894 research outputs found
PT-Symmetric Representations of Fermionic Algebras
A recent paper by Jones-Smith and Mathur extends PT-symmetric quantum
mechanics from bosonic systems (systems for which ) to fermionic systems
(systems for which ). The current paper shows how the formalism
developed by Jones-Smith and Mathur can be used to construct PT-symmetric
matrix representations for operator algebras of the form ,
, , where
. It is easy to construct matrix
representations for the Grassmann algebra (). However, one can only
construct matrix representations for the fermionic operator algebra
() if ; a matrix representation does not exist for the
conventional value .Comment: 5 pages, 2 figure
Quantum Phase Transition of Ground-state Entanglement in a Heisenberg Spin Chain Simulated in an NMR Quantum Computer
Using an NMR quantum computer, we experimentally simulate the quantum phase
transition of a Heisenberg spin chain. The Hamiltonian is generated by a
multiple pulse sequence, the nuclear spin system is prepared in its
(pseudo-pure) ground state and the effective Hamiltonian varied in such a way
that the Heisenberg chain is taken from a product state to an entangled state
and finally to a different product state.Comment: 5 pages, 5 eps figures. Accepted in Phys. Rev.
Quantum teleportation scheme by selecting one of multiple output ports
The scheme of quantum teleportation, where Bob has multiple (N) output ports
and obtains the teleported state by simply selecting one of the N ports, is
thoroughly studied. We consider both deterministic version and probabilistic
version of the teleportation scheme aiming to teleport an unknown state of a
qubit. Moreover, we consider two cases for each version: (i) the state employed
for the teleportation is fixed to a maximally entangled state, and (ii) the
state is also optimized as well as Alice's measurement. We analytically
determine the optimal protocols for all the four cases, and show the
corresponding optimal fidelity or optimal success probability. All these
protocols can achieve the perfect teleportation in the asymptotic limit of
. The entanglement properties of the teleportation scheme are also
discussed.Comment: 14 pages, 4 figure
Dynamical critical scaling and effective thermalization in quantum quenches: the role of the initial state
We explore the robustness of universal dynamical scaling behavior in a
quantum system near criticality with respect to initialization in a large class
of states with finite energy. By focusing on a homogeneous XY quantum spin
chain in a transverse field, we characterize the non-equilibrium response under
adiabatic and sudden quench processes originating from a pure as well as a
mixed excited initial state, and involving either a regular quantum critical or
a multicritical point. We find that the critical exponents of the ground-state
quantum phase transition can be encoded in the dynamical scaling exponents
despite the finite energy of the initial state. In particular, we identify
conditions on the initial distribution of quasi-particle excitation which
ensure Kibble-Zurek scaling to persist. The emergence of effective thermal
equilibrium behavior following a sudden quench towards criticality is also
investigated, with focus on the long-time dynamics of the quasi-particle
excitation. For a quench to a regular quantum critical point, this observable
is found to behave thermally provided that the system is prepared at
sufficiently high temperature, whereas thermalization fails to occur in
quenches taking the system towards a multi-critical point. We argue that the
observed lack of thermalization originates in this case in the asymmetry of the
impulse region that is also responsible for anomalous multicritical dynamical
scaling.Comment: 18 pages, 13 eps color figures, published versio
Stable black hole solutions with non-Abelian fields
We construct finite mass, asymptotically flat black hole solutions in d=4
Einstein-Yang-Mills theory augmented with higher order curvature terms of the
gauge field. They possess non-Abelian hair in addition to Coulomb electric
charge, and, below some non-zero critical temperature, they are
thermodynamically preferred over the Reissner-Nordstrom solution. Our results
indicate the existence of hairy non-Abelian black holes which are stable under
linear, spherically symmetric perturbations.Comment: 8 pages, 3 figure
Asymptotically flat, stable black hole solutions in Einstein--Yang-Mills--Chern-Simons theory
We construct finite mass, asymptotically flat black hole solutions in d=5
Einstein--Yang-Mills--Chern-Simons theory. Our results indicate the existence
of a second order phase transition between Reissner-Nordstrom solutions and the
non-Abelian black holes which generically are thermodynamically preferred. Some
of the non-Abelian configurations are also stable under linear, spherically
symmetric perturbations. In addition a solution in closed form describing an
extremal black hole with non-Abelian hair is found for a special value of the
Chern-Simons coupling constant.Comment: 9 pages, 3 figure
Observation of the ground-state-geometric phase in a Heisenberg XY model
Geometric phases play a central role in a variety of quantum phenomena,
especially in condensed matter physics. Recently, it was shown that this
fundamental concept exhibits a connection to quantum phase transitions where
the system undergoes a qualitative change in the ground state when a control
parameter in its Hamiltonian is varied. Here we report the first experimental
study using the geometric phase as a topological test of quantum transitions of
the ground state in a Heisenberg XY spin model. Using NMR interferometry, we
measure the geometric phase for different adiabatic circuits that do not pass
through points of degeneracy.Comment: manuscript (4 pages, 3 figures) + supporting online material (6 pages
+ 7 figures), to be published in Phys. Rev. Lett. (2010
Coordinate space proton-deuteron scattering calculations including Coulomb force effects
We present a practical method to solve the proton-deuteron scattering problem
at energies above the three-body breakup threshold, in which we treat
three-body integral equations in coordinate space accommodating long-range
proton-proton Coulomb interactions. The method is examined for phase shift
parameters, and then applied to calculations of differential cross sections in
elastic and breakup reactions, analyzing powers, etc. with a realistic
nucleon-nucleon force and three-nucleon forces. Effects of the Coulomb force
and the three-nucleon forces on these observables are discussed in comparing
with experimental data.Comment: 15 pages, 14 figures, submitted to PR
Geometric phase for an adiabatically evolving open quantum system
We derive an elegant solution for a two-level system evolving adiabatically
under the influence of a driving field with a time-dependent phase, which
includes open system effects such as dephasing and spontaneous emission. This
solution, which is obtained by working in the representation corresponding to
the eigenstates of the time-dependent Hermitian Hamiltonian, enables the
dynamic and geometric phases of the evolving density matrix to be separated and
relatively easily calculated.Comment: 10 pages, 0 figure
Conductance calculations for quantum wires and interfaces: mode matching and Green functions
Landauer's formula relates the conductance of a quantum wire or interface to
transmission probabilities. Total transmission probabilities are frequently
calculated using Green function techniques and an expression first derived by
Caroli. Alternatively, partial transmission probabilities can be calculated
from the scattering wave functions that are obtained by matching the wave
functions in the scattering region to the Bloch modes of ideal bulk leads. An
elegant technique for doing this, formulated originally by Ando, is here
generalized to any Hamiltonian that can be represented in tight-binding form. A
more compact expression for the transmission matrix elements is derived and it
is shown how all the Green function results can be derived from the mode
matching technique. We illustrate this for a simple model which can be studied
analytically, and for an Fe|vacuum|Fe tunnel junction which we study using
first-principles calculations.Comment: 14 pages, 5 figure
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