Measurement Scheme for the Macroscopic Quantum Coherence Experiment

An experimental strategy to test for macroscopic quantum coherence in a rf SQUID is described. The back action of broadband noise sources in a conventional dc SQUID magnetometer measurement system destroy the coherent state. A novel measurement scheme is suggested in which the rf SQUID is weakly coupled to an intermediate bistable system. This system consists of a pair of dc SQUIDs biased below threshold. The combined rf and dc threshold SQUID system is described by a single potential. The form of this potential is varied by changes in the external bias to the dc SQUIDs at the moment in which the measurement is to occur. This causes a transition in the dc SQUID system which serves to freeze the coherent oscillations. Amplification by a conventional SQUID linear amplifier chain then follows. © 1987 The Japan Society of Applied Physics.