Circuit QED and sudden phase switching in a superconducting qubit array

Energy conversion from environmental fluctuations to coherent fields by Cooper-pair box quantum meta-materials

Electromagnetic waves propagating in open Cooper-pair boxes (CPBs) system is studied by using Maxwell-Bloch equations and Lindblad master equation. The results demonstrate an ensemble of CPBs as highly non-linear meta-material for electromagnetic waves. Incorporating the CPBs in a ring resonator or a Fabry-Perot cavity, one finds that: (1) With weak environmental couplings and CPBs in superconducting phase dominant regime, the non-linearity is enhanced and the system exhibits regular optical hysteresis. (2) With finite environmental couplings and CPBs in charge dominant regime, the Josephson effect and environmental effect can constructively interplay to produce a gain. In the later case, the electromagnetic field would be amplified by the CPB medium, indicating energy conversion from the environment to coherent fields mediated by CPBs.

Implementation of a quantum metamaterial using superconducting qubits

The key issue for the implementation of a metamaterial is to demonstrate the existence of collective modes corresponding to coherent oscillations of the meta-atoms. Atoms of natural materials interact with electromagnetic fields as quantum two-level systems. Artificial quantum two-level systems can be made, for example, using superconducting nonlinear resonators cooled down to their ground state. Here we perform an experiment in which 20 of these quantum meta-atoms, so-called flux qubits, are embedded into a microwave resonator. We observe the dispersive shift of the resonator frequency imposed by the qubit metamaterial and the collective resonant coupling of eight qubits. The realized prototype represents a mesoscopic limit of naturally occurring spin ensembles and as such we demonstrate the AC-Zeeman shift of a resonant qubit ensemble. The studied system constitutes the implementation of a basic quantum metamaterial in the sense that many artificial atoms are coupled collectively to the quantized mode of a photon field.

Efficient quantum algorithm for computing n-time correlation functions

We propose a method for computing n-time correlation functions of arbitrary spinorial, fermionic, and bosonic operators, consisting of an efficient quantum algorithm that encodes these correlations in an initially added ancillary qubit for probe and control tasks. For spinorial and fermionic systems, the reconstruction of arbitrary n-time correlation functions requires the measurement of two ancilla observables, while for bosonic variables time derivatives of the same observables are needed. Finally, we provide examples applicable to different quantum platforms in the frame of the linear response theory.

Quantum phases in circuit QED with a superconducting qubit array

Circuit QED on a chip has become a powerful platform for simulating complex many-body physics. In this report, we realize a Dicke-Ising model with an antiferromagnetic nearest-neighbor spin-spin interaction in circuit QED with a superconducting qubit array. We show that this system exhibits a competition between the collective spin-photon interaction and the antiferromagnetic nearest-neighbor spin-spin interaction, and then predict four quantum phases, including: a paramagnetic normal phase, an antiferromagnetic normal phase, a paramagnetic superradiant phase, and an antiferromagnetic superradiant phase. The antiferromagnetic normal phase and the antiferromagnetic superradiant phase are new phases in many-body quantum optics. In the antiferromagnetic superradiant phase, both the antiferromagnetic and superradiant orders can coexist, and thus the system possesses symmetry. Moreover, we find an unconventional photon signature in this phase. In future experiments, these predicted quantum phases could be distinguished by detecting both the mean-photon number and the magnetization.

Observing the nonequilibrium dynamics of the quantum transverse-field Ising chain in circuit QED

We show how a quantum Ising spin chain in a time-dependent transverse magnetic field can be simulated and experimentally probed in the framework of circuit QED with current technology. The proposed setup provides a new platform for observing the nonequilibrium dynamics of interacting many-body systems. We calculate its spectrum to offer a guideline for its initial experimental characterization. We demonstrate that quench dynamics and the propagation of localized excitations can be observed with the proposed setup and discuss further possible applications and modifications of this circuit QED quantum simulator.