Measurement of quantum superposition states of a single spin with a superconducting resonator

Abstract

We propose a method of measuring quantum superposition states of a single spin with a superconducting resonator. Following the von Neumann's postulate of the reduction, it seems that we cannot obtain more information about the superposition states other than the eigenvalue by a single observation. However, the indirect quantum measurement has the possibility to measure the ratio of components of the superposition states by transferring the coherence of the spin to high energy macroscopic states of a superconducting harmonic resonator. This transfer enables quantum amplification of the coherence, and the maximum amplitude of the induced current is approximately proportional to the square root of the number of quanta in the initial states. In this work we also discuss the cases containing more than one spin and classical mixed states.