QIC Abstracts

 Vol.5 No.4/No.5 July 1, 2005
Quantum Information and Quantum Control  (pp273-274)
         P. Brumer, D. Lidar, H-K Lo and A Steinberg
Research Articles:
Minimum entangled state dimension required for pseudo-telepathy (pp275-284)
         G. Brassard, A.A. Methot and A. Tapp
Pseudo-telepathy provides an intuitive way of looking at Bell's inequalities, in which it is often obvious that feats achievable by use of quantum entanglement would be classically impossible. A~two-player pseudo-telepathy game proceeds as follows: Alice and Bob are individually asked a question and they must provide an answer. They are \emph{not} allowed any form of communication once the questions are asked, but they may have agreed on a common strategy prior to the execution of the game. We~say that they \emph{win} the game if the questions and answers fulfil a specific relation. A~game exhibits \emph{pseudo-telepathy} if there is a quantum strategy that makes Alice and Bob win the game for all possible questions, provided they share prior entanglement, whereas it would be impossible to win this game systematically in a classical setting. In~this paper, we show that any two-player pseudo-telepathy game requires the quantum players to share an entangled quantum system of dimension at least~\mbox{$3 \times 3$}. This is optimal for two-player games, but the most efficient pseudo-telepathy game possible, in terms of total dimension, involves \emph{three} players who share a quantum system of dimension~\mbox{$2 \times 2 \times 2$}.

Universal dynamical control of decay and decoherence for  weak and strong system-bath coupling (pp285-317)
         G. Gordon, G. Kurizki, A.G. Kofman and S. Pellegrin
A unified theory is given of dynamically modified decay and decoherence in driven two-level and multilevel quantum systems that are weakly coupled to arbitrary finite-temperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of decoherence suppression and the limitations of this approach are obtained. For a driven qubit that is strongly coupled to the continuum edge of reservoir's spectrum, we demonstrate that only an appropriately ordered sequence of abrupt changes of the resonance frequency, near the continuum edge, can effectively protect the qubit state from decoherence.

Coherent optical control of electronic excitations in functionalized semiconductor nanostructures (pp318-334)
         L.G.C. Rego, S.G. Abuabara and V.S. Batista
The feasibility of creating and manipulating coherent quantum states on surfaces of functionalized semiconductor nanostructures is computationally investigated. Quantum dynamics simulations of electron-hole transfer between catechol molecules adsorbed on TiO_2 -anatase nanostructures under cryogenic and vacuum conditions indicate that laser induced coherent excitations can be prepared and manipulated to exhibit controllable spatial Rabi oscillations. The presented computational methods and results are particularly relevant to explore the basic model components of quantum-information electro-optic devices based on inexpensive and readily available semiconductor materials.

On-site localization of excitations (pp335-349)
         M.I. Dykman, L.F. Santos, M. Shapiro and F.M. Izrailev
 We demonstrate that, in a quantum computer with perpetually coupled qubits, all excitations can be confined to their sites (qubits) even without refocusing. The on-site localization is obtained by constructing a sequence of qubit energies that efficiently suppresses resonant hopping. The time during which a many-excitation state remains strongly localized in an infinite chain can exceed the reciprocal hopping frequency by $\agt 10^5$ already for a moderate bandwidth of qubit energies. The proposed energy sequence is also convenient for performing quantum operations on the qubits.

Stabilizing qubit coherence via tracking-control (pp350-363)
         D.A. Lidar and S. Schneider
We consider the problem of stabilizing the coherence of a single qubit subject to Markovian decoherence, via the application of a control Hamiltonian, without any additional resources. In this case neither quantum error correction/avoidance, nor dynamical decoupling applies. We show that using tracking-control, i.e., the conditioning of the control field on the state of the qubit, it is possible to maintain coherence for finite time durations, until the control field diverges.

Phase control for entanglement preparation in two-qubit systems (pp364-379)
         V.S. Malinovsky and I.R. Sola
The theory of Quantum Control is starting to lay bridges with the field of Quantum Information and Quantum Computation. Using key ideas of laser control of the dynamics by means of phase manipulation and adiabatic passage, we review laser schemes that allow entanglement preparation in a two-qubit system. The schemes are based on sequences that use four time-delayed pulses, with or without concerted decay, in or off resonance with the intermediate levels of the qubit space. We show how to control the fidelity and phase of the entanglement, as well as the sensitivity of the preparation to the different pulse parameters. In general the schemes provide an improvement in robustness and in the finesse of the control to phase, with respect to previously proposed schemes based on sequences of $\pi$ pulses.

Bidirectional coherent classical communication (pp380-395)
         A.W. Harrow and D.W.  Leung
A unitary interaction coupling two parties enables quantum or classical communication in both the forward and backward directions. Each communication capacity can be thought of as a tradeoff between the achievable rates of specific types of forward and backward communication. Our first result shows that for any bipartite unitary gate, bidirectional coherent classical communication is no more difficult than bidirectional classical communication --- they have the same achievable rate regions. Previously this result was known only for the unidirectional capacities (i.e., the boundaries of the tradeoff). We then relate the tradeoff for two-way coherent communication to the tradeoff for two-way quantum communication and the tradeoff for coherent communication in one direction and quantum communication in the other.

Information vs. disturbance in dimension D (pp396-412)
         P.O. Boykin and V.P. Roychowdhury
 We show that for Eve to get information in one basis about a state, she must cause errors in \emph{all} bases that are mutually unbiased to that basis. Our result holds in any dimension. We also show that this result holds for \emph{all functions of messages} that are encrypted with a key.

Getting something out of nothing (pp413-418)
         H-K Lo
We study quantum key distribution with standard weak coherent states and show, rather counter-intuitively, that the detection events originated from vacua can contribute to secure key generation rate, over and above the best previous result. Our proof is based on a communication complexity/quantum memory argument. The key observation is that Eve does not have to store anything, if Alice sends out a vacuum state.

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