Robust scheme to generate N-atom W state in two distant cavities

We propose a scheme to realize W states for N-atoms trapped in two distant cavities connected by an optical fiber. In the scheme, the cavity modes and fiber mode are not excited during the process. The quantum information is encoded in two degenerate ground states, so the atom's spontaneous emission can be omitted approximately. Moreover, the operation speed increases with the number of the atoms without a limitation and thus the scheme is extremely robust against decoherence.     

Transferring an N-atom state between two distant cavities via an optical fiber

This paper proposes a scheme for transferring an N-atom state between two distant cavities via an optical fiber.The scheme is based on adiabatic passage along a dark state.In the scheme,all the atoms are always in ground state,the field mode of the fiber remains in vacuum state,and the field mode of the cavities being excited can be negligible under certain conditions.Therefore,the scheme is very robust against decoherence.The successful probability of implementing the quantum state transfer increases with increasing number of atoms.Furthermore,the interaction time does not need to be accurately adjusted as long as the adiabaticity condition is fulfilled.     

Arbitrary state controlled-unitary gate between two remote atomic qubits via adiabatic passage

We generalize the scheme of Lacour et al. [X. Lacour, N. Sangouard, S. Guerin, H.R. Jauslin, Phys. Rev. A 73 (2006) 042321] to the case of nonlocal qubits, which makes the resultant gate suitable for distributed quantum computation. In our scheme, two remote atomic qubits are separately trapped in two distant cavities connected by an optical fiber. Based on adiabatic passage, our scheme is immune to the decoherence due to spontaneous emission and to photon decay from the cavity modes and the fiber mode. Moreover, our scheme can work robustly beyond the Lamb–Dicke limit. It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.98.     

通过绝热过程制备W态

提出一种方案制备W态,方案基于暗态绝热过程。制备过程中,所有原子都处于基态,光纤模保持在真空态,在一定条件下可以忽略腔场激发,因此,方案非常抗消相干。方案的另一个优点是：只要满足绝热条件,不必要精确调节相互作用时间。方案成功的几率随原子数的增加而增加。     

通过绝热过程制备W态

提出一种方案制备W态,方案基于暗态绝热过程.制备过程中,所有原子都处于基态,光纤模保持在真空态,在一定条件下可以忽略腔场激发,因此,方案非常抗消相干.方案的另一个优点是:只要满足绝热条件,不必要精确调节相互作用时间.方案成功的几率随原子数的增加而增加.     

Generation of NOON Sates for Two Atomic Samples Trapped in Two Distant Cavities

A scheme is proposed for generating NOON states for two atomic samples trapped in two distant cavities connected by a third cavity and optical fibers. In the scheme, all the atoms are always populated in the two degenerate ground states, so the atoms’ spontaneous emission can be omitted approximately. During the operation neither the cavity modes nor fiber modes are excited, which is important in view of decoherence. The scheme does not include projective measurement and the NOON state is generated deterministically.     

Robust Scheme for Entangling Two Distant Atoms

A robust scheme is presented for realizing entangled states for two atoms trapped in separate cavities connected by an optical fiber. The first atom is initially in a superposition of the excited state and an auxiliary ground state not coupled to the first cavity, while the second one is initially in the ground state coupled to the second cavity. The scheme involves two atom-cavity-fiber interactions accompanied by the monitoring of the cavity decay and atomic spontaneous emission. The two atoms evolve to an entangled state through exchanging an excitation after the first interaction. The states with the excitation failing to be transferred are eliminated when a photon is detected during the second interaction. Therefore, the scheme is insensitive to the decoherence effect and detection inefficiency.     

Entangled atomic state generation via adiabatic evolution of dark eigenstates in cavity QED

We propose a scheme for generating a two-atom entangled state and an N-atom W state using adiabatic evolution of dark eigenstates in cavity QED. The time required to complete the process does not need precise control. Since the cavity modes are never excited during the operations by engineering adiabatic evolution and controlling the atom–cavity couplings, the decoherence of the cavity decay can be suppressed.     

Preparation of W State with Superconducting Quantum-Interference Devices in a Cavity via Adiabatic Passage

We propose an alternative scheme to prepare W state by using superconducting quantum-interference devices （SQUIDs） coupled to a largely-detuned cavity. The present scheme is based on evolution by adiabatic passage, where only by tuning adiabatically the Rabi frequencies of the classical microwave pulses we can obtain the standard W state without measurement or any auxiliary SQUIDs. Thus the procedure is simplified and the scheme can be achieved with very high success probability since the errors in dynamical or geometric ways can be avoided. In addition, the SQUID system and the cavity have no probability of being excited state. Thus decoherence caused by the excited-level spontaneous emission or the cavity decay is suppressed.     

Quantum State Transfer Between Any Pair of Qubits in a Quantum Network via Optical Fibers

We propose scheme for transferring quantum state between any pair of nodes in a quantum network. Each node consists of an atom and a cavity, with the atom acting as the quantum bit. Any two adjacent nodes are connected by an optical fiber. During the operation neither the atomic system nor the fibers are excited, which is important in view of decoherence. Under certain conditions, the probability that the cavities are excited is negligible. The method has an inherent robustness against the fluctuation perturbations in the classical control parameters and the randomness in the atomic position. The scheme can be generalized to implement quantum phase gate between any two remote qubits.     

