Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback

摘要

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 measurements, 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 feedback. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity. 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 amplifier, which allows realtime 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 indefinitely. Such an ability permits the active suppression of deco-herence and enables a method of quantum error correction based on weak continuous measurements. Other applications include quantum state stabilization, entanglement generation using measurement, state purification and adaptive measurements.%通过对一个量子态进行“弱测量”，有可能降低其波函数的坍缩速度，以便能够逐渐获得关于该量子态的信息。这样的信息可被用来利用反馈连续跟踪和引导该量子态。这篇论文报告了对耦合到一个微波腔的一个超导量子位的量子反馈控制。该量子位发生能被无限加快、减慢或继续的相干振荡。这种能够主动抑制去相干的能力在量子纠错、量子态稳定化和纯化、纠缠的产生及适应性测量等方面会找到很多应用。