Ai-Dong Zhu

Simple implementation of discrete quantum Fourier transform via cavity quantum electrodynamics

We propose a scheme for implementing discrete quantum Fourier transform in cavity quantum electrodynamics (QED). The experimental implementation is appealingly simple because the combined effect of the complex controlled-Rk gate and SWAP gate operations required for implementing discrete quantum Fourier transform in the naive quantum circuit is replaced by the one-step CRkS gate, which can be directly implemented via atom–cavity interaction with the assistance of classical fields. We propose the detailed experimental procedure and analyze the experimental feasibility. The experimental implementation of the scheme would show the full power of quantum algorithm and would open wide prospects for more complicated quantum computation with atoms in QED.

Local conversion of four Einstein-Podolsky-Rosen photon pairs into four-photon polarization-entangled decoherence-free states with non-photon-number-resolving detectors

We propose a linear-optics-based scheme for local conversion of four Einstein-Podolsky-Rosen photon pairs distributed among five parties into four-photon polarization-entangled decoherence-free states using local operations and classical communication. The proposed setup involves simple linear optical elements and non-photon-number-resolving detectors that can only distinguish between the presence and absence of photons, and no information on the exact number of photons can be obtained. This greatly simplifies the experimental realization for linear optical quantum computation and quantum information processing.

Deterministic CNOT gate and entanglement swapping for photonic qubits using a quantum-dot spin in a double-sided optical microcavity

Abstract Abstract We propose a deterministic and scalable scheme to construct a two-qubit controlled-NOT (CNOT) gate and realize entanglement swapping between photonic qubits using a quantum-dot (QD) spin in a double-sided optical microcavity. The scheme is based on spin selective photon reflection from the cavity and can be achieved in a nondestructive and heralded way. We assess the feasibility of the scheme and show that the scheme can work in both the weak coupling and the strong coupling regimes. The scheme opens promising perspectives for long-distance photonic quantum communication and distributed quantum information processing.

One-step implementation of a multiqubit phase gate with one control qubit and multiple target qubits in coupled cavities.

We propose a one-step scheme to implement a multiqubit controlled phase gate with one qubit simultaneously controlling multiple qubits with three-level atoms at distant nodes in coupled cavity arrays. Selective qubit-qubit couplings are achieved by adiabatically eliminating the atomic excited states and photonic states, and the required phase shifts between the control qubit and any target qubit can be realized through suitable choices of the parameters of the external fields. Moreover, the effective model is robust against decoherence because neither the atoms nor the field modes are excited during the gate operation, leading to a useful step toward scalable quantum computing networks.

Deterministic controlled-phase gate and preparation of cluster states via singly charged quantum dots in cavity quantum electrodynamics

A controlled-phase gate is constructed deterministically in a quantum dot (QD)-cavity system based on Faraday rotation via singly charged QDs in the strong-coupling regime. The fidelity of the gate can reach relatively high values even if cavity decay and leakage are considered. Furthermore, we present two schemes for implementing N-qubit cluster states for electron spins and photons, respectively, by exploiting this gate. The schemes are very simple and can be easily realized in the current experiment.

Physical optimization of quantum error correction circuits with spatially separated quantum dot spins.

We propose an efficient protocol for optimizing the physical implementation of three-qubit quantum error correction with spatially separated quantum dot spins via virtual-photon-induced process. In the protocol, each quantum dot is trapped in an individual cavity and each two cavities are connected by an optical fiber. We propose the optimal quantum circuits and describe the physical implementation for correcting both the bit flip and phase flip errors by applying a series of one-bit unitary rotation gates and two-bit quantum iSWAP gates that are produced by the long-range interaction between two distributed quantum dot spins mediated by the vacuum fields of the fiber and cavity. The protocol opens promising perspectives for long distance quantum communication and distributed quantum computation networks.

Fast and effective implementation of discrete quantum Fourier transform via virtual-photon-induced process in separate cavities

We present a fast and effective scheme to implement the multiqubit discrete quantum Fourier transform (DQFT) for distant atoms trapped in separate cavities connected by optical fibers via a virtual-photon-induced process. The effective coupling between two distributed atoms is achieved without exciting and transporting photons through the optical fiber, and the gate operation is robust against the decoherence effect when the thermal photons in the environment are negligible. The implementation of the scheme is appealingly simple because the complex combination of quantum gate operations, which act on each two qubits in the rearranged DQFT circuit, is achieved only in one step through the interaction controlled by optical switches between two adjacent cavities. The scheme opens promising perspectives for scalable quantum communication networks and distributed quantum computation.

Steady-state mechanical squeezing in a double-cavity optomechanical system

We study the physical properties of double-cavity optomechanical system in which the mechanical resonator interacts with one of the coupled cavities and another cavity is used as an auxiliary cavity. The model can be expected to achieve the strong optomechanical coupling strength and overcome the optomechanical cavity decay, simultaneously. Through the coherent auxiliary cavity interferences, the steady-state squeezing of mechanical resonator can be generated in highly unresolved sideband regime. The validity of the scheme is assessed by numerical simulation and theoretical analysis of the steady-state variance of the mechanical displacement quadrature. The scheme provides a platform for the mechanical squeezing beyond the resolved sideband limit and solves the restricted experimental bounds at present.

Robust entanglement between a movable mirror and atomic ensemble and entanglement transfer in coupled optomechanical system

We propose a scheme for the creation of robust entanglement between a movable mirror and atomic ensemble at the macroscopic level in coupled optomechanical system. We numerically simulate the degree of entanglement of the bipartite macroscopic entanglement and show that it depends on the coupling strength between the cavities and is robust with respect to the certain environment temperature. Inspiringly and surprisingly, according to the reported relation between the mechanical damping rate and the mechanical frequency of the movable mirror, the numerical simulation result shows that such bipartite macroscopic entanglement persists for environment temperature up to 170 K, which breaks the liquid nitrogen cooling and liquid helium cooling and largely lowers down the experiment cost. We also investigate the entanglement transfer based on this coupled system. The scheme can be used for the realization of quantum memories for continuous variable quantum information processing and quantum-limited displacement measurements.

Effective W-state fusion strategies for electronic and photonic qubits via the quantum-dot-microcavity coupled system

We propose effective fusion schemes for stationary electronic W state and flying photonic W state, respectively, by using the quantum-dot-microcavity coupled system. The present schemes can fuse a n-qubit W state and a m-qubit W state to a (m + n − 1)-qubit W state, that is, these schemes can be used to not only create large W state with small ones, but also to prepare 3-qubit W states with Bell states. The schemes are based on the optical selection rules and the transmission and reflection rules of the cavity and can be achieved with high probability. We evaluate the effect of experimental imperfections and the feasibility of the schemes, which shows that the present schemes can be realized with high fidelity in both the weak coupling and the strong coupling regimes. These schemes may be meaningful for the large-scale solid-state-based quantum computation and the photon-qubit-based quantum communication.