Xiao-Qiang Shao

One-step implementation of the genuine Fredkin gate in high-Q coupled three-cavity arrays

We present two efficient methods for implementing the Fredkin gate with atoms separately trapped in an array of three high-Q coupled cavities. The first proposal is based on the resonant dynamics, which leads to a fast resonant interaction in a certain subspace while leaving others unchanged, and the second one utilizes a dispersive interaction such that an effective long-distance dipole–dipole interaction between two distributed target qubits is achieved by a virtually excited process. Both schemes can achieve the standard form of the Fredkin gate in a single step without any subsequent single-qubit operation. The effects of decoherence on the performance of the gate are also analyzed in virtue of the master equation, and strictly numerical simulation reveals that the average fidelity of the quantum gate is high.

Fast synthesis of the Fredkin gate via quantum Zeno dynamics

We propose a scheme for fast synthesizing the Fredkin gate with rf SQUID qubits. This scheme utilizes the quantum Zeno dynamics induced by continuous couplings and the non-identical couplings between SQUIDs and superconducting cavity. The effects of decoherence on the performance for the gate are analyzed in virtue of master equation and non-unitary evolution with full Hamiltonian. The strictly numerical simulation shows that the fidelity of this Fredkin gate is relatively high corresponding to current typical experimental parameters. Furthermore, an equivalent physical model is also constructed in an array of coupled cavities.

Robust Toffoli gate originating from Stark shifts

A method for synthesizing the Toffoli gate is proposed based solely on the Stark shifts of three superconducting quantum interference devices (SQUIDs). This scheme is robust against the effect of decoherence, since it operates with no excitation of SQUIDs and the coplanar waveguide cavity. The obtained fidelity of the Toffoli gate is high, corresponding to the current typical experimental parameters, and an equivalent physical model for conveniently addressing qubits is also constructed in the coupled-cavity array system.

Entangled state fusion with Rydberg atoms

We propose a scheme for preparation of large-scale entangled GHZ states and W states with neutral Rydberg atoms. The scheme mainly depends on Rydberg antiblockade effect, i.e., as the Rydberg–Rydberg interaction strength and the detuning between the atom transition frequency and the classical laser frequency satisfies some certain conditions, the effective Rabi oscillation between the two ground states and the two excitation Rydberg states would be generated. The prominent advantage is that both two multiparticle GHZ states and two multiparticle W states can be fused in this model, especially the success probability for fusion of GHZ states can reach unit. In addition, the imperfections induced by the spontaneous emission is also discussed through numerical simulation.

Fusing atomic W states via quantum Zeno dynamics

We propose a scheme for preparation of large-scale entangled W states based on the fusion mechanism via quantum Zeno dynamics. By sending two atoms belonging to an n-atom W state and an m-atom W state, respectively, into a vacuum cavity (or two separate cavities), we may obtain a (n + m − 2)-atom W state via detecting the two-atom state after interaction. The present scheme is robust against both spontaneous emission of atoms and decay of cavity, and the feasibility analysis indicates that it can also be realized in experiment.

One-step achievement of robust multipartite Greenberger–Horne–Zeilinger state and controlled-phase gate via Rydberg interaction

We present a proposal for generation of a robust tripartite Greenberger–Horne–Zeilinger state among three individual neutral Rydberg atoms. By modulating the relation between two-photon detuning and Rydberg interaction strength Uij(r), an effective Raman coupling is obtained between the hyperfine ground states |F=2,M=2〉 of three Rb87 atoms and the Rydberg states |rrr〉 via the third-order perturbation theory. This method is also capable of implementing a three-qubit controlled-phase gate with each qubit encoded into the hyperfine ground states |F=1,M=1〉 and |F=2,M=2〉. As an extension, we generalize our scheme to the case of the multipartite GHZ state and quantum gate in virtue of high-order perturbation theory.

Robust preparation of four-qubit decoherence-free states for superconducting quantum interference devices against collective amplitude damping

Based on the quantum Zeno dynamics, we present an approach for deterministic preparation of arbitrary four-qubit decoherence-free state of superconducting quantum interference devices with respective to collective amplitude damping in a decoherence-free way, namely, not only the form of the target state is free of decoherence, but also the whole process for preparation. The operation is fast and convenient since we only need to manipulate three weak laser pulses sequentially. Other decoherence effects such as cavity decay and the spontaneous emission of qubits are also taken into account in virtue of master equation, and the strictly numerical simulation signifies the final fidelity is high corresponding to the current experimental technology.