International Journal of Quantum Information

Quantum memory channels are attracting growing interest, motivated by both realistic possibilities of transferring information by means of quantum hardware and inadequacies of the memoryless approximation. In fact, subsequent uses of the same quantum transmission resource can be significantly correlated. In this paper we review two Hamiltonian models describing memory effects in a purely dephasing spin-boson channel and in a channel with damping visualized by a micromaser system, respectively. In both cases, we show that the quantum information transmission rates are higher than in the memoryless limit.

We show that the optimal rate of lossless quantum data compression is closely related to Berthiaume, van Dam and Laplante's quantum Kolmogorov complexity. We show that:

• The expected quantum Kolmogorov complexity of a mixture is close to the optimal rate of lossless data compression of that mixture.

• If quantum Kolmogorov complexity obeys some inequality, then so does the optimal rate of lossless quantum data compression.

While many scenarios of anonymous communication can be solved by various famous classical techniques including mix nets or dining cryptographers (DC), an abundant amount of anonymous tasks remains open. In this paper, we present a solution to one such task. Our protocol serves as a private communication channel for quantum messages while ensuring that both the sender and the receiver stay anonymous in a set of all parties. Our protocol is a continuation of our previous work on anonymous transmission of quantum information. The protocol does not require trusted parties, and is secure against an active adversary. It can also withstand denial of service attacks. The protocol exhibits an advantage of quantum resources over classical because no classical solution is known (in the same security settings). Entanglement swapping technique is used to create "symmetry" in the protocol, which makes it very transparent and easier to analyze. The security analysis we present is heavily based on our previous work on quantum anonymous communication. The task we implement is also a nice illustration of a cryptographic task where two parties in a multi-party protocol can be dishonest at the same time, but are not cooperating.

We investigate the effect of the Dzyaloshinskii–Moriya anisotropic antisymmetric interaction on entanglement in a three-qubit Ising model with intrinsic decoherence. The entanglement of the nearest and the next-nearest neighbor qubit is calculated. The results show that, taking into account the intrinsic decoherence, when the qubits are initially in the maximal entangled state, the concurrence of the system decreases with the increasing Dzyaloshinskii–Moriya interaction following the evolution of the time t. When the qubits are initially in the disentangled state, the destructive effect of intrinsic decoherence on entanglement can be moderated by the Dzyaloshinskii–Moriya interaction, the concurrence increases with the increasing Dzyaloshinskii–Moriya interaction following the evolution of the time t.

Taking nonlinear effect into account, we study theoretically the transmission properties of photons in a one-dimensional coupled cavity, the cavity located at the center of the cavity array being coupled to a two-level system. By using the traditional scattering theory and the mean-field approximation, we calculate the transmission rate of photons along the cavities, and discuss the effect of nonlinearity and the cavity-atom coupling on the photon transport. The results show that the cavity-atom couplings affect the coherent transport of photons. The dynamics of such a system is also studied by numerical simulations, the effect of the atom-field detuning and nonlinearity on the dynamics is shown and discussed.

We give a construction for quantum codes from linear and cyclic codes over . We derive Hermitian self-orthogonal codes over as Gray images of linear and cyclic codes over . In particular, we use two binary codes associated with a cyclic code over of odd length to determine the parameters of the corresponding quantum code.

Two continuous-variable quantum identity authentication schemes, which are utilized to prevent active attack, are presented by using Gaussian-modulated squeezed states. The proposed schemes can both verify user's identity as well as distribute an updated new key as the authentication key. A new defined fidelity parameter is proposed to verify the identity of the communicator and detect eavesdropping. The analytical results show the feasibility of these two identity authentication schemes and the security under the attack of general Gaussian-cloner strategy and collective attack.

Reversible logic is an emerging area of research, having applications in nanotechnology, low power CMOS design, quantum computing, and DNA computing. In this paper, two different parity-preserving reversible error coding and detection circuits are studied. First we propose two new reversible Hamming code generator circuits. One of them is parity-preserve. We also propose a new parity-preserving reversible Hamming code error detector circuit. The proposed parity-preserving reversible Hamming code generator (PPHCG) and error detector circuits provide single error correction–double error detection (SEC–DED). The designs are better than the existing counterparts in terms of quantum cost (QC), number of constant inputs, and number of garbage outputs. Then we propose parity-preserving reversible cyclic code encoder/decoder circuits for the first time. A parity-preserving reversible D flip-flop is also proposed. Equivalent quantum representation of two parity-preserving 4 ∗ 4 reversible gates, IG, and PPHCG, are also proposed. We show for the first time that IG has a QC of only 7 and PPHCG has a QC of only 6.

