7,445 research outputs found
Programmable networks for quantum algorithms
The implementation of a quantum computer requires the realization of a large
number of N-qubit unitary operations which represent the possible oracles or
which are part of the quantum algorithm. Until now there are no standard ways
to uniformly generate whole classes of N-qubit gates. We have developed a
method to generate arbitrary controlled phase shift operations with a single
network of one-qubit and two-qubit operations. This kind of network can be
adapted to various physical implementations of quantum computing and is
suitable to realize the Deutsch-Jozsa algorithm as well as Grover's search
algorithm.Comment: 4 pages. Accepted version; Journal-ref. adde
Quantum Analogue Computing
We briefly review what a quantum computer is, what it promises to do for us,
and why it is so hard to build one. Among the first applications anticipated to
bear fruit is quantum simulation of quantum systems. While most quantum
computation is an extension of classical digital computation, quantum
simulation differs fundamentally in how the data is encoded in the quantum
computer. To perform a quantum simulation, the Hilbert space of the system to
be simulated is mapped directly onto the Hilbert space of the (logical) qubits
in the quantum computer. This type of direct correspondence is how data is
encoded in a classical analogue computer. There is no binary encoding, and
increasing precision becomes exponentially costly: an extra bit of precision
doubles the size of the computer. This has important consequences for both the
precision and error correction requirements of quantum simulation, and
significant open questions remain about its practicality. It also means that
the quantum version of analogue computers, continuous variable quantum
computers (CVQC) becomes an equally efficient architecture for quantum
simulation. Lessons from past use of classical analogue computers can help us
to build better quantum simulators in future.Comment: 10 pages, to appear in the Visions 2010 issue of Phil. Trans. Roy.
Soc.
Sideband cooling while preserving coherences in the nuclear spin state in group-II-like atoms
We propose a method for laser cooling group-II-like atoms without changing
the quantum state of their nuclear spins, thus preserving coherences that are
usually destroyed by optical pumping. As group-II-like atoms have a
closed-shell ground state, nuclear spin and electronic degrees of freedom are
decoupled, allowing for independent manipulation. The hyperfine interaction
that couples these degrees of freedom in excited states can be suppressed
through the application of external magnetic fields. Our protocol employs
resolved-sideband cooling on the forbidden clock transition, , with quenching via coupling to the rapidly decaying state,
deep in the Paschen-Back regime. This makes it possible to laser cool neutral
atomic qubits without destroying the quantum information stored in their
nuclear spins, as shown in two examples, Yb and Sr.Comment: 4 pages, 3 figures v4: minor changes in text, changes in the
references, published versio
Efficient and accurate three dimensional Poisson solver for surface problems
We present a method that gives highly accurate electrostatic potentials for
systems where we have periodic boundary conditions in two spatial directions
but free boundary conditions in the third direction. These boundary conditions
are needed for all kind of surface problems. Our method has an O(N log N)
computational cost, where N is the number of grid points, with a very small
prefactor. This Poisson solver is primarily intended for real space methods
where the charge density and the potential are given on a uniform grid.Comment: 6 pages, 2 figure
Adiabatic Quantum Computation in Open Systems
We analyze the performance of adiabatic quantum computation (AQC) under the
effect of decoherence. To this end, we introduce an inherently open-systems
approach, based on a recent generalization of the adiabatic approximation. In
contrast to closed systems, we show that a system may initially be in an
adiabatic regime, but then undergo a transition to a regime where adiabaticity
breaks down. As a consequence, the success of AQC depends sensitively on the
competition between various pertinent rates, giving rise to optimality
criteria.Comment: v2: 4 pages, 1 figure. Published versio
Conditional Quantum Dynamics and Logic Gates
Quantum logic gates provide fundamental examples of conditional quantum
dynamics. They could form the building blocks of general quantum information
processing systems which have recently been shown to have many interesting
non--classical properties. We describe a simple quantum logic gate, the quantum
controlled--NOT, and analyse some of its applications. We discuss two possible
physical realisations of the gate; one based on Ramsey atomic interferometry
and the other on the selective driving of optical resonances of two subsystems
undergoing a dipole--dipole interaction.Comment: 5 pages, RevTeX, two figures in a uuencoded, compressed fil
The Meinunger "Nicht Rote" Objects
Four high-latitude slow variable stars have been noted by Meinunger (1972) as
"nicht rote" ("not red") objects and thus curious. We have previously reported
(Margon & Deutsch 1997) that one of these objects, CC Boo, is in fact a QSO.
