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IEEE Transactions on Applied Superconductivity

Numerical Simulation of Multi-Junction Bias Circuits for Superconducting Detectors

K. Segall, J. J. Mazo, Terry P. Orlando

Published Jun 1, 2005• 3 citations
Publisher

Abstract

We present numerical simulations of a new biasing circuit for single photon detectors based on superconducting tunnel junctions. A single detector junction is replaced with a circuit of three junctions to achieve biasing of a detector junction at subgap currents without the use of an external magnetic field. The biasing occurs through the nonlinear interaction of the three junctions. We show nonlinear numerical simulation for different values of junction parameters and demonstrate that the biasing state is numerically stable against external fluctuations. The elimination of the external magnetic field potentially increases the capability of these types of photon detectors and eases constraints involved in the fabrication of large detector arrays.

DOI
10.1109/tasc.2005.849948

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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.
The magnetic field dependence of the Josephson current of a Josephson tunnel junction whose shape was a normal distribution is analyzed for superconducting tunnel junction (STJ) photon detector applications. It was found that the Josephson current of a normal distribution junction is very sensitive to the external magnetic field and follows a function of exp (-B 2), while a conventional rectangular STJ exhibits the well-known Fraunhofer pattern of sin x/x. Numerical calculations show that the Josephson current of the normal distribution junction can be suppressed by an external magnetic field that is one tenth of that needed for conventional rectangular junctions. Calculations of a normal distribution junction approximated with a 1 µm mesh also show that the Josephson current is sufficiently suppressed by a small magnetic field. The effect of the misalignment of the angle between the junction and the magnetic field was evaluated. It was found that for a normal-distribution-shaped junction, misalignment of the magnetic field is allowed within ± 5°, suppressing the Josephson effect by two orders of magnitude more than that of a rectangular junction.
Detection of light dark matter, such as axion-like particles, puts stringent requirements on the efficiency and dark-count rates of microwave-photon detectors. The possibility of operating a current-biased Josephson junction as a single-microwave photon-detector was investigated through numerical simulations, and through an initial characterization of two Al junctions fabricated by shadow mask evaporation, done in a dilution refrigerator by measuring escape currents at different temperatures, from 40 mK up to the Al transition temperature. The escape dynamics of the junctions were reproduced in the simulation, including the dissipative effects. Inhibition of thermal activation was observed, leaving the macroscopic quantum tunneling as the dominant effect well beyond the crossover temperature.
We describe a new superconductive three-terminal device that acts as a fast pulse amplifier and discriminator. The device is based on a parallel nano-wire configuration. The nano-wires are properly biased near their critical current. When a current pulse of sufficient amplitude is injected into one of the wires, a transition to the normal state of all the wires occurs, thus generating a large output current pulse. This device could be used as a pulse amplifier for signals generated by nano-wire detectors or by RSFQ circuits. The parallel nano-wire structure can be realized with the same fabrication process used for NbN Superconducting Single Photon Detector (SSPD), so its integration into the existing technology is straightforward. Here we present numerical simulations of the device, with special attention to circuit requirement for the correct operation.
Current-biased Josephson junctions can act as detectors of electromagnetic radiation. At optimal conditions, their sensitivity is limited by fluctuations causing stochastic switching from the superconducting to the resistive state. This work provides a quantitative description of a stochastic switching current detector, based on an underdamped Josephson junction. It is shown that activation of a Josephson plasma resonance can greatly enhance the detector responsivity in proportion to the quality factor of the junction. The ways of tuning the detector for achieving optimal operation are discussed. For realistic parameters of Nb / Al O x / Nb tunnel junctions, the sensitivity and noise-equivalent power (NEP) can reach values of S ≃ 5 × 10 12 (V/W) and NEP ≃ 2 × 10 − 23 ( WHz − 1 / 2 ), respectively. These outstanding characteristics facilitate both bolometric and single-photon detection in microwave and terahertz ranges. Published by the American Physical Society 2024