Demonstration of macroscopic coherence and decoherence by adiabatic inversion, application to the SQUID

Abstract A procedure for demonstrating quantum coherence and measuring decoherence times between different states of a macroscopic system is described. We consider its application in particular to the fluxoid states of the SQUID. The procedure uses “adiabatic inversion” where a quantum state is inverted, in analogy to a spin “following” a slowly moving external field. For the SQUID, one macroscopic fluxoid state is smoothly transferred into its opposite by sweeping an applied flux. The success of the inversion depends on a well-defined quantum phase between the two macroscopic states, and it is blocked when phase coherence is lost. Hence varying the speed of the sweep relative to the decoherence time permits one to move from the quantum regime, where such a well-defined phase exists, to the classical regime where it is lost and the inversion is inhibited. Thus in observing when inversion has taken place or not as a function of sweep speed one determines the decoherence time. Estimates of some relevant parameters, particularly the relaxation time in the SQUID, indicate the scheme should be feasible.