Abstract: The dc Superconducting QUantum Interference Device (SQUID) consists of two Josephson junctions connected in parallel on a superconducting loop. A current passed through the junctions develops a voltage V across them. This voltage oscillates as a function of a magnetic flux applied to the loop with a period of one flux quantum 0 = h/2e; h is the Planck constant and e the electronic charge. I briefly review various SQUID applications. To make an amplifier, one couples the SQUID loop inductively to a superconducting input circuit in series with an oscillating voltage source and detects the amplified voltage developed across the SQUID. I discuss the theory of intrinsic noise in junctions and SQUIDs. The noise at the signal frequency arises from two components: one at the signal frequency and the other at the much higher Josephson frequency 2eV/h, mixed down to the signal frequency by the nonlinearity of the junctions. I illustrate the second term for a single junction with the direct observation of zero-point fluctuations. The theory of the noise temperature for a quantum limited SQUID amplifier yields TQ = hf/kB at signal frequency f, where kB is the Boltzmann constant. I demonstrate this result experimentally. Finally, I discuss ADMX (Axion Dark Matter eXperiment) which was enabled by the development of the quantum limited SQUID amplifier.
Professor John Clarke, University of California, Berkeley
Host: CFP
Keywords: Physics, Center for Fundamental Physics
Laura Nevins
(847) 467-6678
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