Measuring the size of a quantum superposition of many-body states

Marquardt, Florian; Abel, Benjamin; von Delft, Jan

doi:10.1103/PhysRevA.78.012109

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Measuring the size of a quantum superposition of many-body states

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Marquardt, F., Abel, B., & von Delft, J. (2008). Measuring the size of a quantum superposition of many-body states. Physical Review A, 78(1): 012109. doi:10.1103/PhysRevA.78.012109.

Cite as: https://hdl.handle.net/21.11116/0000-0001-D8BA-A

Abstract

We propose a measure for the "size" of a quantum superposition of two many-body states with (supposedly) macroscopically distinct properties by counting how many single-particle operations are needed to map one state onto the other. This definition gives sensible results for simple, analytically tractable cases and is consistent with a previous definition restricted to Greenberger-Horne-Zeilinger-like states. We apply our measure to the experimentally relevant, nontrivial example of a superconducting three-junction flux qubit put into a superposition of left- and right-circulating supercurrent states, and we find the size of this superposition to be surprisingly small.