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:
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Journal of Low Temperature Physics
- Pub Date:
- December 2007
- DOI:
- 10.1007/s10909-007-9512-9
- arXiv:
- arXiv:cond-mat/0701719
- Bibcode:
- 2007JLTP..149..261K
- Keywords:
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- Condensed Matter - Superconductivity
- E-Print:
- submitted to J. Low Temp. Phys