430 research outputs found
Composite Geometric Phase for Multipartite Entangled States
When an entangled state evolves under local unitaries, the entanglement in
the state remains fixed. Here we show the dynamical phase acquired by an
entangled state in such a scenario can always be understood as the sum of the
dynamical phases of its subsystems. In contrast, the equivalent statement for
the geometric phase is not generally true unless the state is separable. For an
entangled state an additional term is present, the mutual geometric phase, that
measures the change the additional correlations present in the entangled state
make to the geometry of the state space. For qubit states we find this
change can be explained solely by classical correlations for states with a
Schmidt decomposition and solely by quantum correlations for W states.Comment: 4 pages, 1 figure, improved presentation, results and conclusions
unchanged from v1. Accepted for publication in PR
Geometric phases in dressed state quantum computation
Geometric phases arise naturally in a variety of quantum systems with
observable consequences. They also arise in quantum computations when dressed
states are used in gating operations. Here we show how they arise in these
gating operations and how one may take advantage of the dressed states
producing them. Specifically, we show that that for a given, but arbitrary
Hamiltonian, and at an arbitrary time {\tau}, there always exists a set of
dressed states such that a given gate operation can be performed by the
Hamiltonian up to a phase {\phi}. The phase is a sum of a dynamical phase and a
geometric phase. We illustrate the new phase for several systems.Comment: 4 pages, 2 figure
Geometric phases in superconducting qubits beyond the two-level-approximation
Geometric phases, which accompany the evolution of a quantum system and
depend only on its trajectory in state space, are commonly studied in two-level
systems. Here, however, we study the adiabatic geometric phase in a weakly
anharmonic and strongly driven multi-level system, realised as a
superconducting transmon-type circuit. We measure the contribution of the
second excited state to the two-level geometric phase and find good agreement
with theory treating higher energy levels perturbatively. By changing the
evolution time, we confirm the independence of the geometric phase of time and
explore the validity of the adiabatic approximation at the transition to the
non-adiabatic regime.Comment: 5 pages, 3 figure
Geometric phases and quantum phase transitions
Quantum phase transition is one of the main interests in the field of
condensed matter physics, while geometric phase is a fundamental concept and
has attracted considerable interest in the field of quantum mechanics. However,
no relevant relation was recognized before recent work. In this paper, we
present a review of the connection recently established between these two
interesting fields: investigations in the geometric phase of the many-body
systems have revealed so-called "criticality of geometric phase", in which
geometric phase associated with the many-body ground state exhibits
universality, or scaling behavior in the vicinity of the critical point. In
addition, we address the recent advances on the connection of some other
geometric quantities and quantum phase transitions. The closed relation
recently recognized between quantum phase transitions and some of geometric
quantities may open attractive avenues and fruitful dialog between different
scientific communities.Comment: Invited review article for IJMPB; material covered till June 2007; 10
page
Spin-1/2 geometric phase driven by decohering quantum fields
We calculate the geometric phase of a spin-1/2 system driven by a one and two
mode quantum field subject to decoherence. Using the quantum jump approach, we
show that the corrections to the phase in the no-jump trajectory are different
when considering an adiabatic and non-adiabatic evolution. We discuss the
implications of our results from both the fundamental as well as quantum
computational perspective.Comment: 4 page
Black Hole Production by Cosmic Rays
Ultra-high energy cosmic rays create black holes in scenarios with extra
dimensions and TeV-scale gravity. In particular, cosmic neutrinos will produce
black holes deep in the atmosphere, initiating quasi-horizontal showers far
above the standard model rate. At the Auger Observatory, hundreds of black hole
events may be observed, providing evidence for extra dimensions and the first
opportunity for experimental study of microscopic black holes. If no black
holes are found, the fundamental Planck scale must be above 2 TeV for any
number of extra dimensions.Comment: 4 pages, 4 figures, PRL versio
Pushmepullyou: An efficient micro-swimmer
The swimming of a pair of spherical bladders that change their volumes and
mutual distance is efficient at low Reynolds numbers and is superior to other
models of artificial swimmers. The change of shape resembles the wriggling
motion known as {\it metaboly} of certain protozoa.Comment: Minor rephrasing and changes in style; short explanations adde
Nexus between quantum criticality and the chemical potential pinning in high- cuprates
For strongly correlated electrons the relation between total number of charge
carriers and the chemical potential reveals for large Coulomb
energy the apparently paradoxical pinning of within the Mott gap, as
observed in high- cuprates. By unravelling consequences of the non-trivial
topology of the charge gauge U(1) group and the associated ground state
degeneracy we found a close kinship between the pinning of and the
zero-temperature divergence of the charge compressibility , which marks a novel quantum criticality governed by
topological charges rather than Landau principle of the symmetry breaking.Comment: 4+ pages, 2 figures, typos corrected, version as publishe
The Quantum Geometric Phase between Orthogonal States
We show that the geometric phase between any two states, including orthogonal
states, can be computed and measured using the notion of projective
measurement, and we show that a topological number can be extracted in the
geometric phase change in an infinitesimal loop near an orthogonal state. Also,
the Pancharatnam phase change during the passage through an orthogonal state is
shown to be either or zero (mod ). All the off-diagonal geometric
phases can be obtained from the projective geometric phase calculated with our
generalized connection
Validity of adiabaticity in Cavity QED
This paper deals with the concept of adiabaticity for fully quantum
mechanically cavity QED models. The physically interesting cases of Gaussian
and standing wave shapes of the cavity mode are considered. An analytical
approximate measure for adiabaticity is given and compared with numerical wave
packet simulations. Good agreement is obtained where the approximations are
expected to be valid. Usually for cavity QED systems, the large atom-field
detuning case is considered as the adiabatic limit. We, however, show that
adiabaticity is also valid, for the Gaussian mode shape, in the opposite limit.
Effective semiclassical time dependent models, which do not take into account
the shape of the wave packet, are derived. Corrections to such an effective
theory, which are purely quantum mechanical, are discussed. It is shown that
many of the results presented can be applied to time dependent two-level
systems.Comment: 10 pages, 9 figure
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