14,561 research outputs found

### Gauge parameter dependence in gauge theories (revised: subsection 2.3)

Dependence on the gauge parameters is an important issue in gauge theories:
physical quantities have to be independent. Extending BRS transformations by
variation of the gauge parameter into a Grassmann variable one can control
gauge parameter dependence algebraically. As application we discuss the anomaly
coefficient in the Slavnov-Taylor identity, $S$-matrix elements, the vector
two-point-function and the coefficients of renormalization group and
Callan-Symanzik equation.Comment: 6, MPI-PhT/94-34, BUTP-94/1

### When a quantum measurement can be implemented locally ... and when it cannot

Local operations on subsystems and classical communication between parties
(LOCC) constitute the most general protocols available on spatially separated
quantum systems. Every LOCC protocol implements a separable generalized
measurement -- a complete measurement for which every outcome corresponds to a
tensor product of operators on individual subsystems -- but it is known that
there exist separable measurements that cannot be implemented by LOCC. A
longstanding problem in quantum information theory is to understand the
difference between LOCC and the full set of separable measurements. In this
paper, we show how to construct an LOCC protocol to implement an arbitrary
separable measurement, except that with those measurements for which no LOCC
protocol exists, the method shows explicitly that this is the case.Comment: 21 pages, 7 figures. Extensively revised to include details of all
arguments, explicitly proving all results in full rigor. Version 3 has
sections reordered and other restructuring, but otherwise contains the same
discussion as version

### Experimental scheme for qubit and qutrit symmetric informationally complete positive operator-valued measurements using multiport devices

It is crucial for various quantum information processing tasks that the state
of a quantum system can be determined reliably and efficiently from general
quantum measurements. One important class of measurements for this purpose is
symmetric informationally complete positive operator-valued measurements
(SIC-POVMs). SIC-POVMs have the advantage of providing an unbiased estimator
for the quantum state with the minimal number of outcomes needed for full
tomography. By virtue of Naimark's dilation theorem, any POVM can always be
realized with a suitable coupling between the system and an auxiliary system
and by performing a projective measurement on the joint system. In practice,
finding the appropriate coupling is rather non-trivial. Here we propose an
experimental design for directly implementing SIC-POVMs using multiport devices
and path-encoded qubits and qutrits, the utility of which has recently been
demonstrated by several experimental groups around the world. Furthermore, we
describe how these multiports can be attained in practice with an integrated
photonic system composed of nested linear optical elements.Comment: 7 pages, 5 figures; v2 published versio

### Semidirect product of CCR and CAR algebras and asymptotic states in quantum electrodynamics

A C*-algebra containing the CCR and CAR algebras as its subalgebras and
naturally described as the semidirect product of these algebras is discussed. A
particular example of this structure is considered as a model for the algebra
of asymptotic fields in quantum electrodynamics, in which Gauss' law is
respected. The appearence in this algebra of a phase variable related to
electromagnetic potential leads to the universal charge quantization.
Translationally covariant representations of this algebra with energy-momentum
spectrum in the future lightcone are investigated. It is shown that vacuum
representations are necessarily nonregular with respect to total
electromagnetic field. However, a class of translationally covariant,
irreducible representations is constructed excplicitly, which remain as close
as possible to the vacuum, but are regular at the same time. The spectrum of
energy-momentum fills the whole future lightcone, but there are no vectors with
energy-momentum lying on a mass hyperboloid or in the origin.Comment: 42 pages, LaTeX; minor corrections, a reference adde

### Simple Derivation of the Lindblad Equation

The Lindblad equation is an evolution equation for the density matrix in
quantum theory. It is the general linear, Markovian, form which ensures that
the density matrix is hermitian, trace 1, positive and completely positive.
Some elementary examples of the Lindblad equation are given. The derivation of
the Lindblad equation presented here is "simple" in that all it uses is the
expression of a hermitian matrix in terms of its orthonormal eigenvectors and
real eigenvalues. Thus, it is appropriate for students who have learned the
algebra of quantum theory. Where helpful, arguments are first given in a
two-dimensional hilbert space.Comment: To be published in the European Journal of Physic

