Quantum back-action of variable-strength measurement

  1. M. Hatridge,
  2. S. Shankar,
  3. M. Mirrahimi,
  4. F. Schackert,
  5. K. Geerlings,
  6. T. Brecht,
  7. K. M. Sliwa,
  8. B. Abdo,
  9. L. Frunzio,
  10. S. M. Girvin,
  11. R. J. Schoelkopf,
  12. and M. H. Devoret
Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depends on the quantity which is measured. In a partial measurement performed by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrate this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures is observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.
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