434 research outputs found
Metric Operators for Quasi-Hermitian Hamiltonians and Symmetries of Equivalent Hermitian Hamiltonians
We give a simple proof of the fact that every diagonalizable operator that
has a real spectrum is quasi-Hermitian and show how the metric operators
associated with a quasi-Hermitian Hamiltonian are related to the symmetry
generators of an equivalent Hermitian Hamiltonian.Comment: 6 pages, published versio
Delta-Function Potential with a Complex Coupling
We explore the Hamiltonian operator H=-d^2/dx^2 + z \delta(x) where x is
real, \delta(x) is the Dirac delta function, and z is an arbitrary complex
coupling constant. For a purely imaginary z, H has a (real) spectral
singularity at E=-z^2/4. For \Re(z)<0, H has an eigenvalue at E=-z^2/4. For the
case that \Re(z)>0, H has a real, positive, continuous spectrum that is free
from spectral singularities. For this latter case, we construct an associated
biorthonormal system and use it to perform a perturbative calculation of a
positive-definite inner product that renders H self-adjoint. This allows us to
address the intriguing question of the nonlocal aspects of the equivalent
Hermitian Hamiltonian for the system. In particular, we compute the energy
expectation values for various Gaussian wave packets to show that the
non-Hermiticity effect diminishes rapidly outside an effective interaction
region.Comment: Published version, 14 pages, 2 figure
Conceptual Aspects of PT-Symmetry and Pseudo-Hermiticity: A status report
We survey some of the main conceptual developments in the study of
PT-symmetric and pseudo-Hermitian Hamiltonian operators that have taken place
during the past ten years or so. We offer a precise mathematical description of
a quantum system and its representations that allows us to describe the idea of
unitarization of a quantum system by modifying the inner product of the Hilbert
space. We discuss the role and importance of the quantum-to-classical
correspondence principle that provides the physical interpretation of the
observables in quantum mechanics. Finally, we address the problem of
constructing an underlying classical Hamiltonian for a unitary quantum system
defined by an a priori non-Hermitian Hamiltonian.Comment: 11 page
Application of Pseudo-Hermitian Quantum Mechanics to a Complex Scattering Potential with Point Interactions
We present a generalization of the perturbative construction of the metric
operator for non-Hermitian Hamiltonians with more than one perturbation
parameter. We use this method to study the non-Hermitian scattering
Hamiltonian: H=p^2/2m+\zeta_-\delta(x+a)+\zeta_+\delta(x-a), where \zeta_\pm
and a are respectively complex and real parameters and \delta(x) is the Dirac
delta function. For regions in the space of coupling constants \zeta_\pm where
H is quasi-Hermitian and there are no complex bound states or spectral
singularities, we construct a (positive-definite) metric operator \eta and the
corresponding equivalent Hermitian Hamiltonian h. \eta turns out to be a
(perturbatively) bounded operator for the cases that the imaginary part of the
coupling constants have opposite sign, \Im(\zeta_+) = -\Im(\zeta_-). This in
particular contains the PT-symmetric case: \zeta_+ = \zeta_-^*. We also
calculate the energy expectation values for certain Gaussian wave packets to
study the nonlocal nature of \rh or equivalently the non-Hermitian nature of
\rH. We show that these physical quantities are not directly sensitive to the
presence of PT-symmetry.Comment: 22 pages, 4 figure
Exact PT-Symmetry Is Equivalent to Hermiticity
We show that a quantum system possessing an exact antilinear symmetry, in
particular PT-symmetry, is equivalent to a quantum system having a Hermitian
Hamiltonian. We construct the unitary operator relating an arbitrary
non-Hermitian Hamiltonian with exact PT-symmetry to a Hermitian Hamiltonian. We
apply our general results to PT-symmetry in finite-dimensions and give the
explicit form of the above-mentioned unitary operator and Hermitian Hamiltonian
in two dimensions. Our findings lead to the conjecture that non-Hermitian
CPT-symmetric field theories are equivalent to certain nonlocal Hermitian field
theories.Comment: Few typos have been corrected and a reference update
Physical Aspects of Pseudo-Hermitian and -Symmetric Quantum Mechanics
For a non-Hermitian Hamiltonian H possessing a real spectrum, we introduce a
canonical orthonormal basis in which a previously introduced unitary mapping of
H to a Hermitian Hamiltonian h takes a simple form. We use this basis to
construct the observables O of the quantum mechanics based on H. In particular,
we introduce pseudo-Hermitian position and momentum operators and a
pseudo-Hermitian quantization scheme that relates the latter to the ordinary
classical position and momentum observables. These allow us to address the
problem of determining the conserved probability density and the underlying
classical system for pseudo-Hermitian and in particular PT-symmetric quantum
systems. As a concrete example we construct the Hermitian Hamiltonian h, the
physical observables O, the localized states, and the conserved probability
