A Zero-Bias Superconducting Voltage Amplifier Based on the Bipolar Thermoelectric Effect

TL;DR

This paper presents a superconducting voltage amplifier that operates without external bias by utilizing the bipolar thermoelectric effect in an asymmetric SIS junction, achieving significant gain and low noise at cryogenic temperatures.

Contribution

It introduces a novel zero-bias superconducting amplifier based on the bipolar thermoelectric effect, demonstrating practical design and simulation results for broadband cryogenic applications.

Findings

Voltage gain of 20 dB predicted by simulations

Achieves a 1 dB compression point at 2 μV input

Bandwidth extends up to approximately 180 MHz

Abstract

We introduce a zero-bias superconducting voltage amplifier that harvests energy from a thermal gradient by exploiting negative differential resistance (NDR) in an asymmetric tunnel junction. The device is based on an asymmetric superconductor-insulator-superconductor (SIS) junction with an energy-gap ratio of $\Delta_1/\Delta_2 = 0.5$, connected in series with a load resistor. Owing to the superconducting bipolar thermoelectric effect, the current-voltage characteristic of the junction exhibits a region of NDR, in which the net current flows opposite to the applied voltage. This mechanism enables voltage amplification in the absence of any external electrical bias, relying solely on the temperature difference between the electrodes ($T_H \simeq 1$ K, $T_B \simeq 20$ mK). Numerical simulations predict a voltage gain of 20 dB, a 1 dB compression point at an input amplitude of 2 $\mu$V, and a total harmonic distortion below $-50$ dB. The input-referred noise is approximately 1 nV/$\sqrt{Hz}$, with an associated thermal load on the order of nanowatts. The frequency response is broadband from near DC, with a $-3$ dB cutoff around 180 MHz, set by the RC time constant of the junction. Using Al-, Al-Cu-, and AlO$_x$-based technologies, the amplifier is compatible with conventional superconducting circuit fabrication processes. These findings demonstrate that thermoelectric superconducting junctions can deliver bias-free voltage amplification from near DC up to about 200 MHz, making them promising candidates for transition-edge sensor readout, quantum circuit instrumentation, and low-frequency cryogenic signal processing.