A scheme for teleporting Schroedinger—cat states via the dispersive atom—cavity—field interaction

A proposal is presented for teleporting Schroeding-Cat states,the process of the teleportation is achieved through the dispersive atom-cavity-field interaction.In this proposal.only measurement on the cavity field and on the singlet atomic states are used.     

Simple Scheme for Directly Measuring Concurrence of Two-Qubit Pure States in One Step

In the present work, a simple scheme for the direct measurement of the concurrence of two-qubit pure states is proposed. The scheme is based on trapped ions and only needs one step when the two identical pure states are given. The vibrational mode in our proposal is only virtually excited, which is important in view of decoherence.Furthermore, the scheme is feasible based on current technologies.     

A Simple Scheme for the Preparation of Four-Photon GHZ States Based on Cavity QED

A simple scheme is presented for generating four-photon Greenberger-Horne-Zeilinger states with a large detuned interaction between a three-level atom and two bimodal cavities. In the proposed protocol, the quantum information is encoded on Fock states of the cavity-fields, and the atomic spontaneous emission can be effectively suppressed due to the fact that the excited state of the atom is adiabatically eliminated under the large detuning condition. The detection of the atom can collapse the cavity to the desired state. The experimental feasibility of our proposal is also discussed.     

A scheme for implementing quantum game in cavity QED

In this paper, we propose a scheme for implementing quantum game (QG) in cavity quantum electrodynamics(QED). In the scheme, the cavity is only virtually excited and thus the proposal is insensitive to the cavity fields states and cavity decay. So our proposal can be experimentally realized in the range of current cavity QED techniques.     

Bell Theorem without Inequality for Some Generalized GHZ and W States

Bell＇s theorem without inequalities is applied for some general Greenberger-Horn-Zeilinger （GHZ） states and W states and a wide range of such states can exhibit all-versus-nothing conflict between local realism and quantum theory. The case of standard GHZ state is contained in our proposal. For some generalized GHZ states more intensive violation on local realism is manifested.     

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.     

Preservation of quantum states via a super-Zeno effect on ensemble quantum computers

Following a recent proposal by Dhar et al (2006 Phys. Rev. Lett. 96 100405), we demonstrate experimentally the preservation of quantum states in a two-qubit system based on a super-Zeno effect using liquid-state nuclear magnetic resonance techniques. Using inverting radiofrequency pulses and delicately selecting time intervals between two pulses, we suppress the effect of decoherence of quantum states. We observe that preservation of the quantum state |11\rg with the super-Zeno effect is three times more efficient than the ordinary one with the standard Zeno effect.     

A Robust Scheme for Two－Qubit Grover Quantum Search Alogrithm Based on the Motional and Internal States of a Single Cold Trapped Ion

We propose a scheme to implement a two-qubit Grover quantum search algorithm. The novelty in the proposal is that the motional state is introduced into the computation and the internal state within a single cold trapped ion.The motional and internal states of the ion are manipulated as two qubits by the laser pulses to accomplish anexample of a Grover algorithm based on the two qubits. The composite laser pulses that are applied to implementthe Grover algorithm have been designed in detail. The issues concerning measurement and decoherence arediscussed.     

Transferring entangled states of photonic cat-state qubits in circuit QED

We propose a method for transferring quantum entangled states of two photonic cat-state qubits(cqubits)from two microwave cavities to the other two microwave cavities.This proposal is realized by using four microwave cavities coupled to a superconducting flux qutrit.Because of using four cavities with different frequencies,the inter-cavity crosstalk is significantly reduced.Since only one coupler qutrit is used,the circuit resource is minimized.The entanglement transfer is completed with a singlestep operation only,thus this proposal is quite simple.The third energy level of the coupler qutrit is not populated during the state transfer,therefore decoherence from the higher energy level is greatly suppressed.Our numerical simulations show that high-fidelity transfer of two-cqubit entangled states from two transmission line resonators to the other two transmission line resonators is feasible with current circuit QED technology.This proposal is universal and can be applied to accomplish the same task in a wide range of physical systems,such as four microwave or optical cavities,which are coupled to a natural or artificial three-level atom.     

Controlling of the Polarization States of Electromagnetic Waves Using Epsilon-near-Zero Metamaterials

We demonstrate theoretically that the epsilon-near-zero materials can be utilized to control effectively the polarization conversion of an electromagnetic wave through reflection. The significant feature differing from all other means based on whatever natural materials or metamaterials is that for TM incident wave, the reflected phase is a constant, while for TE wave, the reflected phase is a linear function of the incident angle. The phase difference between them covers the range from-180° to 0°, and the polarization conversions from linear states to elliptical or circular states can be obtained by only adjusting the incident angle. Because no complex structures are employed, our proposal promises a simple approach for manipulating polarization conversion at both terahertz and optical frequencies.     

