A Numerical Treatment of the rf SQUID: II. Noise Temperature
Abstract
We investigate rf SQUIDs (Superconducting QUantum Interference Devices), coupled to a resonant input circuit, a readout tank circuit and a preamplifier, by numerically solving the corresponding Langevin equations. The quantity of interest is the noise temperature T _{ N }. We use an analytical expression T _{ N0,opt}, which is already optimized for the parameters of the input circuit, and vary the model parameters of the remaining circuit to minimize T _{ N0,opt}. We also compare T _{ N0,opt} to numerical simulations of the full circuit and find good agreement. The best device performance is obtained when β'_{ L }≡2 π LI _{0}/ Φ _{0} is in the range 0.5 0.9; L is the SQUID inductance, I _{0} the junction critical current and Φ _{0} the flux quantum. For a tuned input circuit we find an optimal noise temperature T _{ N0,opt}≈3 Tf/ f _{ c }, where T, f and f _{ c } denote temperature, signal frequency and junction characteristic frequency, respectively. This value is close to the optimal noise temperatures obtained by approximate analytical theories carried out previously in the limit β'_{ L }≲1. We study the dependence of T _{ N0,opt} on various model parameters away from their optimum values, and often find much lower values of T _{ N0,opt} than predicted by the analytical theory. We finally discuss implications for devices that can be implemented experimentally.
 Publication:

Journal of Low Temperature Physics
 Pub Date:
 December 2007
 DOI:
 10.1007/s1090900795129
 arXiv:
 arXiv:condmat/0701719
 Bibcode:
 2007JLTP..149..261K
 Keywords:

 Condensed Matter  Superconductivity
 EPrint:
 submitted to J. Low Temp. Phys