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Journal de Physique IV (Proceedings)

Performance of a SQUID read-out system for superconducting tunnel junctions

Didier D. E. Martin, Anthony J. Peacock

Published Jun 1, 1998• 1 citation
Publisher

Abstract

Superconducting Tunnel Junctions (STJs) have traditionally been read-out using low noise JFET based charge sensitive amplifiers. In some applications, low temperature SQUIDs could be used instead. The advantages are that the first amplification stage is also cooled to cryogenic temperatures and can be physically closer to the STJ, the detector can be voltage biased by means of a small fixed resistor and the detected current pulse shapes can be measured more directly. This work will outline the actual SQUID amplifier configuration and explain its virtues as well as its limitations.

DOI
10.1051/jp4:1998349

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Superconducting Tunnel Junctions (STJs) have traditionally been read-out using low noise JFET based charge sensitive amplifiers. In some applications, low temperature SQUIDs could be used instead. The advantages are that the first amplification stage is also cooled to cryogenic temperatures and can be physically closer to the STJ, the detector can be voltage biased by means of a small fixed resistor and the detected current pulse shapes can be measured more directly. This work will outline the actual SQUID amplifier configuration and explain its virtues as well as its limitations.
The new generation of astronomical detectors based on Superconducting Tunnel Junctions (STJs) is capable of detecting photons in a wide range of wavelengths, and, in some cases, even of counting single photons. Since these detectors operate at very low temperature (down to 100 mK), a natural candidate for the readout electronics is the dc-SQUID, a superconducting device that can work as a current preamplifier, reaching very good noise performance. In this paper we present measurements on a homemade dc-SQUID, showing the response of the device to small current pulses, which simulate the real signals from an STJ. An optimal filtering processing of the data is performed, in order to extract from the noisy data the best estimate for the pulse height, which, in a real experiment with single optical photons, would be the parameter that carries the spectroscopic information.
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Abstract We report on the development of a current amplifier for measuring small currents from mesoscopic electronic devices at low temperatures down to the milli-Kelvin range. In our setup, a superconducting quantum interference device (SQUID) located at the mixing chamber stage of the dilution refrigerator is used as the first-stage current amplifier, thereby improving the noise floor down to 8 × 10 −27 A 2 /Hz, which is one order of magnitude as low as those obtained by the conventional methods that utilize a semiconductor-based cryogenic current amplifier. We show the configuration of this setup and demonstrate the amplification of the current generated by a quantum point contact. This approach can open a new way to examine solid-state phenomena that are elusive owing to their small current.
Lateral superconductor/graphene structures can be used to make Josephson junctions with low contact resistances and gate-tuneable critical currents. These junctions have the potential to provide new functionality for superconducting devices. For most devices, e.g. transmon qubits and SQUID sensors, it is important to quantify the intrinsic noise of the junctions. The voltage noise of low resistance junctions is typically below the noise floor of room temperature amplifiers. By cross-correlating the signals from two parallel amplifiers, we can detect signals down to 100 pV/√Hz, well below the noise floor of each amplifier. Using this technique, we characterise the voltage noise of a NbTi DC SQUID with graphene junctions in a frequency range from Hz to kHz. Combined with measurements of the SQUID's gain, we map its sensitivity across a range of operating conditions and find that the best-case sensitivity of the device is similar to traditional low temperature SQUIDs with oxide tunnel junctions. This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No 785219.
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Superconducting tunnel junctions (STJs) have been successfully used as single-photon detectors but require the use of a magnetic field to operate. A recent paper has proposed the idea to use a circuit of three junctions in place of a single junction in order to achieve the necessary biasing without applying a magnetic field. The nonlinear interaction between the different junctions in the circuit causes the existence of a stable subgap state for one of the junctions, which acts as the detector junction. In this paper, we present the first measurements demonstrating the existence of such a biasing state feasible for STJ detectors. Single junction measurements with an applied magnetic field help determine the functional form of the subgap current versus voltage; then the operating point of a three-junction circuit is measured and fit to theory. The excellent match between theory and experiment demonstrates the existence of the subgap biasing state. The outlook for possible use in detector applications is discussed.