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-neighborspin-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 $Z_{2}^{z}\otimes Z_{2}$\ 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.
The generation and control of quantum states of spatially-separated qubits distributed in different cavities constitute fundamental tasks in cavity quantum electrodynamics. An interestingquestion in this context is how to prepare entanglement and realize quantum information transfer between qubits located at different cavities, which are important in large-scale quantum information processing. In this paper, we consider a physical system consisting of two cavities and three qubits. Two of the qubits are placed in two different cavities while the remaining one acts as a coupler, which is used to connect the two cavities. We propose an approach for generating quantum entanglement and implementing quantum information transfer between the two spatially-separated intercavity qubits. The quantum operations involved in this proposal are performed by a virtual photon process, and thus the cavity decay is greatly suppressed during the operations. In addition, to complete the present tasks, only one coupler qubit and one operation step are needed. Moreover, there is no need of applying classical pulses, so that the engineering complexity is much reduced and the operation procedure is greatly simplified. Finally, our numerical results illustrate that high-fidelity implementation of this proposal using superconducting phase qubits and one-dimenstion transmision line resonators is feasible for current circuit QED implementations. This proposal can also be applied to other types of superconducting qubits, including flux and charge qubits.
We consider the feedback stabilization of Rabi oscillations in a superconducting qubit which is coupled to a microwave readout cavity. The signal is readout by homodyne detection ofthe in-phase quadrature amplitude of the weak measurement output. By multiplying the time-delayed Rabi reference, one can extract the signal, with maximum signal-to-noise ratio, from the noise. We further track and stabilize the Rabi oscillations by using Lyapunov feedback control to properly adjust the input Rabi drives. Theoretical and simulation results illustrate the effectiveness of the proposed control law.
We explore theoretically photon-mediated transport processes in a hybrid circuit-QED structure consisting of two double quantum dots coupled to a common microwave cavity. Under suitableresonance conditions, electron transport in one double quantum dot is facilitated by the transport in the other dot via photon-mediated processes through the cavity. We calculate the average current in the quantum dots, the mean cavity photon occupation, and the current cross-correlations using a recursive perturbation scheme that allows us to include the influence of the cavity order-by-order in the couplings between the cavity and the quantum dot systems. Within this framework we can clearly identify the photon-mediated processes in the transport.
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of theflux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.
We propose a simple feedback-control scheme for adiabatic quantum computation
with superconducting flux qubits. The proposed method makes use of existing
on-chip hardware to monitorthe ground-state curvature, which is then used to
control the computation speed to maximize the success probability. We show that
this scheme can provide a polynomial speed-up in performance and that it is
possible to choose a suitable set of feedback-control parameters for an
arbitrary problem Hamiltonian.
We propose an experimentally realizable hybrid quantum circuit for achieving
a strong coupling between a spin ensemble and a transmission-line resonator via
a superconducting flux qubitused as a data bus. The resulting coupling can be
used to transfer quantum information between the spin ensemble and the
resonator. More importantly, in contrast to the direct coupling without a data
bus, our approach requires far less spins to achieve a strong coupling between
the spin ensemble and the resonator (e.g., 3 to 4 orders of magnitude less).
This drastic reduction of the number of spins in the ensemble can greatly
improve the quantum coherence of the spin ensemble. This proposed hybrid
quantum circuit could enable a long-time quantum memory when storing
information in the spin ensemble.
Hybrid quantum circuits combine two or more physical systems, with the goal
of harnessing the advantages and strengths of the different systems in order to
better explore new phenomenaand potentially bring about novel quantum
technologies. This article presents a brief overview of the progress achieved
so far in the field of hybrid circuits involving atoms, spins and solid-state
devices (including superconducting and nanomechanical systems). We discuss how
these circuits combine elements from atomic physics, quantum optics, condensed
matter physics, and nanoscience, and we present different possible approaches
for integrating various systems into a single circuit. In particular, hybrid
quantum circuits can be fabricated on a chip, facilitating their future
scalability, which is crucial for building future quantum technologies,
including quantum detectors, simulators and computers.
A strong photon-photon nonlinear interaction is a necessary condition for
photon blockade. Moreover, this nonlinearity can also result a bistable
behavior in the cavity field. We analyzethe relation between detecting field
and photon blockade in a superconducting circuit QED system, and show that the
photon blockade cannot occur when the detecting field is in the bistable
regime. We further demonstrate that the photon transmission through such system
can be controlled (from photon blockade to transparency) by the detecting
field. Numerical simulations show that our proposal is experimentally
realizable with current technology.
We show a systematic construction for implementing general measurements on a
single qubit, including both strong (or projection) and weak measurements. We
mainly focus on linear opticalqubits. The present approach is composed of
simple and feasible elements, i.e., beam splitters, wave plates, and polarizing
beam splitters. We show how the parameters characterizing the measurement
operators are controlled by the linear optical elements. We also propose a
method for the implementation of general measurements in solid-state qubits.