QIC Abstracts

 Vol.2 No.4 June 15, 2002 (print: July 15, 2002)
Transport of quantum states and separation of ions in a dual RF ion trap (pp257-271)
        M.A. Rowe, A. Ben-Kish, B. DeMarco, D. Leibfried, V. Meyer, J. Beall, J. Britton, J. Hughes, W.M. Itano, B. Jelenkovic, C. Langer, T. Rosenband, and D.J. Wineland
We have investigated ion dynamics associated with a dual linear ion trap where ions can be stored in and moved between two distinct locations. Such a trap is a building block for a system to engineer arbitrary quantum states of ion ensembles. Specifically, this trap is the unit cell in a strategy for scalable quantum computing using a series of interconnected ion traps. We have transferred an ion between trap locations 1.2 mm apart in 50 $\mu$s with near unit efficiency ($> 10^{6}$ consecutive transfers) and negligible motional heating, while maintaining internal-state coherence. In addition, we have separated two ions held in a common trap into two distinct traps.

Quantum statistical properties in a single trapped ion interacting with a laser fields (pp272-284)
        M. Abdel-Aty, S. Furuichi, and S. Nakamura
 In this paper we use the quasi-mutual entropy, which does not  depend on quantum channel, as a measure of information content of ionic state due to ion-laser interaction in a single trapped ion. It is found that the quasi-mutual entropy is a good measure of the entanglement degree of the ionic state at any time. We examine numerically the population inversion, the photon number distribution and degree of entanglement due to quasi-mutual entropy for a single trapped ion interacting with a laser field. During the transition from collapse to revival and vice-versa we have a minimum degree of entanglement value. Successive revival peaks show a lowering of the local maximum points indicating a dissipative irreversible change in the ionic state. We propose a generation of Bell-type states having a simple initial state preparation of a single trapped ion.

Time--optimal Hamiltonian simulation and gate synthesis using homogeneous local unitaries (pp285-296)
        Ll. Masanes, G. Vidal, and J.I. Latorre
Motivated by experimental limitations commonly met in the design of solid state quantum computers, we study the problems of non--local Hamiltonian simulation and non--local gate synthesis when only {\em homogeneous} local unitaries are performed in order to tailor the available interaction. Homogeneous (i.e. identical for all subsystems) local manipulation implies a more refined classification of interaction Hamiltonians than the inhomogeneous case, as well as the loss of universality in Hamiltonian simulation. For the case of symmetric two--qubit interactions, we provide time--optimal protocols for both Hamiltonian simulation and gate synthesis.  

Quantum computer architecture for fast entropy extraction (pp297-306)
        A.M. Steane
If a quantum computer is stabilized by fault-tolerant quantum error correction (QEC), then most of its resources (qubits and operations) are dedicated to the extraction of error information. Analysis of this process leads to a set of central requirements for candidate computing devices, in addition to the basic ones of stable qubits and controllable gates and measurements. The logical structure of the extraction process has a natural geometry and hierarchy of communication needs; a computer whose physical architecture is designed to reflect this will be able to tolerate the most noise. The relevant networks are dominated by quantum information transport, therefore to assess a computing device it is necessary to characterize its ability to transport quantum information, in addition to assessing the performance of conditional logic on nearest neighbours and the passive stability of the memory. The transport distances involved in QEC networks are estimated, and it is found that a device relying on swap operations for information transport must have those operations an order of magnitude more precise than the controlled gates of a device which can transport information at low cost.

Stabilizer codes can be realized as graph codes (pp307-323)
        D Schlingemann
We establish the connection between a recent new construction  technique for quantum error correcting codes, based on graphs, and the so-called stabilizer codes: Each stabilizer code can be realized as a graph code and vice versa.

ROM-based computation: quantum versus classical (pp324-332)
        B.C. Travaglione, M.A. Nielsen, H.M. Wiseman and A. Ambainis
We introduce a model of computation based on \emph{read only memory} (ROM), which allows us to compare the space-efficiency of reversible, error-free classical computation with reversible, error-free quantum computation. We show that a ROM-based quantum computer with one writable qubit is universal, whilst two writable bits are required for a universal classical ROM-based computer. We also comment on the time-efficiency advantages of quantum computation within this model.

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