QIC Abstracts

 Vol.3 No.1  January 1, 2003
Between entropy and subentropy (pp1-14)
        S.R. Nichols and W.K. Wootters
The von Neumann entropy and the subentropy of a mixed quantum state are upper and lower bounds, respectively, on the accessible information of any ensemble consistent with the given mixed state. Here we define and investigate a set of quantities intermediate between entropy and subentropy.

Simulation of quantum dynamics with quantum optical systems (pp15-37)
        E. Jané, G. Vidal, W. Dür, P. Zoller, J.I. Cirac
We propose the use of quantum optical systems to perform universal simulation of quantum dynamics. Two specific implementations that require present technology are put forward for illustrative purposes. The first scheme consists of neutral atoms stored in optical lattices, while the second scheme consists of ions stored in an array of micro--traps. Each atom (ion) supports a two--level system, on which local unitary operations can be performed through a laser beam. A raw interaction between neighboring two--level systems is achieved by conditionally displacing the corresponding atoms (ions) Then, average Hamiltonian techniques are used to achieve evolutions in time according to a large class of Hamiltonians.

Dynamics of distillability (pp38-47)
        W. Wu, W. Wang, and X. X. Yi
The time evolution of a maximally entangled bipartite systems is presented in this paper. The distillability criterion is given in terms of Kraus operators. Using the criterion, we discuss the distillability of 2x2 and nxn (n>2) systems in their evolution process. There are two distinguished processes, dissipation and decoherence, which may destroy the distillability. We discuss the effects of those processes on distillability in details.

On the problem of authentication in a quantum protocol to detect traffic analysis (pp48-54)
        J. Muller-Quade and R. Steinwandt
Recently, a basic design for a quantum protocol to detect a perpetual traffic analysis of a communication channel has been proposed\cite{SJB01}. As it stands, this protocol does not take the problem of authentication into account. We show that a `standard' authentication procedure does not apply in this context, as the attacker is only interested in the number of transmitted bits. Moreover, we demonstrate how to use a one-time pad like construction for solving this authentication problem.

On the structure of a reversible entanglement generating set  for tripartite states (pp55-63)
        A. Acin, G. Vidal and J.I. Cirac
We show that Einstein--Podolsky--Rosen--Bohm (EPR) and Greenberger--Horne--Zeilinger--Mermin (GHZ) states can not generate, through local manipulation and in the asymptotic limit, all forms of tripartite pure--state entanglement in a reversible way. The techniques that we use indicate that there is a connection between this result and the irreversibility that occurs in the asymptotic preparation and distillation of bipartite mixed states.

Local vs. joint measurements for the entanglement of assistance (pp64-83)
        T. Laustsen, F. Verstraete, and S. J. van Enk
We consider a variant of the entanglement of assistance, as independently introduced by D.P. DiVincenzo et al. (quant-ph/9803033) and O. Cohen (Phys. Rev. Lett. 80, 2493 (1998)). Instead of considering three-party states in which one of the parties, the assistant, performs a measurement such that the remaining two parties are left with on average as much entanglement as possible, we consider four-party states where two parties play the role of assistants. We answer several questions that arise naturally in this scenario, such as (i) how much more entanglement can be produced when the assistants are allowed to perform joint measurements, (ii) for what type of states are local measurements sufficient, (iii) is it necessary for the second assistant to know the measurement outcome of the first, and (iv) are projective measurements sufficient or are more general POVMs needed?

Both Toffoli and Controlled-NOT need little help to universal quantum computing (pp84-92)
        Y-Y Shi
What additional gates are needed for a set of classical universal gates to do universal quantum computation? We prove that any single-qubit real gate suffices, except those that preserve the computational basis. The Gottesman-Knill Theorem implies that any quantum circuit involving only the Controlled-NOT and Hadamard gates can be efficiently simulated by a classical circuit. In contrast, we prove that Controlled-NOT plus any single-qubit real gate that does not preserve the computational basis and is not Hadamard (or its like) are universal for quantum computing. Previously only a generic gate, namely a rotation by an angle incommensurate with \pi, is known to be sufficient in both problems, if only one  single-qubit gate is added.

Book Review:
On “An Introduction to Quantum Computing Algorithms” by A.O. Pittenger,   “Quantum Computing” by M. Hirvensalo, and "Classical and Quantum Computation” by A. Yu. Kitaev, A. Shen, and M.N. Vyalyi  (pp93-96)
        R. de Wolf

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