The stationary SQUID

TL;DR

This paper proposes a stationary SQUID design that avoids oscillatory electromagnetic fields, enabling flux measurement with high resolution by analyzing a superconducting loop without Josephson junctions.

Contribution

It introduces a novel stationary SQUID circuit design and derives analytical expressions for its behavior, including flux dependence, power dissipation, and fluctuation effects.

Findings

Achieves flux resolution of about 10^{-5}Φ_0/Hz^{1/2} at 0.9T_c

Demonstrates the circuit reaches a stationary regime with flux-dependent voltage

Provides analytical expressions valid without specific order parameter evolution models

Abstract

In the customary mode of operation of a SQUID, the electromagnetic field in the SQUID is an oscillatory function of time. In this situation, electromagnetic radiation is emitted, and couples to the sample. This is a back-action that can alter the state that we intend to measure. A circuit that could perform as a stationary SQUID consists of a loop of superconducting material that encloses the magnetic flux, connected to a superconducting and to a normal electrode. This circuit does not contain Josephson junctions, or any other miniature feature. We study the evolution of the order parameter and of the electrochemical potential in this circuit; they converge to a stationary regime and the voltage between the electrodes depends on the enclosed flux. We obtain expressions for the power dissipation and for the heat transported by the electric current; the validity of these expressions does not rely on a particular evolution model for the order parameter. We evaluate the influence of fluctuations. For a SQUID perimeter of the order of 1$\mu$m and temperature $0.9T_c$, we obtain a flux resolution of the order of $10^{-5}\Phi_0/$Hz$^{1/2}$; the resolution is expected to improve as the temperature is lowered.