Here we present observations demonstrating that the remaining three are also
highly variable active galactic nuclei. The most interesting object of the four
is perhaps S 10765 (= NGP9 F324-0276706), which proves to be a resolved galaxy
at z=0.063. Despite the rapid and large reported variability amplitude (~1.6
mag), the spectrum is that of a perfectly normal galaxy, with no emission lines
or evident nonthermal continuum. We also present new spectroscopic and
photometric observations for AR CVn, suggested by Meinunger to be an RR Lyrae
star despite its very faint magnitude (=19.4). The object is indeed one of
the most distant RR Lyrae stars known, at a galactocentric distance of ~40 kpc.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific, Volume 111, January 1999; 14 pages including 4 figures and 1
tabl
Quantum Ballistic Evolution in Quantum Mechanics: Application to Quantum Computers
Quantum computers are important examples of processes whose evolution can be
described in terms of iterations of single step operators or their adjoints.
Based on this, Hamiltonian evolution of processes with associated step
operators is investigated here. The main limitation of this paper is to
processes which evolve quantum ballistically, i.e. motion restricted to a
collection of nonintersecting or distinct paths on an arbitrary basis. The main
goal of this paper is proof of a theorem which gives necessary and sufficient
conditions that T must satisfy so that there exists a Hamiltonian description
of quantum ballistic evolution for the process, namely, that T is a partial
isometry and is orthogonality preserving and stable on some basis. Simple
examples of quantum ballistic evolution for quantum Turing machines with one
and with more than one type of elementary step are discussed. It is seen that
for nondeterministic machines the basis set can be quite complex with much
entanglement present. It is also proved that, given a step operator T for an
arbitrary deterministic quantum Turing machine, it is decidable if T is stable
and orthogonality preserving, and if quantum ballistic evolution is possible.
The proof fails if T is a step operator for a nondeterministic machine. It is
an open question if such a decision procedure exists for nondeterministic
machines. This problem does not occur in classical mechanics.Comment: 37 pages Latexwith 2 postscript figures tar+gzip+uuencoded, to be
published in Phys. Rev.
The Hidden Subgroup Problem and Eigenvalue Estimation on a Quantum Computer
A quantum computer can efficiently find the order of an element in a group,
factors of composite integers, discrete logarithms, stabilisers in Abelian
groups, and `hidden' or `unknown' subgroups of Abelian groups. It is already
known how to phrase the first four problems as the estimation of eigenvalues of
certain unitary operators. Here we show how the solution to the more general
Abelian `hidden subgroup problem' can also be described and analysed as such.
We then point out how certain instances of these problems can be solved with
only one control qubit, or `flying qubits', instead of entire registers of
control qubits.Comment: 16 pages, 3 figures, LaTeX2e, to appear in Proceedings of the 1st
NASA International Conference on Quantum Computing and Quantum Communication
(Springer-Verlag
Experimental application of decoherence-free subspaces in a quantum-computing algorithm
For a practical quantum computer to operate, it will be essential to properly
manage decoherence. One important technique for doing this is the use of
"decoherence-free subspaces" (DFSs), which have recently been demonstrated.
Here we present the first use of DFSs to improve the performance of a quantum
algorithm. An optical implementation of the Deutsch-Jozsa algorithm can be made
insensitive to a particular class of phase noise by encoding information in the
appropriate subspaces; we observe a reduction of the error rate from 35% to
essentially its pre-noise value of 8%.Comment: 11 pages, 4 figures, submitted to PR
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