Reversal of the weak measurement of a quantum state in a superconducting phase qubit

Phys Rev Lett. 2008 Nov 14;101(20):200401. doi: 10.1103/PhysRevLett.101.200401. Epub 2008 Nov 10.

Abstract

We demonstrate in a superconducting qubit the conditional recovery (uncollapsing) of a quantum state after a partial-collapse measurement. A weak measurement extracts information and results in a nonunitary transformation of the qubit state. However, by adding a rotation and a second partial measurement with the same strength, we erase the extracted information, canceling the effect of both measurements. The fidelity of the state recovery is measured using quantum process tomography and found to be above 70% for partial-collapse strength less than 0.6.