Direct current superconducting quantum interferometers with asymmetric shunt resistors

TL;DR

This study demonstrates that asymmetric shunting in dc SQUIDs significantly improves their energy resolution and noise performance, confirmed through experiments matching simulation predictions.

Contribution

The paper introduces a novel asymmetric shunt configuration in dc SQUIDs that enhances their energy resolution by a factor of 3-4 over symmetric designs.

Findings

Asymmetric SQUIDs achieved an energy resolution of approximately 32ħ.

Symmetric SQUIDs had an energy resolution of about 110ħ.

Experimental results confirmed simulation predictions of improved performance.

Abstract

We have investigated asymmetrically shunted Nb/Al-AlO$_x$/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance $R$ identical to a comparable symmetric SQUID with $R^{-1} = R_1^{-1} + R_2^{-1}$, we shunted only one of the two Josephson junctions with $R = R_{1,2}/2$. Simulations predict that the optimum energy resolution $\epsilon$ and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2\,K yielded $\epsilon \approx 32\,\hbar$ for an asymmetric SQUID with an inductance of $22\,\rm{pH}$. For a comparable symmetric device $\epsilon = 110\,\hbar$ was achieved, confirming our simulation results.