1,678 research outputs found
Against Quantum Noise
This is a brief description of how to protect quantum states from dissipation
and decoherence that arise due to uncontrolled interactions with the
environment. We discuss recoherence and stabilisation of quantum states based
on two techniques known as "symmetrisation" and "quantum error correction". We
illustrate our considerations with the most popular quantum-optical model of
the system-environment interaction, commonly used to describe spontaneous
emission, and show the benefits of quantum error correction in this case.Comment: 12 pages. Presented at the International Conference "Quantum Optics
IV", Jaszowiec, Poland, June 17-24 1997. An introductory overview of quantum
dissipation and error correction. Late submission to the archive due to
requests and the limited availability of the journa
Universality in Quantum Computation
We show that in quantum computation almost every gate that operates on two or
more bits is a universal gate. We discuss various physical considerations
bearing on the proper definition of universality for computational components
such as logic gates.Comment: 11 pages, LaTe
Quantum Algorithms: Entanglement Enhanced Information Processing
We discuss the fundamental role of entanglement as the essential nonclassical
feature providing the computational speed-up in the known quantum algorithms.
We review the construction of the Fourier transform on an Abelian group and the
principles underlying the fast Fourier transform algorithm. We describe the
implementation of the FFT algorithm for the group of integers modulo 2^n in the
quantum context, showing how the group-theoretic formalism leads to the
standard quantum network and identifying the property of entanglement that
gives rise to the exponential speedup (compared to the classical FFT). Finally
we outline the use of the Fourier transform in extracting periodicities, which
underlies its utility in the known quantum algorithms.Comment: 17 pages latex, no figures. To appear in Phil. Trans. Roy. Soc.
(Lond.) 1998, Proceedings of Royal Society Discussion Meeting ``Quantum
Computation: Theory and Experiment'', held in November 199
A Universal Two--Bit Gate for Quantum Computation
We prove the existence of a class of two--input, two--output gates any one of
which is universal for quantum computation. This is done by explicitly
constructing the three--bit gate introduced by Deutsch [Proc.~R.~Soc.~London.~A
{\bf 425}, 73 (1989)] as a network consisting of replicas of a single two--bit
gate.Comment: 3 pages, RevTeX, two figures in a uuencoded fil
From quantum circuits to adiabatic algorithms
This paper explores several aspects of the adiabatic quantum computation
model. We first show a way that directly maps any arbitrary circuit in the
standard quantum computing model to an adiabatic algorithm of the same depth.
Specifically, we look for a smooth time-dependent Hamiltonian whose unique
ground state slowly changes from the initial state of the circuit to its final
state. Since this construction requires in general an n-local Hamiltonian, we
will study whether approximation is possible using previous results on ground
state entanglement and perturbation theory. Finally we will point out how the
adiabatic model can be relaxed in various ways to allow for 2-local partially
adiabatic algorithms as well as 2-local holonomic quantum algorithms.Comment: Version accepted by and to appear in Phys. Rev.
Mirror Inversion of Quantum States in Linear Registers
Transfer of data in linear quantum registers can be significantly simplified
with pre-engineered but not dynamically controlled inter-qubit couplings. We
show how to implement a mirror inversion of the state of the register in each
excitation subspace with respect to the centre of the register. Our
construction is especially appealing as it requires no dynamical control over
individual inter-qubit interactions. If, however, individual control of the
interactions is available then the mirror inversion operation can be performed
on any substring of qubits in the register. In this case a sequence of mirror
inversions can generate any permutation of a quantum state of the involved
qubits.Comment: 4 page
Geometric phases induced in auxiliary qubits by many-body systems near its critical points
The geometric phase induced in an auxiliary qubit by a many-body system is
calculated and discussed. Two kinds of coupling between the auxiliary qubit and
the many-body system are considered, which lead to dephasing and dissipation in
the qubit, respectively. As an example, we consider the XY spin-chain
dephasingly couple to a qubit, the geometric phase induced in the qubit is
presented and discussed. The results show that the geometric phase might be
used to signal the critical points of the many-body system, and it tends to
zero with the parameters of the many-body system going away from the critical
points
Stabilisation of Quantum Computations by Symmetrisation
We propose a method for the stabilisation of quantum computations (including
quantum state storage). The method is based on the operation of projection into
, the symmetric subspace of the full state space of redundant
copies of the computer. We describe an efficient algorithm and quantum network
effecting --projection and discuss the stabilising effect of the
proposed method in the context of unitary errors generated by hardware
imprecision, and nonunitary errors arising from external environmental
interaction. Finally, limitations of the method are discussed.Comment: 20 pages LaTeX, 2 postscript figure
On Quantum Algorithms
Quantum computers use the quantum interference of different computational
paths to enhance correct outcomes and suppress erroneous outcomes of
computations. In effect, they follow the same logical paradigm as
(multi-particle) interferometers. We show how most known quantum algorithms,
including quantum algorithms for factorising and counting, may be cast in this
manner. Quantum searching is described as inducing a desired relative phase
between two eigenvectors to yield constructive interference on the sought
elements and destructive interference on the remaining terms.Comment: 15 pages, 8 figure
Optimal State Discrimination Using Particle Statistics
We present an application of particle statistics to the problem of optimal
ambiguous discrimination of quantum states. The states to be discriminated are
encoded in the internal degrees of freedom of identical particles, and we use
the bunching and antibunching of the external degrees of freedom to
discriminate between various internal states. We show that we can achieve the
optimal single-shot discrimination probability using only the effects of
particle statistics. We discuss interesting applications of our method to
detecting entanglement and purifying mixed states. Our scheme can easily be
implemented with the current technology
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