Coherent State Evolution in a Superconducting Qubit from Partial-Collapse Measurement
N. Katz,1
M. Ansmann,1
Radoslaw C. Bialczak,1
Erik Lucero,1
R. McDermott,1
Matthew Neeley,1
Matthias Steffen,1
E. M. Weig,1
A. N. Cleland,1
John M. Martinis,1*
A. N. Korotkov2
Measurement is one of the fundamental building blocks of quantum-information
processing systems. Partial measurement, where full wavefunction
collapse is not the only outcome, provides a detailed test of
the measurement process. We introduce quantum-state tomography
in a superconducting qubit that exhibits high-fidelity single-shot
measurement. For the two probabilistic outcomes of partial measurement,
we find either a full collapse or a coherent yet nonunitary
evolution of the state. This latter behavior explicitly confirms
modern quantum-measurement theory and may prove important for
error-correction algorithms in quantum computation.
2 Department of Electrical Engineering, University of California, Riverside, CA 92521, USA.
* To whom correspondence should be addressed. E-mail: martinis{at}physics.ucsb.edu
