Broadband SQUID amplifiers for photonic applications

The dynamics and noise of a de SQUID (the Superconducting Quantum Interfere nce Device), with the Mc Cumber parameter beta(c) = 2 pi R-2/C-c/Phi(0) clo se to the unity (where Ic, R, C is the critical current, the shunt resistan ce and the capacitance of the Josephson junctions comprising the SQUID, res pectively., Phi(0) = 2.07 10(-15) Wb is the magnetic flux quantum) integrat ed with a planar spiral input coil, have been experimentally studied. The l ength of the spiral input coil was chosen so as to match its lambda/4 micro wave resonance frequency to the plasma resonance frequency of the SQUID, Th e input coil resonance is found to enhance the overall quality factor Q of the Josephson oscillations in the SQUID and, as a result, to increase the d ynamic resistance Rd and the gradient of the flux-to-voltage characteristic s, partial derivative V/partial derivative Phi without hysteresis. A de SQU ID with a loop inductance L=30.5 pH, beta(c)=0.72, and a 6 turn input coil demonstrated a non-distorted quasi-sinusoidal flux-to-voltage transfer func tion with an exceptionally large modulation depth of 140 mu V peak-to-peak. The spectral density of the magnetic flux noise was as low as 3.5x10(-7) P hi(0)/Hz(1/2) in the double stage configuration, measured at a temperature 4.2K using direct read out electronics. In combination with an intermediary transformer, the current resolution of the SQUID is as low as 1.25 pA/Hz(1 /2) with an input coil inductance of 100 nH.