Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors, The scheme may be realized based on current technologies.     

Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors. The scheme may be realized based on current technologies.     

Quantum Dense Coding Based on Entangled Bell State in Cavity QED

We investigate quantum dense coding based on entangled Bell states in cavity QED. We implement a experimentally feasible new scheme in cavity QED with atomic qubits where the atoms interact with a highly detuned cavity mode with the assistance of a classical field. The scheme is insensitive to the cavity decay and the thermal field. Based on cavity QED techniques, the scheme can be realizable.     

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.     

Scheme for Implementing an N-Qubit Phase Gate via Selective Atom--Field Interaction with Cavity Quantum Electrodynamics

A scheme for implementing a two-qubit phase gate with atoms sent through a high-Q optical cavity is proposed by choosing nonidentical coupling constants between the atoms and cavity. The atomic spontaneous emission can be suppressed due to the large atom-field detuning. Moreover, the scheme can be generalized to implement an N-qubit phase gate and the gating time does not change with an increase of the number of qubits.     

Implementation of n-qubit Deutsch--Jozsa algorithm using resonant interaction in cavity QED

We propose a scheme to implement the n-qubit Deutsch--Jozsa algorithm based on resonant interaction between the atoms and a single-mode cavity. In the scheme, the resonant transitions between two ground states and one excited state of an atom are changed alternately by adjusting the cavity frequency appropriately, and the operations required to complete the algorithm can be significantly simplified following the increment of the number of qubits. The implementation of the scheme in experiment would show the full power of quantum algorithm and would be significative and important for more complicated quantum algorithm in cavity quantum electrodynamics.     

A simple scheme for implementing four-atom quantum dense coding in cavity QED

An experimentally feasible scheme for implementing four-atom quantum dense coding of an atom--cavity system is proposed. The cavity is only virtually excited and no quantum information will be transferred from the atoms to the cavity. Thus the scheme is insensitive to cavity decay and the thermal field. In the scheme, Alice can send faithfully 4 bits of classical information to Bob by sending two qubits. Generalized Bell states can be exactly distinguished by detecting the atomic state, and quantum dense coding can be realized in a simple way.     

腔QED中相位协变量子克隆机的实现及纠错

应用N个二能级原子和单模真空腔场相互作用,提出了一个1→2的相位协变量子克隆机的方案.同时,基于这种克隆机,我们也提出了量子纠错方案,考虑了由相位和比特反转错误所产生的消相干影响,通过对后两个比特位进行Bell测量,并沿着合适的轴旋转第一比特,就可以恢复初始态.     

Entanglement dynamics of two-bipartite system under the influence of dissipative environments

An experimental scheme is suggested that permits a direct measure of entanglement in a two-qubit cavity system. It is realized in the cavity-QED technology utilizing atoms as flying qubits. With this scheme we generate two different measures of entanglement, namely logarithmic negativity and concurrence. The phenomenon of sudden death entanglement (ESD) in a bipartite system subjected to dissipative environment is examined. We show that the sudden death time of the entangled states depends on the initial preparation of the entangled state and the temperature of the reservoir.     

Quantum Logic Gates and Cluster States with Four-level Atoms in Cavity QED

We propose a model to implement the two-qubit quantum logic gates, i.e., the quantum phase gate and the Controlled-NOT gate, and generate the atomic qubits cluster states with a large detuned interaction between four-level atoms and a single-mode cavity field. In the presented protocol, the quantum information is encoded on the stable ground states of the atoms, and the effect of decoherence from atomic spontaneous emission is negligible. In addition, the interaction between atoms and the cavity is large detuned, and the cavity is only virtually excited. Therefore, the scheme is insensitive to the cavity decay. The experimental feasibility of our proposal is also discussed.     

Fast quantum search algorithm for databases of arbitrary size and its implementation in a cavity QED system

We propose a method for implementing the Grover search algorithm directly in a database containing any number of items based on multi-level systems. Compared with the searching procedure in the database with qubits encoding, our modified algorithm needs fewer iteration steps to find the marked item and uses the carriers of the information more economically. Furthermore, we illustrate how to realize our idea in cavity QED using Zeeman?s level structure of atoms. And the numerical simulation under the influence of the cavity and atom decays shows that the scheme could be achieved efficiently within current state-of-the-art technology.     

