Feng M, Zhong Y P, Liu T, Yan L L, Yang W L, Twamley J, Wang H
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China.
Department of Physics, Zhejiang University, Hangzhou 310027, China.
Nat Commun. 2015 May 14;6:7111. doi: 10.1038/ncomms8111.
Quantum phase transitions play an important role in many-body systems and have been a research focus in conventional condensed-matter physics over the past few decades. Artificial atoms, such as superconducting qubits that can be individually manipulated, provide a new paradigm of realising and exploring quantum phase transitions by engineering an on-chip quantum simulator. Here we demonstrate experimentally the quantum critical behaviour in a highly controllable superconducting circuit, consisting of four qubits coupled to a common resonator mode. By off-resonantly driving the system to renormalize the critical spin-field coupling strength, we have observed a four-qubit nonequilibrium quantum phase transition in a dynamical manner; that is, we sweep the critical coupling strength over time and monitor the four-qubit scaled moments for a signature of a structural change of the system's eigenstates. Our observation of the nonequilibrium quantum phase transition, which is in good agreement with the driven Tavis-Cummings theory under decoherence, offers new experimental approaches towards exploring quantum phase transition-related science, such as scaling behaviours, parity breaking and long-range quantum correlations.
量子相变在多体系统中起着重要作用，并且在过去几十年一直是传统凝聚态物理的研究重点。人工原子，例如可单独操控的超导量子比特，通过构建片上量子模拟器为实现和探索量子相变提供了一种新的范例。在此，我们通过实验证明了在一个高度可控的超导电路中的量子临界行为，该电路由四个耦合到一个公共谐振器模式的量子比特组成。通过非共振驱动系统来重整临界自旋 - 场耦合强度，我们以动态方式观测到了一个四量子比特非平衡量子相变；也就是说，我们随时间扫描临界耦合强度，并监测四量子比特的标度矩以寻找系统本征态结构变化的特征。我们对非平衡量子相变的观测结果与退相干下的驱动塔维斯 - 卡明斯理论高度吻合，为探索与量子相变相关的科学，如标度行为、宇称破缺和长程量子关联，提供了新的实验方法。
