with a sparsity-based tomographic method that allows factoring out of the optical absorption. We discuss the implication of our model system for the understanding of meander-type SSPDs.
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.
We present an experimental method to characterize multi-photon detectors with a small overall detection efficiency. We do this by separating the nonlinear action of the multiphoton detection event from linear losses in the detector. Such a characterization is a necessary step for quantum information protocols with single and multiphoton detectors and can provide quantitative information to understand the underlying physics of a given detector. This characterization is applied to a superconducting multiphoton nanodetector, consisting of an NbN nanowire with a bowtie-shaped subwavelength constriction. Depending on the bias current, this detector has regimes with single and multiphoton sensitivity. We present the first full experimental characterization of such a detector.
We demonstrate the use of a flexible and highly accurate Markov chain Monte Carlo quantum detector tomography method as a minimization algorithm to best describe the response of an efficient 120 nm wide NbTiN superconducting nanobridge single photon detector. Separation of the internal quantum efficiency and external quantum efficiency is possible due to the difference in saturation behavior of an ideal one-photon threshold detector as compared to a detector with non-unity one-photon internal quantum efficiency. From a statistical analysis of our measurements (at T=(4.23 ± 0.01) K, I=(29.4 ± 0.1) μA, I/Iswitch=(0.90 ± 0.01)), we find an external quantum efficiency of η=(1.60 ± 0.05)×10−6, a one-photon internal quantum efficiency of p1=0.568 ± 0.008, and a unity multi-photon (two or more) internal quantum efficiency.
The quantum efficiency of NbN and NbTiN superconducting single photon detectors drops with decreasing photon energy. A lower gap material would enable single photon detection deeper in the infrared. We have fabricated a NbSi detector and compare its characteristics with a NbTiN device. NbSi (TC≃2 K) has a smaller superconducting gap than NbTiN or NbN (TC≃15 K). We measure the detection efficiency for a wavelength range from 1100 to 1900 nm. In this range the NbSi detector shows a 10-fold increase in relative efficiency with respect to the NbTiN detector.