Implementation of a many-qubit Grover search by cavity QED

We explore the possibility of an N-qubit (N>3) Grover search in cavity QED, based on a fast operation of an N-qubit controlled phase-flip with atoms in resonance with the cavity mode. We demonstrate both analytically and numerically that our scheme can be achieved efficiently to find a marked state with high fidelity and high success probability. As an example, a ten-qubit Grover search is simulated specifically under the discussion of experimental feasibility and challenge. We argue that our scheme is applicable to the case involving an arbitrary number of qubits. As cavity decay is involved in our quantum trajectory treatment, we can analytically understand the implementation of a Grover search subject to dissipation, which will be very helpful for relevant experiments.     

Three‐Photon Polarization‐Spatial Hyperparallel Quantum Fredkin Gate Assisted by Diamond Nitrogen Vacancy Center in Optical Cavity

The construction of a near‐deterministic photonic hyperparallel quantum Fredkin (hyper‐Fredkin) gate is investigated for a three‐photon system with the optical property of a diamond nitrogen vacancy center embedded in an optical cavity (cavity‐NV center system). This hyper‐Fredkin gate can be used to perform double Fredkin gate operations on both the polarization and spatial‐mode degrees of freedom (DOFs) of a three‐photon system with a near‐unit success probability, compared with those on the double three‐photon systems in one DOF. In this proposal, the hybrid quantum logic gate operations are the key elements of the hyper‐Fredkin gate, and only two cavity‐NV center systems are required. Moreover, the possibility of constructing a high‐fidelity and high‐efficiency hyper‐Fredkin gate in the experimental environment of a cavity‐NV center system is discussed, which may be used to implement high‐fidelity photonic computational tasks in two DOFs with a high efficiency.     

Heralded Universal Quantum Gate and Entangler Assisted by Imperfect Double‐Sided Quantum‐Dot‐Microcavity Systems

Efficient ways are presented to accomplish photonic controlled‐phase‐flip gate and entangler with the assistance of imperfect double‐sided quantum‐dot‐microcavity systems, but without ancillary qubits. Compact quantum circuits for implementing entanglement swapping between photon pairs and electron pairs are then designed. Unity fidelities of the schemes can be achieved, and physical imperfections in the construction processes are detected by single‐photon detectors. Also, the efficiencies of the schemes can be further improved by repeating the operation processes when the undesired performances are detected. The evaluations show that the schemes are possible with current experiment parameters.     

Realization of quantum gates by Lyapunov control

We propose a Lyapunov control design to achieve specific (or a family of) unitary time-evolution operators, i.e., quantum gates in the Schrödinger picture by tracking control. Two examples are presented. In the first, we illustrate how to realize the Hadamard gate in a single-qubit system, while in the second, the controlled-NOT (CNOT) gate is implemented in two-qubit systems with the Ising and Heisenberg interactions. Furthermore, we demonstrate that the control can drive the time-evolution operator into the local equivalence class of the CNOT gate and the operator keeps in this class forever with the existence of Ising coupling.     

Efficient generation of two-dimensional cluster states in a cavity QED

We propose a scheme to achieve a kind of nontrivial multipartite pair-wise controlled phase operation in a cavity QED setup. The operation implemented is of geometrical nature and is not sensitive to the thermal state of the cavity. In particular, we have managed to avoid the conventional dispersive coupling so that high speed gate operation is achieved which is very important in view of decoherence. We show that this multipartite pair-wise controlled phase operation makes the generation of two-dimensional cluster states very efficient.     

Approximate Toffoli Gate Originated from a Single Resonant Interaction of Cavity Dissipation and Atomic Spontaneous Emission

We propose a potentially practical scheme to implement an approximate three-qubit Toffoli gate by a single resonant interaction in dissipative cavity QED in which the cavity mode decay and atomic spontaneous emission are considered. The scheme does not require two-qubit controlled-NOT gates but uses a three-qubit phase gate and two Hadamard gates, where the approximate phase gate can be implemented by only a single dissipative resonant interaction of atoms with the cavity mode. Discussions are made for the advantages and the experimental feasibility of our scheme.     

