6,666 research outputs found
Deutsch-Jozsa algorithm as a test of quantum computation
A redundancy in the existing Deutsch-Jozsa quantum algorithm is removed and a
refined algorithm, which reduces the size of the register and simplifies the
function evaluation, is proposed. The refined version allows a simpler analysis
of the use of entanglement between the qubits in the algorithm and provides
criteria for deciding when the Deutsch-Jozsa algorithm constitutes a meaningful
test of quantum computation.Comment: 10 pages, 2 figures, RevTex, Approved for publication in Phys Rev
Quantum Faraday Effect in Double-Dot Aharonov-Bohm Ring
We investigate Faraday's law of induction manifested in the quantum state of
Aharonov-Bohm loops. In particular, we propose a flux-switching experiment for
a double-dot AB ring to verify the phase shift induced by Faraday's law. We
show that the induced {\em Faraday phase} is geometric and nontopological. Our
study demonstrates that the relation between the local phases of a ring at
different fluxes is not arbitrary but is instead determined by Faraday's
inductive law, which is in strong contrast to the arbitrary local phase of an
Aharonov-Bohm ring for a given flux.Comment: Submitted to Phys. Rev. Let
Approximate quantum error correction can lead to better codes
We present relaxed criteria for quantum error correction which are useful
when the specific dominant noise process is known. These criteria have no
classical analogue. As an example, we provide a four-bit code which corrects
for a single amplitude damping error. This code violates the usual Hamming
bound calculated for a Pauli description of the error process, and does not fit
into the GF(4) classification.Comment: 7 pages, 2 figures, submitted to Phys. Rev.
Full characterization of a three-photon GHZ state using quantum state tomography
We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.Comment: 3 pages, 3 figure
Combined Error Correction Techniques for Quantum Computing Architectures
Proposals for quantum computing devices are many and varied. They each have
unique noise processes that make none of them fully reliable at this time.
There are several error correction/avoidance techniques which are valuable for
reducing or eliminating errors, but not one, alone, will serve as a panacea.
One must therefore take advantage of the strength of each of these techniques
so that we may extend the coherence times of the quantum systems and create
more reliable computing devices. To this end we give a general strategy for
using dynamical decoupling operations on encoded subspaces. These encodings may
be of any form; of particular importance are decoherence-free subspaces and
quantum error correction codes. We then give means for empirically determining
an appropriate set of dynamical decoupling operations for a given experiment.
Using these techniques, we then propose a comprehensive encoding solution to
many of the problems of quantum computing proposals which use exchange-type
interactions. This uses a decoherence-free subspace and an efficient set of
dynamical decoupling operations. It also addresses the problems of
controllability in solid state quantum dot devices.Comment: Contribution to Proceedings of the 2002 Physics of Quantum
Electronics Conference", to be published in J. Mod. Optics. This paper
provides a summary and review of quant-ph/0205156 and quant-ph/0112054, and
some new result
Scheme for direct measurement of a general two-qubit Hamiltonian
The construction of two-qubit gates appropriate for universal quantum
computation is of enormous importance to quantum information processing.
Building such gates is dependent on accurate knowledge of the interaction
dynamics between two qubit systems. This letter will present a systematic
method for reconstructing the full two-qubit interaction Hamiltonian through
experimental measures of concurrence. This not only gives a convenient method
for constructing two qubit quantum gates, but can also be used to
experimentally determine various Hamiltonian parameters in physical systems. We
show explicitly how this method can be employed to determine the first and
second order spin-orbit corrections to the exchange coupling in quantum dots.Comment: 4 Pages, 1 Figur
Unified model for vortex-string network evolution
We describe and numerically test the velocity-dependent one-scale (VOS)
string evolution model, a simple analytic approach describing a string network
with the averaged correlation length and velocity. We show that it accurately
reproduces the large-scale behaviour (in particular the scaling laws) of
numerical simulations of both Goto-Nambu and field theory string networks. We
explicitly demonstrate the relation between the high-energy physics approach
and the damped and non-relativistic limits which are relevant for condensed
matter physics. We also reproduce experimental results in this context and show
that the vortex-string density is significantly reduced by loop production, an
effect not included in the usual `coarse-grained' approach.Comment: 5 pages; v2: cosmetic changes, version to appear in PR
Simulating quantum operations with mixed environments
We study the physical resources required to implement general quantum
operations, and provide new bounds on the minimum possible size which an
environment must be in order to perform certain quantum operations. We prove
that contrary to a previous conjecture, not all quantum operations on a
single-qubit can be implemented with a single-qubit environment, even if that
environment is initially prepared in a mixed state. We show that a mixed
single-qutrit environment is sufficient to implement a special class of
operations, the generalized depolarizing channels.Comment: 4 pages Revtex + 1 fig, pictures at
http://stout.physics.ucla.edu/~smolin/tetrahedron .Several small correction
A cryogenic surface-electrode elliptical ion trap for quantum simulation
Two-dimensional crystals of trapped ions are a promising system with which to
implement quantum simulations of challenging problems such as spin frustration.
Here, we present a design for a surface-electrode elliptical ion trap which
produces a 2-D ion crystal and is amenable to microfabrication, which would
enable higher simulated coupling rates, as well as interactions based on
magnetic forces generated by on-chip currents. Working in an 11 K cryogenic
environment, we experimentally verify to within 5% a numerical model of the
structure of ion crystals in the trap. We also explore the possibility of
implementing quantum simulation using magnetic forces, and calculate J-coupling
rates on the order of 10^3 / s for an ion crystal height of 10 microns, using a
current of 1 A
The Exotic Statistics of Leapfrogging Smoke Rings
The leapfrogging motion of smoke rings is a three dimensional version of the
motion that in two dimensions leads to exotic exchange statistics. The
statistical phase factor can be computed using the hydrodynamical Euler
equation, which is a universal law for describing the properties of a large
class of fluids. This suggests that three dimensional exotic exchange
statistics is a common property of closed vortex loops in a variety of quantum
liquids and gases, from helium superfluids to Bose-Einstein condensed alkali
gases, metallic hydrogen in its liquid phases and maybe even nuclear matter in
extreme conditions.Comment: 9 pages 1 figur
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