基于六光子量子避错码的量子密钥分发方案

量子信道中不可避免存在的噪声将扭曲被传输的信息,对通信造成危害。目前克服量子信道噪声的较好方案是量子避错码（QEAC）。将量子避错码思想用于量子密钥分发,能有效克服信道中的噪声,且无需复杂的系统。用六光子构造了量子避错码,提出了一种丛于六光子避错码的量子密钥分发（QDK）方案。以提高量子密钥分发的量子比特效率和安全性为前提,对六光子避错码的所有可能态进行组合,得到一种六光子避错码的最优组合方法,可将两比特信息编码在一个态中,根据测肇结果和分组信息进行解码,得到正确信息的平均概率为7／16。与最近的基于四光子避错码的克服量子信道噪声的量子密钥分发方案相比,该方案的量子比特效率提高了16．67％,密钥分发安全性足它的3．5倍。     

Degree of fourth-order coherence by double Hanbury Brownben Twiss detections

Photon quantum statistics of light can be shown by the high-order coherence. The fourth-order coherences of various quantum states including Fock states, coherent states, thermal states and squeezed vacuum states are investigated based on a double Hanbury Brown–Twiss (HBT) scheme. The analytical results are obtained by taking the overall efficiency and background into account.     

Efficient scheme for the preparation of symmetric Dicke states via cross-Kerr nonlinearity

A scheme is proposed for generating maximally entangled Dicke states among four modes. The scheme only uses Kerr medium and homodyne measurements on coherent light fields, which can be efficiently made in quantum optical laboratories. The scheme can be generalized to produce maximally entangled 2k-qubit states.     

Robust scheme for the preparation of symmetric Dicke states with coherence state via cross-Kerr nonlinearity

A scheme is proposed for generating multiphoton maximally entangled states among four modes. These schemes only use Kerr medium and polarization beam splitters and P homodyne measurements on coherent light fields, which can be efficiently implemented in quantum optical laboratories. It's comparatively easy to realize symmetric Dicke state of light fields in the scheme. The scheme can be generalized to produce N-qubit maximally entangled states.     

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.     

Quantum secret sharing based on quantum error-correcting codes

Quantum secret sharing(QSS) is a procedure of sharing classical information or quantum information by using quantum states.This paper presents how to use a [2k-1,1,k] quantum error-correcting code(QECC) to implement a quantum(k,2k 1) threshold scheme.It also takes advantage of classical enhancement of the [2k-1,1,k] QECC to establish a QSS scheme which can share classical information and quantum information simultaneously.Because information is encoded into QECC,these schemes can prevent intercept-resend attacks and be implemented on some noisy channels.     

A Blind Quantum Signature Scheme with χ-type Entangled States

A blind quantum signature scheme with χ-type entangled states is proposed, which can be applied to E-voting system. In this scheme, the particles in χ-type state sequence are used for quantum key distribution first, and then for quantum signature. Our scheme is characterized by its blindness, impossibility of forgery, impossibility of disavowal. In addition, our scheme can perform an audit program with respect to the validity of the verification process in the light of actual requirements. The security of the scheme is also analyzed.     

A Novel Deterministic Quantum Communication Scheme Using Stabilizer Quantum Code

Exploiting the encoding process of the stabilizer quantum code [[n, k, d]], a deterministic quantum communication scheme, in which n - 1 photons are distributed forward and backward in two-way channel, is proposed to transmit the secret messages with unconditional security. The present scheme can be implemented to distribute the secret quantum （or classical） messages with great capacity in imperfect quantum channel since the utilized code encodes k-qubit messages for each scheme run.     

Vortex phase qubit: generating arbitrary, counterrotating, coherent superpositions in Bose-Einstein condensates via optical angular momentum beams

We propose a scheme for the generation of arbitrary coherent superpositions of vortex states in Bose-Einstein condensates (BEC) using the orbital-angular-momentum states of light. We devise a scheme to generate coherent superpositions of two such counterrotating states of light using well-known experimental techniques. We show that a specially designed Raman scheme allows for transfer of the optical vortex-superposition state onto an initially nonrotating BEC. This creates an arbitrary and coherent superposition of a vortex and antivortex pair in the BEC. The ideas presented here could be extended to generate entangled vortex states, design memories for the orbital-angular-momentum states of light, and perform other quantum information tasks. Applications to inertial sensing are also discussed.     

Quantum memory for images with feedback

A spatially multimode scheme of the quantum memory for light is presented that can be considered to be a thin hologram, whose long-lived spin subsystem of a multiatomic ensemble stores image peculiarities, such as the multimode superposition and entangled quantum states, which is unattainable for ordinary holography. In the recording process, the measurement of polarization parameters of the transmitted light wave and the feedback are used for the action on an ensemble of fixed atoms (obtained, for example, by means of the laser cooling). The information capacity of a quantum hologram with a feedback will considerably exceed that of a spatially singlemode quantum memory.     

Secure deterministic communication scheme based on quantum remote state preparation

Based on the techniques of the quantum remote state preparation via a deterministic way, this paper proposes a quantum communication scheme to distribute the secret messages in two phases, i.e., the carrier state checking phase and the message state transmitting phase. In the first phase, the secret messages are encoded by the sender using a stabilizer quantum code and then transmitted to the receiver by implementing three CNOT gates. In the second phase, the communicators check the perfectness of the entanglement of the transmitted states. The messages can be distributed to the receiver even if some of the transmitted qubits are destroyed.     

