The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this ‘instantaneous’ process can be stretched using weak measurements1,2, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback3,4,5,6,7,8. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity9. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier10,11, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely12. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements13,14. Other applications include quantum state stabilization4,7,15, entanglement generation using measurement16, state purification17 and adaptive measurements18,19.
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We thank M. Sarovar for several discussions and Z. Minev for assistance with numerical simulations. This research was supported in part (R.V., C.M. and I.S.) by the US Army Research Office (W911NF-11-1-0029) and the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), through the Army Research Office (K.W.M., S.J.W. and A.N.K.). All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI or the US government. D.H.S. acknowledges support from a Hertz Foundation Fellowship endowed by Big George Ventures. A.N.K. also acknowledges funding from an ARO MURI.
The authors declare no competing financial interests.
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Vijay, R., Macklin, C., Slichter, D. et al. Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback. Nature 490, 77–80 (2012) doi:10.1038/nature11505
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