International Journal of Quantum Information

We show that classical and quantum Kolmogorov complexity of binary strings agree up to an additive constant. Both complexities are defined as the minimal length of any (classical resp. quantum) computer program that outputs the corresponding string.

It follows that quantum complexity is an extension of classical complexity to the domain of quantum states. This is true even if we allow a small probabilistic error in the quantum computer's output. We outline a mathematical proof of this statement, based on an inequality for outputs of quantum operations and a classical program for the simulation of a universal quantum computer.

We explicitate the relation between Hamiltonians for networks of interacting qubits in the XYZ model and graph Laplacians. We then study evolution in networks in which all sites can communicate with each other. These are modeled by the complete graphK_n and called all-to-all networks. It turns out that K_n does not exhibit perfect state transfer (PST). However, we prove that deleting an edge in K_n allows PST between the two non-adjacent sites, when n is a multiple of four. An application is routing a qubit over n different sites, by switching off the link between the sites that we wish to put in communication. Additionally, we observe that, in certain cases, the unitary inducing evolution in K_n is equivalent to the Grover operator.

We study the effect of an environment consisting of noninteracting two level systems on Landau-Zener transitions with an interest on the performance of an adiabatic quantum computer. We show that if the environment is initially at zero temperature, it does not affect the transition probability. An excited environment, however, will always increase the probability of making a transition out of the ground state. For the case of equal intermediate gaps, we find an analytical upper bound for the transition probability in the limit of large number of environmental spins. We show that such an environment will only suppress the probability of success for adiabatic quantum computation by at most a factor close to 1/2.

We present a stable and deterministic quantum key distribution (QKD) system based on differential phase shift. With three cascaded Mach-Zehnder interferometers with different arm-length differences for creating key, its key creation efficiency can be improved to be 7/8, more than other systems. Any birefringence effects and polarization-dependent losses in the long-distance fiber are automatically compensated with a Faraday mirror. Added an eavesdropping check, this system is more secure than some other phase-coding-based QKD systems. Moreover, the classical information exchanged is reduced largely and the modulation of phase shifts is simplified. All these features make this QKD system more convenient than others in a practical application.

A scheme is proposed for realizing simultaneous negative permittivity and negative permeability based on quantum coherence in a four-level dense atomic system here. Under some parametric conditions the system shows that simultaneous negative permittivity and negative permeability (i.e. Left-handness) can be achieved in a wider frequency band because of quantum coherence. And the novelty properties of gain and dispersion near the resonance frequency may have some potential applications.

In the past decades, various schemes of teleportation of quantum states through different types of quantum channels (a prior shared entangled state between the sender and the receiver), e.g. EPR pairs, generalized Bell states, qubit GHZ states, standard W states and its variations, genuine multiqubit entanglement states, etc., have been developed. Recently, three-qutrit quantum states and two-qudit quantum states have also been considered as quantum channels for teleportation. In this paper, we investigate the teleportation of an unknown qudit using a d level GHZ state, i.e. a three-qudit maximally entangled state, as quantum channel. We design a general scheme of faithful teleportation of an unknown qudit using a d-level GHZ state shared between the sender and the receiver, or among the sender, the receiver and the controller; an unknown two-qudit of Schmidt form using a d level GHZ state shared between the sender and the receiver; as well as an unknown arbitrary two-qudit using two shared d level GHZ states between the sender, the receiver and the controller, or using one shared d level GHZ state and one shared generalized Bell state. We obtain the general formulas of Alice's measurement basis, Charlie's measurement basis and Bob's unitary operations to recover the input state of Alice. It is intuitionistic to generalize the protocols of teleporting an arbitrary two-qudit state to teleporting an arbitrary n-qudit state.

The interaction between a three-level cascade atom and an initial coherent field with a time-varying frequency has been investigated, including the dynamic behavior of the atomic level occupation probabilities and the entanglement between the atom and the field. The frequency of the field is modulated in the forms of sine and rectangle. It is found that the field-frequency variation has a great influence on the properties of the generalized Jaynes-Cummings model. Appropriate frequency modulation can enhance and stabilize the degree of entanglement between the atom and the field.

We present an efficient protocol for polarization entanglement generation from spatially correlated photons in spontaneous parametric down-conversion. Compare with the protocol proposed by Shimuzu et al. (Phys. Rev. A77 032338), only one polarizing beam splitter is used in this protocol, which makes it simpler and more convenient in a practical application. By increasing the number of the slits, we provide a way for generation of a hyperentangled state in both the spatial and polarization degrees of freedom.

The information energy is proposed as a possible measure for electron correlation energy in atoms and molecules. For neutral atoms and their cations, we proposed an expression for correlation energy with explicit dependence on the information energy and atomic number. The obtained correlation energies have been used to compute the first ionization potentials of the ground state of the main group elements from hydrogen through krypton.

It is shown that the information energy of a molecule can be well-estimated by use of the information energies of its constituent atoms. On the basis of the molecular information energy and number of electrons in the molecule, a simple model has been proposed for evaluation of molecular correlation energy.

Large scale quantum information processing requires stable and long-lived quantum memories. Here, using atom-photon entanglement, we propose an experimentally feasible scheme to realize decoherence-free quantum memory with atomic ensembles, and show one of its applications, remote transfer of unknown quantum state, based on laser manipulation of atomic ensembles, photonic state operation through optical elements, and single-photon detection with moderate efficiency. The scheme, with inherent fault-tolerance to the practical noise and imperfections, allows one to retrieve the information in the memory for further quantum information processing within the reach of current technology.

We propose a scheme for long-distance teleportation of an unknown atomic state between two atoms that are trapped separately in optical cavities. Our scheme is based on the effects of quantum statistics of indistinguishable photons emitted from the trapped atoms. In this scheme, the probability of success is independent of the state to be teleported. Moreover, in the Lamb-Dicke limit, the requirement of the simultaneous clicks of the detectors is not necessary.

We consider a class of entangled states of a quantum system (S) and a second system (A) where pure states of the former are correlated with mixed states of the latter, and work out the entanglement measure with reference to the nearest separable state. Such "pure-mixed" entanglement is expected when the system S interacts with a macroscopic measuring apparatus in a quantum measurement, where the quantum correlation is destroyed in the process of environment-induced decoherence whereafter only the classical correlation between S and A remains, the latter being large compared to the former. We present numerical evidence that the entangled S–A state drifts towards the nearest separable state through decoherence, with an additional tendency of equimixing among relevant groups of apparatus states.