LOW-NOISE MULTIWASHER SUPERCONDUCTING INTERFEROMETER

The dc-superconducting quantum interference device (SQUID) is a low-no ise converter from magnetic flux to voltage which can have, in princip le, an energy sensitivity near the quantum limit of (h) over bar/2. A critical parameter for the ideal performance is the device inductance, which must be kept as small as possible. Minimizing the SQUID inducta nce, however, is a major concern for a practical device; this requirem ent implies a small SQUID ring and hence magnetic coupling with an ext ernal signal is more difficult to achieve. Here we present an original scheme (called multiwasher) to circumvent this problem, and its imple mentation in an all-refractory thin-film device. Our scheme not only p rovides good magnetic coupling with a large input coil (0.5 mu H) and very low SQUID inductance, but also shielding from outside uniform fie lds, such as those generated by ambient disturbances. The measured cou pled spectral energy sensitivity in the white region at about 1 kHz is 28(h) over bar. at 4.2 K and <5.5(h)over bar> in a pumped helium bath at 0.9 K. The flux noise spectral density at 0.1 Hz and 0.9 K is Phi( n) = 1 x 10(-6) Phi(0)/root Hz. (C) 1998 American Institute of Physics .