density for the non-Hermitian PT-symmetric square well. We achieve this by
employing an appropriate perturbation scheme. For this system, we conduct a
comprehensive study of both the kinematical and dynamical effects of the
non-Hermiticity of the Hamiltonian on various physical quantities. In
particular, we show that these effects are quantum mechanical in nature and
diminish in the classical limit. Our results provide an objective assessment of
the physical aspects of PT-symmetric quantum mechanics and clarify its
relationship with both the conventional quantum mechanics and the classical
mechanics.Comment: 45 pages, 13 figures, 2 table
PT-Symmetric Versus Hermitian Formulations of Quantum Mechanics
A non-Hermitian Hamiltonian that has an unbroken PT symmetry can be converted
by means of a similarity transformation to a physically equivalent Hermitian
Hamiltonian. This raises the following question: In which form of the quantum
theory, the non-Hermitian or the Hermitian one, is it easier to perform
calculations? This paper compares both forms of a non-Hermitian
quantum-mechanical Hamiltonian and demonstrates that it is much harder to
perform calculations in the Hermitian theory because the perturbation series
for the Hermitian Hamiltonian is constructed from divergent Feynman graphs. For
the Hermitian version of the theory, dimensional continuation is used to
regulate the divergent graphs that contribute to the ground-state energy and
the one-point Green's function. The results that are obtained are identical to
those found much more simply and without divergences in the non-Hermitian
PT-symmetric Hamiltonian. The contribution to the
ground-state energy of the Hermitian version of the theory involves graphs with
overlapping divergences, and these graphs are extremely difficult to regulate.
In contrast, the graphs for the non-Hermitian version of the theory are finite
to all orders and they are very easy to evaluate.Comment: 13 pages, REVTeX, 10 eps figure
Scattering Theory and -Symmetry
We outline a global approach to scattering theory in one dimension that
allows for the description of a large class of scattering systems and their
-, -, and -symmetries. In
particular, we review various relevant concepts such as Jost solutions,
transfer and scattering matrices, reciprocity principle, unidirectional
reflection and invisibility, and spectral singularities. We discuss in some
detail the mathematical conditions that imply or forbid reciprocal
transmission, reciprocal reflection, and the presence of spectral singularities
and their time-reversal. We also derive generalized unitarity relations for
time-reversal-invariant and -symmetric scattering
systems, and explore the consequences of breaking them. The results reported
here apply to the scattering systems defined by a real or complex local
potential as well as those determined by energy-dependent potentials, nonlocal
potentials, and general point interactions.Comment: Slightly expanded revised version, 38 page
A Physical Realization of the Generalized PT-, C-, and CPT-Symmetries and the Position Operator for Klein-Gordon Fields
Generalized parity (P), time-reversal (T), and charge-conjugation
(C)operators were initially definedin the study of the pseudo-Hermitian
Hamiltonians. We construct a concrete realization of these operators for
Klein-Gordon fields and show that in this realization PT and C operators
respectively correspond to the ordinary time-reversal and charge-grading
operations. Furthermore, we present a complete description of the quantum
mechanics of Klein-Gordon fields that is based on the construction of a Hilbert
space with a relativistically invariant, positive-definite, and conserved inner
product. In particular we offer a natural construction of a position operator
and the corresponding localized and coherent states. The restriction of this
position operator to the positive-frequency fields coincides with the
Newton-Wigner operator. Our approach does not rely on the conventional
restriction to positive-frequency fields. Yet it provides a consistent quantum
mechanical description of Klein-Gordon fields with a genuine probabilistic
interpretation.Comment: 20 pages, published versio
Invisibility and PT-symmetry
For a general complex scattering potential defined on a real line, we show
that the equations governing invisibility of the potential are invariant under
the combined action of parity and time-reversal (PT) transformation. We
determine the PT-symmetric an well as non-PT-symmetric invisible configurations
of an easily realizable exactly solvable model that consists of a two-layer
planar slab consisting of optically active material. Our analysis shows that
although PT-symmetry is neither necessary nor sufficient for the invisibility
of a scattering potential, it plays an important role in the characterization
of the invisible configurations. A byproduct of our investigation is the
discovery of certain configurations of our model that are effectively
reflectionless in a spectral range as wide as several hundred nanometers.Comment: 11 pages, 3 figures, revised version, accepted for publication in
Phys.Rev.
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