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arXiv:1703.05284 · Physical Review Applied

Josephson Photodetectors via Temperature-to-Phase Conversion

Pauli Virtanen, Alberto Ronzani, Francesco Giazotto

Published May 17, 2018• 20 citations
arXivHTMLPDF

Abstract

Superconducting radiation detectors are promising for high sensitivity in the terahertz frequency range, as required for applications in astrophysics and quantum information processing. This study considers using a detector readout scheme based on a superconducting quantum interference proximity transistor (SQUIPT), with a detailed analysis of device physics and performance predictions. This interferometer-based scheme could also be used for other temperature-sensing purposes, and has the potential to improve the state of the art in ultrahigh-sensitivity superconducting single-photon detectors.

DOI
10.1103/physrevapplied.9.054027

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Superconducting radiation detectors are promising for high sensitivity in the terahertz frequency range, as required for applications in astrophysics and quantum information processing. This study considers using a detector readout scheme based on a superconducting quantum interference proximity transistor (SQUIPT), with a detailed analysis of device physics and performance predictions. This interferometer-based scheme could also be used for other temperature-sensing purposes, and has the potential to improve the state of the art in ultrahigh-sensitivity superconducting single-photon detectors.
Abstract Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52 mV/Φ0) and flux-to-current (above 12 nA/Φ0) transfer functions, with the best estimated flux sensitivity ~ 2.6 μΦ0/(Hz)1/2 reached under fixed voltage bias, where Φ0 is the flux quantum. The interferometers operate up to Tbath ≃\simeq ≃ ≃ 2 K, with an improvement of 70% of the maximal operating temperature with respect to early SQUIPTs design. The main features of the V-based SQUIPT are described within a simplified theoretical model. Our results open the way to the realization of SQUIPTs that take advantage of the use of higher-gap superconductors for ultra-sensitive nanoscale applications that operate at temperatures well above 1 K.
The design for an inductive superconducting quantum interference proximity transistor with enhanced performance, the L-SQUIPT, is presented and analyzed. The interferometer is based on a double-loop structure, where each ring comprises a superconductor--normal-metal--superconductor mesoscopic Josephson weak link and the readout electrode is implemented in the form of a superconducting tunnel probe. Our design allows us to both improve the coupling of the transistor to the external magnetic field and increase the characteristic magnetic flux transfer functions, thereby leading to an improved ultrasensitive quantum limited magnetometer. The L-SQUIPT behavior is analyzed in both the dissipative and the dissipationless Josephson-like operation modes, in the latter case by exploiting both an inductive and a dispersive readout scheme. The improved performance makes the L-SQUIPT promising for magnetic flux detection as well as for specific applications in quantum technology, where a responsive dispersive magnetometry at millikelvin temperatures is required.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. \n \nThe first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. \n \nWe then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. \n \nIn the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. The first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. We then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. In the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
The authors discuss the possible implementation of a single-photon detector based on a superconducting quantum interference device, or SQUID. The SQUID is initialized in an unstable state through the magnetic flux piercing it. A photon absorbed in a proximized normal-metal link induces a change in the critical current of the link, and a voltage pulse at the extremes of the device. Pulses of several tens of microvolts could result. Such a device allows the discrimination of a vast range of photon frequencies and is robust against thermal noise, making it a valuable candidate for detecting terahertz photons in particular.