Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands.
Nature. 2013 Oct 17;502(7471):350-4. doi: 10.1038/nature12513.
The stochastic evolution of quantum systems during measurement is arguably the most enigmatic feature of quantum mechanics. Measuring a quantum system typically steers it towards a classical state, destroying the coherence of an initial quantum superposition and the entanglement with other quantum systems. Remarkably, the measurement of a shared property between non-interacting quantum systems can generate entanglement, starting from an uncorrelated state. Of special interest in quantum computing is the parity measurement, which projects the state of multiple qubits (quantum bits) to a state with an even or odd number of excited qubits. A parity meter must discern the two qubit-excitation parities with high fidelity while preserving coherence between same-parity states. Despite numerous proposals for atomic, semiconducting and superconducting qubits, realizing a parity meter that creates entanglement for both even and odd measurement results has remained an outstanding challenge. Here we perform a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a three-dimensional circuit quantum electrodynamics architecture and phase-sensitive parametric amplification. Using postselection, we produce entanglement by parity measurement reaching 88 per cent fidelity to the closest Bell state. Incorporating the parity meter in a feedback-control loop, we transform the entanglement generation from probabilistic to fully deterministic, achieving 66 per cent fidelity to a target Bell state on demand. These realizations of a parity meter and a feedback-enabled deterministic measurement protocol provide key ingredients for active quantum error correction in the solid state.
量子系统在测量过程中的随机演化可以说是量子力学最神秘的特征。测量一个量子系统通常会引导它走向经典状态，破坏初始量子叠加的相干性和与其他量子系统的纠缠。值得注意的是，即使是非相互作用的量子系统之间共享的属性进行测量，也可以从非关联状态开始产生纠缠。在量子计算中特别感兴趣的是奇偶校验测量，它将多个量子位（量子位）的状态投影到具有偶数或奇数激发量子位的状态。奇偶校验仪必须以高保真度区分两个量子位激发的奇偶校验，同时保持同奇偶校验状态之间的相干性。尽管有许多关于原子、半导体和超导量子位的提案，但实现一种奇偶校验仪，为偶数和奇数测量结果都能产生纠缠，仍然是一个悬而未决的挑战。在这里，我们使用三维电路量子电动力学架构中的腔和相位敏感参量放大来进行两个超导量子位的时间分辨、连续奇偶校验测量。通过后选择，我们通过奇偶校验测量产生纠缠，达到最接近贝尔态的 88%保真度。在反馈控制环中包含奇偶校验仪，我们将纠缠的产生从概率转变为完全确定，按需达到目标贝尔态的 66%保真度。奇偶校验仪和反馈使能的确定性测量协议的这些实现为固态中的主动量子纠错提供了关键成分。