Tripartite entanglement of atoms trapped in coupled cavities via quantum Zeno dynamics

A scheme is proposed for the generation of a W state for three atoms trapped in spatially separated cavities connected by optical fibers via quantum Zeno dynamics. Our scheme is based on the resulting effective dynamics induced by continuous coupling between the atoms and cavities. The effects of decoherence such as atomic spontaneous emission and the fiber and cavity losses are considered. Numerical results show that the scheme is very robust against the cavity decay due to a tiny excitation probability of the cavity fields during the operation.     

Geometric phase gate with trapped ions in thermal motion by adiabatic passage

We propose a scheme for implementing two-qubit geometric phase gate via the adiabatic evolution for trapped ions in thermal motion, leveraging on the stimulated Raman adiabatic passage with the geometric phase mechanism. Evolution along a dark state makes our scheme not only immune from decoherence due to spontaneous emission from excited states, but also rid off the dynamical phase. Furthermore, due to the opposite detuning of the driving lasers, the vibrational states of the trapped ions are only virtually excited during the operations, so our scheme is also insensitive to the occupation number of the vibrational mode.     

Conditional quantum CNOT gate between two four-level atoms

We propose a scheme to implement a quantum controlled-NOT (CNOT) gate between two four-level atoms inside the detuned optical cavity. The system state is evolved inside the decoherence-free (DF) subspace through stimulated Raman processes, which yields the desired unitary evolution operation for the CNOT.Our scheme is immune to decoherence due to dissipation of cavity excitation and spontaneous emission from the excited atomic level.     

Effective dynamics for two-atom entanglement and quantum information processing by coupled cavity QED systems

We propose a scheme for deterministic generation of entanglement embodied by two L\Lambda -type atoms distributed in two coupled cavities. We study such a system in the dispersive atom-field interactions, where the dynamics of the system operates through the virtual population of both the atomic excited states and the photonic states in the cavities (plus the fiber). We verify the validity of the dynamics, and moreover, study the influences of the decoherence due to the spontaneous emission and photon leakage. We also apply the dynamics for realizing quantum state transfer and quantum phase gates.     

Scheme for n phase gates operation and one-step preparation of highly entangled cluster state

In the system with superconducting quantum interference devices (SQUIDs) in a cavity, we propose a scheme for simultaneous implementing n phase gates and one step preparing the highly entangled cluster states based on the two-channel Raman interaction. In our scheme, the system is independent to the photon number of the cavity field, the cavity field can be initially in an arbitrary state, which is convenient for the experimental operation. The n phase gates operation and the cluster state generation are realized by using only the two lower flux states of the SQUID and the excited state would not be excited so that the influence of the decoherence due to spontaneous emission of the SQUID’s levels is possible to minimize. More importantly, the operation time of the phase gates is independent of the number n of the qubits. Finally, the experimental feasibility is also discussed in detail.     

Entangling Two Atomic Ensembles Trapped in Separated Cavities

We propose a scheme for the generation of two collections of atoms trapped in distant cavities connected by an optical fiber. The virtual photon exchange leads to the entanglement between these two atomic ensembles. During the operation the atomic system, cavity modes, and fiber are not excited, which is important in view of decoherence.     

Generation of Greenberger-Horne-Zeilinger states for multiple atoms trapped in separated cavities

We propose a scheme for generating maximally entangled states for multiple atoms trapped in distant cavities connected by fibers. During the operation neither the atomic system nor the fibers are excited, which is important in view of decoherence. Under certain conditions, the probability that the cavities are excited is negligible. The scheme does not include projective measurement and the GHZ state is generated deterministically. Taking advantage of adiabatic passage, the entanglement fidelity is insensitive to fluctuation of experimental parameters.     

One-Step Realization of SWAP Gate with Superconducting Quantum-Interference Devices and Atoms in Cavity QED

We put forward a simple scheme for one-step realization of a two-qubit SWAP gate with SQUIDs (superconducting quantum-interference devices) in cavity QED via Raman transition. In this scheme, the cavity field is only virtually excited and thus the cavity decay is suppressed. The SWAP gate is realized by using only two lower flux states of the SQUID system and the excited state would not be excited. Therefore, the effect of decoherence caused from the levels of the SQUID system is possibly minimized. The scheme can also be used to implement the SWAP gate with atoms.     

Generation of four-atom Greenberger-Horn-Zeilinger state via adiabatic passage

We propose a scheme to generate a Greenberger-Horn-Zeilinger (GHZ) state of four atoms trapped in a two-mode optical cavity via an adiabatic passage. The scheme is robust against moderate fluctuations of the experimental parameters. Numerical calculations show that the excited probabilities of both the cavity modes and the atoms are tiny and depend on the pulse peaks of the classical laser fields. For certain decoherence due to the atomic spontaneous emission and the cavity decay, there exits a range of pulse peaks to get a high fidelity.     

Measurement of the decoherence of a mesoscopic superposition of motional states of a trapped ion

We propose a scheme to observe the decoherence of a mesoscopic superposition of two coherent states in the motion of a trapped ion. In the scheme the ion is excited by two perpendicular lasers tuned to the ion transition. The decoherence is revealed by the decrease of the correlation between two successive measurements of the internal state of the ion after relevant laser－ion interaction.