International Journal of Quantum Information

We examine a proposed auction algorithm using quantum states to represent bids and distributed adiabatic search to find the winner.¹ When the auctioneer follows the protocol, the final measurement giving the outcome of the auction also destroys the bid states, thereby preserving the privacy of losing bidders. We describe how a dishonest auctioneer could alter the protocol to violate this privacy guarantee, and present methods by which bidders can counter such attacks. We also suggest possible quantum circuit implementations of the auction protocol, and quantum circuits to perpetrate and to counter attacks by a dishonest auctioneer.

Li et al. presented a protocol [International Journal of Quantum Information4(6) (2006) 899–906] for quantum key distribution based on entanglement swapping. In this protocol, they use random and certain bits to construct a classical key, and they claim that this key is secure. In our article, we show that the protocol by Li et al. is insecure, presenting a new type of attack strategy which gives an adversary full information about the key without being detected. This strategy is based on entanglement swapping, too, and manages to preserve the correlation between the measurement results of the legitimate parties. Further, we present a modified version of the protocol and show that it is secure against this new attack strategy.

We investigate the separability of arbitrary-dimensional tripartite systems. By introducing a new operator related to transformations on the subsystems, a necessary condition for the separability of tripartite systems is presented.

We study the thermal entanglement of a 2-qutrit spin chain with nonlinear coupling in the presence of nonuniform magnetic field. Thermal entanglement of an arbitrary (finite-dimensional) m-partite system vanishes at some finite threshold temperature T_s. We investigate the dependence of T_s on the system's parameters, i.e. the nonlinear coupling and the magnetic field, for this 2-qutrit system. In addition, we compare two lower bounds of I-concurrence for this system and also study its dense coding capacity as a function of system's parameters.

We investigate violation of Cauchy–Schwarz and Bell inequalities in two-mode three-level cascade system with injected atomic coherence in the framework of quantum theory of multiwave mixing. We show that Cauchy–Schwarz inequality is strongly violated when there is strong entanglement in the system. It also appears that Bell inequality is violated in the region where there is weak entanglement and well-preserved where there is strong entanglement in the system. We thus note that there are states which are entangled but do not violate Bell inequality. We also show that this system can be used to prepare states that exhibit EPR correlations.

Utilizing W state, which is much more robust than GHZ state, we propose a protocol for three-party controlled quantum secure direct communication (QSDC). The feature of this protocol is that the sender encodes the secret message directly on a sequence of particle states and faithfully transmits them to an arbitrary one of two receivers without revealing any information to a potential eavesdropper. On the other hand, we construct the efficient quantum circuit to implement the QSDC by means of primitive operations in quantum computation.

The Kolmogorov complexity of a string is the length of its shortest description. We define a second quantized Kolmogorov complexity where the length of a description is defined to be the average length of its superposition. We discuss this complexity's basic properties. We define the corresponding prefix complexity and show that the inequalities obeyed by this prefix complexity are also obeyed by von Neumann entropy.

We present a protocol for telecloning an ionic state to M distant users by using linear optical elements. The quantum channel used here is a (M + 1)-particle W state. The most distinct advantage of the current scheme is that it does not need a joint measurement required by the previous telecloning schemes.

Problems of optimization are very common in several areas like engineering, physics, economics and mathematics and usually, they are very difficult to solve. Basically, one has to find the minimum or maximum of an objective function. In this work, we chose two optimization problems and we present quantum algorithms to solve them. The problems are: (1) to find the best decomposition of a unitary operation achievable by a programmable quantum circuit; (2) to find the minimal distance of a linear code. The quantum algorithms used are asymptotic quantum search and their main property is the fact that only one measurement is required.

In the present paper, we study the continuous-time quantum walk on quotient graphs. On such graphs, there is a straightforward reduction of the problem to a subspace that can be considerably smaller than the original one. Along the lines of reductions, by using the idea of calculation of the probability amplitudes for continuous-time quantum walk in terms of the spectral distribution associated with the adjacency matrix of graphs [Jafarizadeh and Salimi (Ann. Phys.322 (2007))], we show that the continuous-time quantum walk on original graph Γ induces a continuous-time quantum walk on quotient graph Γ_H. Finally, for example, we investigate the continuous-time quantum walk on some quotient Cayley graphs.

The dynamics of the population density matrix and polarization density matrix of the biexciton system of single semiconductor quantum dots with pulse excitation were analyzed by using master equations and quantum regression theorem. A model based on the spectral distributions of the cascade photon emissions was proposed to calculate the polarization entanglement of the photon pairs emitted from the non-degenerate biexciton system with spectral filters. The influences of the spectral filter width and the exciton energy splitting between the two orthogonally polarized eigenstates on the entanglement were theoretically analyzed. The dependence of the polarization entanglement on the input pulse area was also discussed.