Dale J. van Harlingen

Dc SQUID near the quantum noise limit

Abstract A series of dc SQUIDs has been fabricated in an attempt to obtain quantum-limited sensitivity. Typically, the SQUID inductance, L, is 2 pH, the capacitance and critical current of each tunnel junction are 0.5 pF and 0.5 mA, and the shunt resistance for each junction is 1Ω. The measured spectral density of the voltage noise S v contains a 1/f component that extends typically to 100 kHz or higher. When the 1/f component is subtracted out, the best energy sensitivity achieved to date is e 1 Hz = S v 2 L ( ∂ V ∂Φ ) 2 2 h , where ∂V/∂Φ is the transfer coefficient.