### Hawking Radiation as Tunneling

We present a short and direct derivation of Hawking radiation as a tunneling
process, based on particles in a dynamical geometry. The imaginary part of the
action for the classically forbidden process is related to the Boltzmann factor
for emission at the Hawking temperature. Because the derivation respects
conservation laws, the exact spectrum is not precisely thermal. We compare and
contrast the problem of spontaneous emission of charged particles from a
charged conductor.Comment: LaTeX, 10 pages; v2. journal version, added section on relation of
black hole radiation to electric charge emission from a charged conducting
sphere; v3. restored cut referenc

### Supersymmetric Yang-Mills theories with local coupling: The supersymmetric gauge

Supersymmetric pure Yang-Mills theory is formulated with a local, i.e.
space-time dependent, complex coupling in superspace. Super-Yang-Mills theories
with local coupling have an anomaly, which has been first investigated in the
Wess-Zumino gauge and there identified as an anomaly of supersymmetry. In a
manifest supersymmetric formulation the anomaly appears in two other
identities: The first one describes the non-renormalization of the topological
term, the second relates the renormalization of the gauge coupling to the
renormalization of the complex supercoupling. Only one of the two identities
can be maintained in perturbation theory. We discuss the two versions and
derive the respective beta function of the local supercoupling, which is
non-holomorphic in the first version, but directly related to the coupling
renormalization, and holomorphic in the second version, but has a non-trivial,
i.e.anomalous, relation to the beta function of the gauge coupling.Comment: References correcte

### Local noise can enhance entanglement teleportation

Recently we have considered two-qubit teleportation via mixed states of four
qubits and defined the generalized singlet fraction. For single-qubit
teleportation, Badziag {\em et al.} [Phys. Rev. A {\bf 62}, 012311 (2000)] and
Bandyopadhyay [Phys. Rev. A {\bf 65}, 022302 (2002)] have obtained a family of
entangled two-qubit mixed states whose teleportation fidelity can be enhanced
by subjecting one of the qubits to dissipative interaction with the environment
via an amplitude damping channel. Here, we show that a dissipative interaction
with the local environment via a pair of time-correlated amplitude damping
channels can enhance fidelity of entanglement teleportation for a class of
entangled four-qubit mixed states. Interestingly, we find that this enhancement
corresponds to an enhancement in the quantum discord for some states.Comment: 10 page

### N=1 SYM Action and BRST Cohomology

The relation between BRST cohomology and the N=1 supersymmetric Yang-Mills
action in 4 dimensions is discussed. In particular, it is shown that both off
and on shell N=1 SYM actions are related to a lower dimensional field
polynomial by solving the descent equations, which is obtained from the
cohomological analysis of linearized Slavnov-Taylor operator \B, in the
framework of Algebraic Renormalization. Furthermore we show that off and on
shell solutions differ only by a \B- exact term, which is a consequence of
the fact that the cohomology of both cases are same.Comment: 14 Pages, LaTex. Revised version. To be published in MPL

### Fidelity and coherence measures from interference

By utilizing single particle interferometry, the fidelity or coherence of a pair of quantum states is identified with their capacity for interference. We consider processes acting on the internal degree of freedom (e.g., spin or polarization) of the interfering particle, preparing it in states ρA or ρB in the respective path of the interferometer. The maximal visibility depends on the choice of interferometer, as well as the locality or nonlocality of the preparations, but otherwise depends only on the states ρA and ρB and not the individual preparation processes themselves. This allows us to define interferometric measures which probe locality and correlation properties of spatially or temporally separated processes, and can be used to differentiate between processes that cannot be distinguished by direct process tomography using only the internal state of the particle

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