Coherent State Evolution in a Superconducting Qubit from Partial-Collapse Measurement

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. T he wave-particle duality in quantum mechanics originates from two distinct ways in which a quantum state may change: a linear (unitary) evolution according to the SchrPdinger wave equation and a non-linear (projective or Bcollapse[) evolution due to measurement (1). In recent years, i