The lossy propagation law (generalization of Lambert-Beer's law for classical radiation loss) for non-classical, dual-mode entangled states is derived from first principles, using an infinite-series of beam splitters to model continuous photon loss. This model is general enough to accommodate stray-photon noise along the propagation, as well as amplitude attenuation. An explicit analytical expression for the density matrix as a function of propagation distance is obtained for completely general input states with bounded photon number in each mode. The result is analyzed numerically for various examples of input states. For N00N state input, the loss of coherence and entanglement is super exponential, as predicted by a number of previous studies. However, for generic input states, where the coefficients are generated randomly, the decay of coherence is very different; in fact no worse than the classical Beer-Lambert law. More surprisingly, there is a plateau at a mid-range interval in propagation distance where the loss is in fact sub-classical, following which it resumes the classical rate. The qualitative behavior of the decay of entanglement for two-mode propagation is also analyzed numerically for ensembles of random states using the behavior of negativity as a function of propagation distance.

Dynamics of the nearest-neighbor concurrence and the block–block entanglement in closed Heisenberg chains with Dzyaloshinskii–Moriya (DM) interactions are simulated numerically. It is found that the nearest-neighbor concurrence and the block–block entanglement can be generated dynamically from an initial antiferromagnetic (AF) state, and DM interactions will enhance the amplitude of oscillation of concurrence and that of block–block entanglement. Furthermore, dynamics of block–block entanglement S_L in the chain with even number spins is coincident with the dynamics of S_{N - L} for D = 0. However, with the consideration of DM interactions, the evolution of S_L will not be consistent with that of S_{N - L} for the chain with odd number spins. In a given time interval, the maximal value of the generated nearest-neighbor concurrence will first decrease with the increase of chain's length, then oscillate with the increase of N. The maximal value and the minimal value of the oscillation becomes stable for a large N, and of the chain with odd spins is larger than that of the chain with even spins for a large N. The influence of DM interaction on the value of becomes unobvious with the increase of chain's length.

We demonstrate that a genuine six-qubit entangled state introduced by Tapiador et al. [J. Phys. A42 (2009) 415301] can be used to realize the deterministic controlled teleportation of an arbitrary three-qubit state by performing only the Bell-state measurements.

A scheme for realizing a multi-qubit phase gate with one control qubit simultaneously controlling n target qubits in a cavity QED system is proposed. The operation time of the gate is independent of the number n of qubits. The realizability of the gate with current technology is also discussed.

In NMR quantum computing, spin states of spin-1/2 nuclei are called qubits. Quantum logic gates are represented by unitary matrices. As a universal gate, controlled-NOT (CNOT) is a two-qubit gate. For the IS (I = 1/2 and S = 1/2) spin system, two-qubit CNOT gate is represented by a 4 × 4 matrix. SWAP logic gate, which exchanges two quantum states, is constructed by CNOT gates. In this study, first, four-qubit CNOT gates are constructed for the IS (I = 3/2, S = 3/2) spin system. Then, by using these CNOT gates, a four-qubit SWAP logic gate is found. As an application and verification, an obtained SWAP logic gate is applied to the matrix representation of product operators for the IS (I = 3/2, S = 3/2) spin system. SWAP logic gate can also be presented by an NMR pulse sequence. By using the product operator theory, the pulse sequence of the SWAP logic gate is applied to product operators of the IS (I = 3/2, S = 3/2) spin system. The expected exchange results are obtained for both the matrix representation and the pulse sequence of SWAP logic gate.

In this paper, we investigate the entanglement dynamics of three-qubit states under a spin environment. From the analysis, we find that the entanglement dynamics of the three-qubit states depends not only on the coupling strength between the system and the environment and the number of degrees of freedom of the environment but also on the tunneling matrix elements of the spin environment and the specific state of concern. Specifically, the entanglement dynamics of the GHZ state, the W state, and the Werner state have been analyzed in detail and the conditions to identify the decoherence-free subspaces of our model have been discussed.

We present a new multiparty controlled quantum secure direct communication protocol with phase encryption, in which the sender's secret message can only be recovered by the receiver under the permission of all the controllers. The security of this scheme is based on the basic principles of quantum mechanics and the secret order of encoding photons.

By joint actions of two separate two-qubit projective measurements, I present a new protocol for remotely preparing a two-qubit state via the shared three bipartite entanglements. The success probability and the classical communication cost are also minutely calculated. Then I concisely generalize it to multi-party case and multi-qubit state case, respectively. In contrast to the previous schemes [Int. J. Theor. Phys.48 (2009) 2005; Int. J. Theor. Phys.46 (2007) 2378; Commun. Theor. Phys.47 (2007) 247], this protocol has distinct advantages overwhelming the above ones as far as quantum resource consumption and quantum operation difficulty or intensity are concerned.