Quantum Nanoelectronics Laboratory, Department of Physics, University of California, Berkeley, California 94720, USA.
Center for Quantum Coherent Science, University of California, Berkeley, California 94720, USA.
Nature. 2016 Oct 27;538(7626):491-494. doi: 10.1038/nature19762. Epub 2016 Oct 5.
In quantum mechanics, measurements cause wavefunction collapse that yields precise outcomes, whereas for non-commuting observables such as position and momentum Heisenberg's uncertainty principle limits the intrinsic precision of a state. Although theoretical work has demonstrated that it should be possible to perform simultaneous non-commuting measurements and has revealed the limits on measurement outcomes, only recently has the dynamics of the quantum state been discussed. To realize this unexplored regime, we simultaneously apply two continuous quantum non-demolition probes of non-commuting observables to a superconducting qubit. We implement multiple readout channels by coupling the qubit to multiple modes of a cavity. To control the measurement observables, we implement a 'single quadrature' measurement by driving the qubit and applying cavity sidebands with a relative phase that sets the observable. Here, we use this approach to show that the uncertainty principle governs the dynamics of the wavefunction by enforcing a lower bound on the measurement-induced disturbance. Consequently, as we transition from measuring identical to measuring non-commuting observables, the dynamics make a smooth transition from standard wavefunction collapse to localized persistent diffusion and then to isotropic persistent diffusion. Although the evolution of the state differs markedly from that of a conventional measurement, information about both non-commuting observables is extracted by keeping track of the time ordering of the measurement record, enabling quantum state tomography without alternating measurements. Our work creates novel capabilities for quantum control, including rapid state purification, adaptive measurement, measurement-based state steering and continuous quantum error correction. As physical systems often interact continuously with their environment via non-commuting degrees of freedom, our work offers a way to study how notions of contemporary quantum foundations arise in such settings.
在量子力学中，测量会导致波函数坍缩，从而产生精确的结果，而对于非对易的可观测量，如位置和动量，海森堡不确定性原理限制了态的固有精度。尽管理论工作已经证明，应该有可能进行同时的非对易测量，并揭示了测量结果的限制，但直到最近才讨论了量子态的动力学。为了实现这个未被探索的领域，我们同时应用两个连续的量子非破坏性探针来测量非对易可观测量，这些探针作用于超导量子比特上。我们通过将量子比特耦合到腔的多个模式，实现了多个读出通道。为了控制测量可观测量，我们通过驱动量子比特并施加具有设定可观测量的相对相位的腔边带来实现“单正交测量”。在这里，我们使用这种方法表明，不确定性原理通过对测量引起的干扰施加下限来控制波函数的动力学。因此，当我们从测量相同的可观测量转变为测量非对易的可观测量时，动力学从标准的波函数坍缩平稳地转变为局域的持续扩散，然后转变为各向同性的持续扩散。尽管状态的演化与传统测量有显著差异，但通过跟踪测量记录的时间顺序，提取了关于两个非对易可观测量的信息，从而无需交替测量即可进行量子态层析成像。我们的工作为量子控制创造了新的能力，包括快速状态纯化、自适应测量、基于测量的状态控制和连续量子纠错。由于物理系统通常通过非对易自由度与环境连续相互作用，我们的工作提供了一种研究在这种情况下当代量子基础概念如何出现的方法。