Efficient high-capacity quantum secret sharing with two-photon entanglement

An efficient high-capacity quantum secret sharing scheme is proposed following some ideas in quantum dense coding with two-photon entanglement. The message sender, Alice prepares and measures the two-photon entangled states, and the two agents, Bob and Charlie code their information on their photons with four local unitary operations, which makes this scheme more convenient for the agents than others. This scheme has a high intrinsic efficiency for qubits and a high capacity.     

Scheme for sharing classical information via tripartite entangled states

We investigate schemes for quantum secret sharing and quantum dense coding via tripartite entangled states. We present a scheme for sharing classical information via entanglement swapping using two tripartite entangled GHZ states. In order to throw light upon the security affairs of the quantum dense coding protocol, we also suggest a secure quantum dense coding scheme via W state by analogy with the theory of sharing information among involved users.     

Transferring Multi-Dimensional Quantum States and Preparing QuantumNetworks in Cavity QED

In this paper we propose a scheme for transferring quantum states andpreparing quantum networks. Compared with the previous schemes, this scheme is more efficient, since three or four-dimensional quantum states can be transferred with a single step and information interchange of three-dimensional quantum states can be realized, which is a significant improvement. It is based on the resonant interaction of a three-mode cavityfield with an atom. As a consequence, the interaction time is shortenedgreatly. Furthermore, we give some discussions about the feasibility of the scheme.     

New Bi-Signature Scheme Based on GHZ States and W States

A new quantum bi-signature scheme based on GHZ states and W states is proposed. In the proposed scheme, Alice and Bob sign one same message and send their signatures to Charlie. Different from some typical quantum signature schemes, the new quantum bi-signature scheme firstly sets up a secure channel and the three parties verify each other with the correlation of GHZ states. Then Alice, Bob and Charlie utilize the measurement outcomes of W states to implement signature and verification. The proposed scheme without any key converts the message with quantum one-way function to improve the security. The new quantum bi-signature scheme can solve the most issues of two-way choice in real life, and analysis results show that the proposed scheme is secure and efficient. Furthermore, the proposed scheme can be implemented with the existing physical technologies.

     

Fault-Tolerate Quantum Private Comparison Based on GHZ States and ECC

There are some quantum private comparison (QPC) schemes proposed previously. In this paper we study these QPC protocols in non-ideal scenario and find that they are not secure there. For resolving the problem, we propose a QPC scheme which could be performed in practical scenario. By the use of Greenberger-Horne-Zeilinger (GHZ) states and error-correcting code (ECC), the scheme has the capability of fault-tolerate.     

Multi-state Quantum Teleportation via One Entanglement State

A multi-sender-controlled quantum teleportation scheme is proposed to teleport several secret quantum states from different senders to a distance receiver based on only one Einstein-Podolsky-Rosen (EPR) pair with controlled-NOT (CNOT) gates. In the present scheme, several secret single-qubit quantum states are encoded into a multi-qubit entangled quantum state. Two communication modes, i.e., the detecting mode and the message mode, are employed so that the eavesdropping can be detected easily and the teleported message may be recovered efficiently. It has an advantage over teleporting several different quantum states for one scheme run with more efficiency than the previous quantum teleportation schemes.     

Quantum fluctuations in holographic teleportation of optical images

We investigate in detail the quantum fluctuations in the quantum holographic teleportation protocol that we recently proposed [11]. This protocol implements a continuous variable teleportation scheme that enables the transfer of the quantum state of spatially multimode electromagnetic fields, preserving their quantum correlations in space-time, and can be used to perform teleportation of 2D optical images. We derive a characteristic functional, which provides any arbitrary spatio-temporal correlation function of the teleported field, and calculate the fidelity of the teleportation scheme for multimode Gaussian input states. We show that for multimode light fields one has to distinguish between a global and a reduced fidelity. While the global fidelity tends to vanish for teleportation of fields with many degrees of freedom, the reduced fidelity can be made close to unity by choosing properly the number of essential degrees of freedom and the spatial bandwidth of the EPR beams used in the teleportation scheme.Received: 16 March 2004, Published online: 11 May 2004PACS: 03.67.-a Quantum information - 03.65.Bz Foundations, theory of measurement, miscellaneous theories (including Aharonov-Bohm effect, Bell inequalities, Berrys phase) - 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements     

Efficient decoherence-free entanglement distribution over lossy quantum channels

We propose and demonstrate a scheme for boosting the efficiency of entanglement distribution based on a decoherence-free subspace over lossy quantum channels. By using backward propagation of a coherent light, our scheme achieves an entanglement-sharing rate that is proportional to the transmittance T of the quantum channel in spite of encoding qubits in multipartite systems for the decoherence-free subspace. We experimentally show that highly entangled states, which can violate the Clauser-Horne-Shimony-Holt inequality, are distributed at a rate proportional to T.     

Manipulating a Bose-Einstein condensate with a single photon

We propose a method to create macroscopic superpositions, so-called Schr?dinger cat states, of different motional states of an ideal Bose-Einstein condensate. The scheme is based on the scattering of a freely expanding condensate by the light field of a high-finesse optical cavity in a quantum superposition state of different photon numbers. The atom-photon interaction creates an entangled state of the motional state of the condensate and the photon number, which can be converted into a pure atomic Schr?dinger cat state by operations only acting on the cavity field. We discuss in detail the fully quantised theory and propose an experimental procedure to implement the scheme using short coherent light pulses. Received 26 June 2000 and Received in final form 2nd October 2000