Singlet-State Preparation for Three Λ-type Atoms with Rydberg Blockade Mechanism

A proposal for the generation of singlet states of three Λ-type Rydberg atoms is presented via the interaction between a separate Rydberg state and an EPR pair. Different from previous schemes, we do not need to couple ground states by using microwave lights but to set appropriate frequency detuning between lasers and two atomic transitions between ground states and Rydberg levels to eliminate the degenerate of the two ground states, making the present protocol more easily in experiment. Moreover, a series of numerical simulations are made to show the feasibility.     

Entanglement capacity of two-qubit unitary operator for rank two mixed states

The entanglement capacity of two-qubit unitary operator acting on rank two mixed states in concurrence is discussed. The condition of perfect entangler is the same as that acting on pure states and the entanglement capacity is the mixing parameter v1. For non-perfect entangler, the upper and lower bound of the entanglement capacity are given.     

Surface States in the AlxGa1-xN Barrier in AlxGa1-xN／GaN Heterostructures

Frequency-dependent capacitance-voltage (C-V) measurements have been performed on modulation-doped Al0.22 Ga0.78N/GaN heterostructures to investigate the characteristics of the surface states in the AlxGa1-xN barrier. Numerical fittings based on the experimental data indicate that there are surface states with high density locating on the AlxGa1-xN barrier. The density of the surface states is about 10^12cm^-2eV^-1, and the time constant is about 1μs. It is found that an insulating layer (Si3N4) between the metal contact and the surface of AlxGa1-xN can passlvate the surface states effectively.     

Quantum computation with surface-state electrons by rapid population passages

Quantum computation requires coherently controlling the evolutions of qubits.Usually,these manipulations are implemented by precisely designing the durations(such as theπ-pulses)of the Rabi oscillations and tunable interbit coupling.Relaxing this requirement,herein we show that the desired population transfers between the logic states can be deterministically realized(and thus quantum computation could be implemented)both adiabatically and non-adiabatically,by performing the duration-insensitive quantum manipulations.Our proposal is specifically demonstrated with the surface-state of electrons floating on the liquid helium,but could also be applied to the other artificially controllable systems for quantum computing.     

Three-qubit quantum-gate operation with an SQUID in a cavity

We propose a method of realizing a three-qubit quantum gate with a superconducting quantum interference device(SQUID) in a cavity.In this proposal,the gate operation involves the SQUID ground-states and the Fock states of cavity modes b and c.The two field-modes act as the controlling qubits,and the two SQUID states form the target qubit.Since only the metastable lower levels are involved in the gate operation,the gate is not affected by the SQUID decay rates.     

Decoherence of elliptical states in phase space

The effects of decoherence on elliptical states which concern the quantum superposition of N coherent states on an ellipse in the α plane are studied.The characteristic decoherence times are determined.The evolutions of the Wigner functions associated with these states are investigated theoretically and the losses of nonclassicality as a result of decoherence are discussed.The result shows that the decoherence of elliptical states is slower than circular states relying on the number of coherent states and the amplitude,and the constructed states have a higher resilience to losses.     

Realization of Three-Qubit Controlled-Phase Gate Operation with Atoms in Cavity QED System

We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.     

Error Tolerance in Constructing Cluster States

We consider the problem often encountered in constructing large cluster states, that is, the σx measurements in the depolarizing noise, and present a regulation to calculate the error propagation and the accumulation. Rohde et al. first pointed out the question [Phys. Rev. A 75 （2007） 010302（R）]. A quantitative analysis is given on the disadvantage influence of the probabilistic gate and noisy environment on the construction of two-dimensional （213） scalable cluster states. Positivity of partial transpose criteria is used to predict whether the final state is distillable or not. A critical value on the error tolerance in constructing cluster states is determined, which could be used as a test-bed for successful construction of scalable 2D cluster states. Furthermore, our quantitative analysis would also be helpful to find optimal approach to suppress the noise accumulation.     

Spectator Model Dynamics in a Leakage Cavity

A spectator system in an atoms-cavity QED model is investigated. The subsystem initial state is prepared as one of the Bell states or one mixed state. It is shown that （i） the dynamics of Bell states are independent on the initial states; （ii） the concurrence dynamics can be engineered by a controlling light field.     

Analysis of the low—lying spectrum of the He—tetramer

There is a type of nodal surface imposed by symmetry on wavefunctions.These surfaces are crucial to the ordering of low-lying states.Based on an analysis of this type of surface and based on existing theoretical results,the feature of the low-lying spectrum of the He-tetramer is studied,and the candidates of bound states and narrow resonances are suggested.