5,540 research outputs found
Quantum Bit Regeneration
Decoherence and loss will limit the practicality of quantum cryptography and
computing unless successful error correction techniques are developed. To this
end, we have discovered a new scheme for perfectly detecting and rejecting the
error caused by loss (amplitude damping to a reservoir at T=0), based on using
a dual-rail representation of a quantum bit. This is possible because (1)
balanced loss does not perform a ``which-path'' measurement in an
interferometer, and (2) balanced quantum nondemolition measurement of the
``total'' photon number can be used to detect loss-induced quantum jumps
without disturbing the quantum coherence essential to the quantum bit. Our
results are immediately applicable to optical quantum computers using single
photonics devices.Comment: 4 pages, postscript only, figures available at
http://feynman.stanford.edu/qcom
Incomplete quantum process tomography and principle of maximal entropy
The main goal of this paper is to extend and apply the principle of maximum
entropy (MaxEnt) to incomplete quantum process estimation tasks. We will define
a so-called process entropy function being the von Neumann entropy of the state
associated with the quantum process via Choi-Jamiolkowski isomorphism. It will
be shown that an arbitrary process estimation experiment can be reformulated in
a unified framework and MaxEnt principle can be consistently exploited. We will
argue that the suggested choice for the process entropy satisfies natural list
of properties and it reduces to the state MaxEnt principle, if applied to
preparator devices.Comment: 8 pages, comments welcome, references adde
Polarization state of a biphoton: quantum ternary logic
Polarization state of biphoton light generated via collinear
frequency-degenerate spontaneous parametric down-conversion is considered. A
biphoton is described by a three-component polarization vector, its arbitrary
transformations relating to the SU(3) group. A subset of such transformations,
available with retardation plates, is realized experimentally. In particular,
two independent orthogonally polarized beams of type-I biphotons are
transformed into a beam of type-II biphotons. Polarized biphotons are suggested
as ternary analogs of two-state quantum systems (qubits)
Wall-crossing, open BPS counting and matrix models
We consider wall-crossing phenomena associated to the counting of D2-branes
attached to D4-branes wrapping lagrangian cycles in Calabi-Yau manifolds, both
from M-theory and matrix model perspective. Firstly, from M-theory viewpoint,
we review that open BPS generating functions in various chambers are given by a
restriction of the modulus square of the open topological string partition
functions. Secondly, we show that these BPS generating functions can be
identified with integrands of matrix models, which naturally arise in the free
fermion formulation of corresponding crystal models. A parameter specifying a
choice of an open BPS chamber has a natural, geometric interpretation in the
crystal model. These results extend previously known relations between open
topological string amplitudes and matrix models to include chamber dependence.Comment: 25 pages, 8 figures, published versio
Avoiding Quantum Chaos in Quantum Computation
We study a one-dimensional chain of nuclear spins in an external
time-dependent magnetic field. This model is considered as a possible candidate
for experimental realization of quantum computation. According to the general
theory of interacting particles, one of the most dangerous effects is quantum
chaos which can destroy the stability of quantum operations. According to the
standard viewpoint, the threshold for the onset of quantum chaos due to an
interaction between spins (qubits) strongly decreases with an increase of the
number of qubits. Contrary to this opinion, we show that the presence of a
magnetic field gradient helps to avoid quantum chaos which turns out to
disappear with an increase of the number of qubits. We give analytical
estimates which explain this effect, together with numerical data supportingComment: RevTex, 5 pages including 3 eps-figure
Defect Formation in Quench-Cooled Superfluid Phase Transition
We use neutron absorption in rotating 3He-B to heat locally a 10
micrometer-size volume into normal phase. When the heated region cools back in
microseconds, vortex lines are formed. We record with NMR the number of lines
as a function of superflow velocity and compare to the Kibble-Zurek theory of
vortex-loop freeze-out from a random network of defects. The measurements
confirm the calculated loop-size distribution and show that also the superfluid
state itself forms as a patchwork of competing A and B phase blobs. This
explains the A to B transition in supercooled neutron-irradiated 3He-A.Comment: RevTex file, 4 pages, 3 figures, resubmitted to Phys. Rev. Let
Doubling of the bands in overdoped Bi2Sr2CaCu2O8-probable evidence for c-axis bilayer coupling
We present high resolution ARPES data of the bilayer superconductor
Bi2Sr2CaCu2O8 (Bi2212) showing a clear doubling of the near EF bands. This
splitting approaches zero along the (0,0)-(pi,pi) nodal line and is not
observed in single layer Bi2Sr2CuO6 (Bi2201), suggesting that the splitting is
due to the long sought after bilayer splitting effect. The splitting has a
magnitude of approximately 75 meV near the middle of the zone, extrapolating to
about 100 meV near the (pi,0) poin
Density of Bloch Waves after a Quench
Production of Bloch waves during a rapid quench is studied by analytical and
numerical methods. The density of Bloch waves decays exponentially with the
quench time. It also strongly depends on temperature. Very few textures are
produced for temperatures lower than a characteristic temperature proportional
to the square of the magnetic field.Comment: 4 pages in RevTex + 3 .ps files; improved presentation; version to
appear in PR
Factoring in a Dissipative Quantum Computer
We describe an array of quantum gates implementing Shor's algorithm for prime
factorization in a quantum computer. The array includes a circuit for modular
exponentiation with several subcomponents (such as controlled multipliers,
adders, etc) which are described in terms of elementary Toffoli gates. We
present a simple analysis of the impact of losses and decoherence on the
performance of this quantum factoring circuit. For that purpose, we simulate a
quantum computer which is running the program to factor N = 15 while
interacting with a dissipative environment. As a consequence of this
interaction randomly selected qubits may spontaneously decay. Using the results
of our numerical simulations we analyze the efficiency of some simple error
correction techniques.Comment: plain tex, 18 pages, 8 postscript figure
Quantum hypercomputation based on the dynamical algebra su(1,1)
An adaptation of Kieu's hypercomputational quantum algorithm (KHQA) is
presented. The method that was used was to replace the Weyl-Heisenberg algebra
by other dynamical algebra of low dimension that admits infinite-dimensional
irreducible representations with naturally defined generalized coherent states.
We have selected the Lie algebra , due to that this algebra
posses the necessary characteristics for to realize the hypercomputation and
also due to that such algebra has been identified as the dynamical algebra
associated to many relatively simple quantum systems. In addition to an
algebraic adaptation of KHQA over the algebra , we
presented an adaptations of KHQA over some concrete physical referents: the
infinite square well, the infinite cylindrical well, the perturbed infinite
cylindrical well, the P{\"o}sch-Teller potentials, the Holstein-Primakoff
system, and the Laguerre oscillator. We conclude that it is possible to have
many physical systems within condensed matter and quantum optics on which it is
possible to consider an implementation of KHQA.Comment: 25 pages, 1 figure, conclusions rewritten, typing and language errors
corrected and latex format changed minor changes elsewhere and
- …