Abstract
Continuous feedback control — monitoring a system and adjusting its dynamics — is widely used to keep systems 'on track'. This approach has now been used to maintain the cycling of a quantum bit almost indefinitely. See Letter p.77 By performing weak measurements of a quantum state, it is possible to slow the rate of collapse of its wavefunction, so that information about the quantum state can be gradually acquired. Such information can be used to continuously track and steer the quantum state using feedback. This paper reports quantum feedback control of a superconducting quantum bit (qubit) coupled to a microwave cavity. The qubit undergoes coherent oscillations that can be made to speed up, slow down or persist indefinitely. This ability to actively suppress decoherence could find many applications in quantum error correction, quantum-state stabilization and purification, entanglement generation and adaptive measurements.
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