Abstract
Nonequilibrium phase transitions exist in damped-driven open quantum systems when the continuous tuning of an external parameter leads to a transition between two robust steady states. In second-order transitions this change is abrupt at a critical point, whereas in first-order transitions the two phases can coexist in a critical hysteresis domain. Here, we report the observation of a first-order dissipative quantum phase transition in a driven circuit quantum electrodynamics system. It takes place when the photon blockade of the driven cavity-atom system is broken by increasing the drive power. The observed experimental signature is a bimodal phase space distribution with varying weights controlled by the drive strength. Our measurements show an improved stabilization of the classical attractors up to the millisecond range when the size of the quantum system is increased from one to three artificial atoms. The formation of such robust pointer states could be used for new quantum measurement schemes or to investigate multiphoton phases of finite-size, nonlinear, open quantum systems.
- Received 13 August 2016
DOI:https://doi.org/10.1103/PhysRevX.7.011012
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Phase transitions, such as the change of liquid water into ice, help elucidate the complex behavior of systems composed of many particles and occur in all areas of physics. Recently, theorists have predicted that a cavity containing only a single atom should transition from opaque to transparent when the input photon flux reaches a critical number. And just as water and ice can coexist at the melting point temperature, the cavity was predicted to be both opaque and transparent close to the critical point, stochastically switching between the two states. This coexistence is a hallmark for a so-called first-order phase transition, which has been observed for the first time in a dissipative quantum system.
Our setup consists of a microchip with a superconducting microwave resonator acting as the cavity and a few superconducting qubits—the quantum equivalent of a digital bit—acting as the atoms. We cool the chip to 0.01 K and send a coherent and continuous microwave tone to the input of the resonator on the chip. On the output side we amplify and detect the transmitted tone. For certain input powers, we detect a real-time telegraph signal between full transmission and zero transmission. Interestingly, the lifetimes of the two states have been observed to be much longer than the coherence times of the individual qubits and the cavity, which points to a significant stabilization of the two phases.
The high photon number limit in systems like ours is relatively poorly understood, and we hope to shed some light on the underlying physics. Our techniques can also be extended to multicavity systems and lattices forming artificial crystals of light. Such setups could be used to simulate certain quantum systems orders of magnitude faster than numerical simulations on a supercomputer cluster.