Phase‐Dependent Quantum Correlation in a Cavity‐Atom System

A scheme to manipulate quantum correlation between output lights of a cavity‐atom system by phase control is proposed. A driving‐field phase is introduced which has a similar value with that of building up quantum correlation in a Hanbury–Brown–Twiss setup. A closed‐loop phase is formed to improve quantum coherence by phase‐dependent electromagnetically induced transparency. The closed‐loop phase has been utilized to realize quantum correlation and even quantum entanglement in the atomic system of previous work. With these two phases, a steady and maximum quantum correlation has been obtained in the scheme here. Moreover, the maximum quantum correlation is free to decoherence of this cavity‐atom system. The study on field‐intensity correlation (quantum correlation) has potential applications on correlated imaging, image encryption transmission, and the improvement of noise resistance in a quantum network.     

以耦合量子点体系模块构造量子逻辑非门

我们使用处于居里温度附近的耦合量子点体系模块,并利用旋进磁场与其相互作用,构造一个二能级量子体系,使用驻波形式的电磁激励使其发生拉比振荡.由于该量子体系在统计力学上本质是一个纯粹系综,通过控制电磁激励作用时间的手段,我们可以实现一个输出信号易于被磁强计检测的量子逻辑非门.特别地,该量子逻辑门具备一定抗干扰性质.     

Selective Atom-Cavity Interaction Scheme for Quantum Controlled-NOT Gate Using Four-Level Atoms in Cavity QED System

We propose a scheme for realization a quantum Controlled-NOT gate operation using two four-level atoms through a selective atom cavity interaction in cavity quantum electrodynamics system. In our protocol, the quantum information is encoded on the stable ground states of the two atoms. During the interaction between atoms and single-mode vacuum cavity-field, the atomic spontaneous emission is negligible as the large atom-cavity detuning effectively suppresses the spontaneous decay of the atoms. The influences of the dissipation and the deviation of interaction time on fidelity and corresponding success probability of the quantum Controlled-NOT gate and the experimental feasibility of our proposal are also discussed.     

Accelerated and Noise‐Resistant Protocol of Dissipation‐Based Knill–Laflamme–Milburn State Generation with Lyapunov Control

A protocol to generate steady Knill–Laflamme–Milburn (KLM) states of two Rydberg atoms with dissipation and Lyapunov control is proposed. Herein, combining with the quantum Zeno dynamics and the Rydberg antiblockade, a unique and steady solution of the master equation of the system is obtained, which satisfies the definition of KLM states. Furthermore, to polish the convergence rate, well‐designed additional coherent control (ACC) fields with Lyapunov control are added into the traditional dissipation process. Numerical simulation reveals that the steady target state generation is prominently accelerated and the protocol is robust against the stochastic noise errors caused by laser fields. It is hoped that the work may provide an alternative protocol to polish the generation of entanglement.     

Efficient Atomic One-Qubit Phase Gate Realized by a Cavity QED and Identical Atoms System

We present a scheme to implement a one-qubit phase gate with a two-level atom crossing an optical cavity in which some identical atoms are trapped. One can conveniently acquire an arbitrary phase shift of the gate by properly choosing the number of atoms trapped in the cavity and the velocity of the atom crossing the cavity. The present scheme provides a very simple and efficient way for implementing one-qubit phase gate.     

Scheme for teleportation of an unknown atomic state via a cluster state in cavity QED

This paper proposes an experimentally feasible scheme for teleportation of an unknown two-atom entangled state, where a cluster state is used as the quantum channel. This scheme does not need any joint measurement. In addition, the successful probability and fidelity of teleportation can both reach 1.0. The current scheme can be realized within the current experimental technology.     

Scheme for teleportation of an unknown atomic state via a cluster state in cavity QED

This paper proposes an experimentally feasible scheme for teleportation of an unknown two-atom entangled state, where a cluster state is used as the quantum channel. This scheme does not need any joint measurement. In addition, the successful probability and fidelity of teleportation can both reach 1.0. The current scheme can be realized within the current experimental technology.     

