A Numerical Treatment of the Rf SQUID: I. General Properties and Noise Energy

TL;DR

This paper numerically analyzes the noise performance of rf SQUIDs, revealing optimal operation near the crossover regime and comparing intrinsic noise energies to dc SQUIDs, with implications for practical applications.

Contribution

It provides a detailed numerical study of rf SQUID noise characteristics, especially in the crossover regime, improving understanding beyond previous analytical models.

Findings

Optimal noise performance near  in the crossover regime

Intrinsic noise energy is about twice previous analytical estimates

rf SQUIDs have lower noise energy than dc SQUIDs at all temperatures

Abstract

We investigate the characteristics and noise performance of rf Superconducting Quantum Interference Devices (SQUIDs) by solving the corresponding Langevin equations numerically and optimizing the model parameters with respect to noise energy. After introducing the basic concepts of the numerical simulations, we give a detailed discussion of the performance of the SQUID as a function of all relevant parameters. The best performance is obtained in the crossover region between the dispersive and dissipative regimes, characterized by an inductance parameter \beta_L'\equiv 2\pi L I_0/\Phi_0\approx 1; L is the loop inductance, I_0 the critical current of the Josephson junction, and \Phi_0 the flux quantum. In this regime, which is not well explored by previous analytical approaches, the lowest (intrinsic) values of noise energy are a factor of about 2 above previous estimates based on analytical approaches. However, several other analytical predictions, such as the inverse proportionality of the noise energy on the tank circuit quality factor and the square of the coupling coefficient between the tank circuit and the SQUID loop, could not be well reproduced. The optimized intrinsic noise energy of the rf SQUID is superior to that of the dc SQUID at all temperatures. Although for technologically achievable parameters this advantage shrinks, particularly at low thermal fluctuation levels, we give an example for realistic parameters that leads to a noise energy comparable to that of the dc SQUID even in this regime.