746 research outputs found
A quantum computer only needs one universe
The nature of quantum computation is discussed. It is argued that, in terms
of the amount of information manipulated in a given time, quantum and classical
computation are equally efficient. Quantum superposition does not permit
quantum computers to ``perform many computations simultaneously'' except in a
highly qualified and to some extent misleading sense. Quantum computation is
therefore not well described by interpretations of quantum mechanics which
invoke the concept of vast numbers of parallel universes. Rather, entanglement
makes available types of computation process which, while not exponentially
larger than classical ones, are unavailable to classical systems. The essence
of quantum computation is that it uses entanglement to generate and manipulate
a physical representation of the correlations between logical entities, without
the need to completely represent the logical entities themselves.Comment: 13 pages. The paper has undergone major changes, in order to
stengthen the argument and cut extraneous material. Schrodinger's Cat has
been cut. The "one-way computer" model is now included, and the other remarks
tightened. A positive statement on what a QC is, as opposed to what it is
not, is adde
Numerical simulation of information recovery in quantum computers
Decoherence is the main problem to be solved before quantum computers can be
built. To control decoherence, it is possible to use error correction methods,
but these methods are themselves noisy quantum computation processes. In this
work we study the ability of Steane's and Shor's fault-tolerant recovering
methods, as well a modification of Steane's ancilla network, to correct errors
in qubits. We test a way to measure correctly ancilla's fidelity for these
methods, and state the possibility of carrying out an effective error
correction through a noisy quantum channel, even using noisy error correction
methods.Comment: 38 pages, Figures included. Accepted in Phys. Rev. A, 200
Active stabilisation, quantum computation and quantum state synthesis
Active stabilisation of a quantum system is the active suppression of noise
(such as decoherence) in the system, without disrupting its unitary evolution.
Quantum error correction suggests the possibility of achieving this, but only
if the recovery network can suppress more noise than it introduces. A general
method of constructing such networks is proposed, which gives a substantial
improvement over previous fault tolerant designs. The construction permits
quantum error correction to be understood as essentially quantum state
synthesis. An approximate analysis implies that algorithms involving very many
computational steps on a quantum computer can thus be made possible.Comment: 8 pages LaTeX plus 4 figures. Submitted to Phys. Rev. Let
Quantum computer architecture for fast entropy extraction
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
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