Remote Implementation of a Fredkin Gate via Virtual Excitation of an Atom-Cavity-Fiber System

Simple operations and robust results are always of interest for any quantum tasks. Herein, a novel scheme is proposed for implementing a Fredkin gate via the virtual excitation of an atom-cavity-fiber system. The scheme is to control the nonlocal state-swap of two spatially separated target atoms according to the state of the control atom at hand. In the scheme, only the control atom at hand needs the laser to drive and the virtual excitation of the atom-cavity-fiber system effectively suppresses the decoherence. By numerical simulations, appreciated parameters are chosen and it is shown that the Fredkin gate can be implemented with high fidelity. Although the operation time error has slightly stronger influence on the fidelity than atom-cavity coupling strength error, the robustness of the scheme can be effectively improved against the operation time error by adopting Gaussian pulse to replace the constant pulse. In addition, the scheme can be generalized to implement alternative Fredkin gates by controlling the non-local state-swap of two remote atoms or of two remote and spatially separated atoms, which will be undoubtedly of benefit to the distributed quantum computation and remote quantum information processing.     

Teleportation of an Unknown Atomic Entangled State Without Any Joint Bell-state Measurement via a Cluster State

An experimentally feasible scheme for teleportation of an unknown two-atom entangled state is proposed. Our scheme uses a duster state as the quantum channel, where we do not need any joint Bell-state measurement. Moreover the successful probability and fidelity of teleportation can both reach 1.0. The current scheme can be realized within the current experimental technology.     

Control of chaos in permanent magnet synchronous motor by using optimal Lyapunov exponents placement

Permanent Magnet Synchronous Motor (PMSM) experiences chaotic behavior for a certain range of its parameters. In this case, since the performance of the PMSM degrades, the chaos should be eliminated. In this Letter, the control of the undesirable chaos in PMSM using Lyapunov exponents (LEs) placement is proposed that is also improved by choosing optimal locations of the LEs in the sense of predefined cost function. Moreover, in order to provide the physical realization of the method, nonlinear parameter estimator for the system is suggested. Finally, to show the effectiveness of the proposed methodology, the simulation results for applying this control strategy are provided.     

Implementation of quantum controlled phase gate and preparation of multiparticle entanglement in cavity QED

Schemes are presented for realizing quantum controlled phase gate and preparing an N-qubit W-like state, which are based on the large-detuned interaction among three-state atoms, dual-mode cavity and a classical pulse. In particular, a class of W states that can be used for perfect teleportation and superdense coding is generated by only one step. Compared with the previous schemes, cavity decay is largely suppressed because the cavity is only virtually excited and always in the vacuum state and the atomic spontaneous emission is strongly restrained due to a large atom-field detuning.     

Efficient scheme of quantum SWAP gate and multi-atom cluster state via cavity QED

In this paper, we propose a physical scheme to realize quantum SWAP gate by using a large-detuned single-mode cavity field and two identical Rydberg atoms. It is shown that the scheme can also be used to create multi-atom cluster state. During the interaction between atom and cavity, the cavity is only virtually excited and thus the scheme is insensitive to the cavity field states and cavity decay. With the help of our scheme it is very simple to prepare the $N$-atom cluster state with perfect fidelity and probability. The practical feasibility of this method is also discussed.     

Efficient scheme of quantum SWAP gate and multi-atom cluster state via cavity QED

In this paper, we propose a physical scheme to realize quantum SWAP gate by using a large-detuned single-mode cavity field and two identical Rydberg atoms. It is shown that the scheme can also be used to create multi-atom cluster state. During the interaction between atom and cavity, the cavity is only virtually excited and thus the scheme is insensitive to the cavity field states and cavity decay. With the help of our scheme it is very simple to prepare the N-atom cluster state with perfect fidelity and probability. The practical feasibility of this method is also discussed.     

Generation of an N-qubit phase gate via atom--cavity nonidentical coupling

A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l （related to atom-cavity coupling constants）, the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time （the gate time is much shorter than the atomic radiative lifetime and photon lifetime）. When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N→∞, the fidelity and success probability infinitely approach 1, but never exceed 1.