Quantum computation based on a quantum switch in cavity QED

A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating “bus”; thus, the effect of atomic spontaneous emission can be neglected and the strict requirements for the cavity can be relaxed.     

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.     

Cavity loss induced generation of W states

The existence of decoherence-free subspace （DFS） has been discussed widely. In this paper, we propose an alternative scheme for generating the four-atom W states by manipulating DF qubits. The atoms are divided into two pairs and trapped in two separate optical cavities. Manipulation of atoms within DFS may generate a two-atom maximally entangled state in an individual cavity, which is a stable state. After driving the system out of DFS, the atoms will interact resonantly with the cavity field. The photons leaking from the cavities interfere at the beamsplitter, which destroys which-path information, and are finally detected by one of the detectors, leading to the generation of a W state. In addition, the numerical simulation indicates that the fidelity of the prepared state can, for a very wide parameter regime, be very close to unity.     

Simple schemes for generation of W-type multipartite entangled states and realization of quantum- information concentration

We propose simple schemes for generating W-type multipartite entangled states in cavity quantum electrodynamics (CQED). Our schemes involve a largely detuned interaction of Λ-type three-level atoms with a single-mode cavity field and a classical laser, and both the symmetric and asymmetric W states can be created in a single step. Our schemes are insensitive to both the cavity decay and atomic spontaneous emission. With the above system, we also propose a scheme for realizing quantum-information concentration which is the reverse process of quantum cloning. In this scheme, quantum-information originally coming from a single qubit, but now distributed into many qubits, is concentrated back to a single qubit in only one step.     

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.     

Cavity loss induced generation of W states

The existence of decoherence-free subspace (DFS) has been discussed widely. In this paper, we propose an alternative scheme for generating the four-atom $W$ states by manipulating DF qubits. The atoms are divided into two pairs and trapped in two separate optical cavities. Manipulation of atoms within DFS may generate a two-atom maximally entangled state in an individual cavity, which is a stable state. After driving the system out of DFS, the atoms will interact resonantly with the cavity field. The photons leaking from the cavities interfere at the beamsplitter, which destroys which-path information, and are finally detected by one of the detectors, leading to the generation of a $W$ state. In addition, the numerical simulation indicates that the fidelity of the prepared state can, for a very wide parameter regime, be very close to unity.     

Schemes for preparing atomic qubit cluster states in cavity QED

Two schemes are proposed for generating atomic qubits cluster states in cavity quantum electrodynamics (QED). In the first scheme, only two-atom-cavity interactions are involved, and cluster states can be directly generated by using constructed two-qubit controlled phase gates. The second scheme needs the assistance of additional single-qubit rotations, but takes less time than the first one for two-atom operations in the cavity. In this scheme, two projective operators are constructed to prepare two-dimension or more complicated configurations of cluster states. Both schemes are insensitive to the cavity decay due to the fact that the cavity is only virtually excited during the interaction between atoms and the cavity. The idea can also be applied to the ion trap system.     

Resonant interaction scheme for GHZ state preparation and quantum phase gate with superconducting qubits in a cavity

A scheme is proposed to generate GHZ state and realize quantum phase gate for superconducting qubits placed in a microwave cavity. This scheme uses resonant interaction between the qubits and the cavity mode, so that the interaction time is short, which is important in view of decoherence. In particular, the phase gate can be realized simply with a single interaction between the qubits and the cavity mode. With cavity decay being considered, the fidelity and success probability are both very close to unity.     

Realization of Toffoli gate operation using four-level atoms in cavity QED system

This paper proposes a scheme for realization of a three-qubit Toffoli gate operation using three four-level atoms by a selective atom--field interaction in a cavity quantum electrodynamics system. In the proposed protocol, the quantum information is encoded on the stable ground states of atoms, and atomic spontaneous emission is negligible as the large atom--cavity detuning effectively suppresses the spontaneous decay of the atoms. The influence of the dissipation on fidelity and success probability of the three-qubit Toffoli gate is also discussed. The scheme can also be applied to realize an N-qubit Toffoli gate and the interaction time required does not rise with increasing the number of qubits.