17,593 research outputs found
Variable-speed Generators with Flux Weakening
A cost-competitive, permanent-magnet 20 kW generator is designed such that the following criteria are satisfied: an (over) load capability of at least 30 kW over the entire speed range of 60-120 rpm, generator weight of about 550 lbs with a maximum radial stator flux density of 0.82 T at low speed, unity power factor operation, acceptably small synchronous reactances and operation without a gear box. To justify this final design four different generator designs are investigated: the first two designs are studied to obtain a speed range from 20 to 200 rpm employing rotor field weakening, and the latter two are investigated to obtain a maximum speed range of 40 to 160 rpm based on field weakening via the stator excitation. The generator reactances and induced voltages are computed using finite element/difference solutions. Generator losses and efficiencies are presented for all four designs at rated temperature of Tr=120C
Information Tradeoff Relations for Finite-Strength Quantum Measurements
In this paper we give a new way to quantify the folklore notion that quantum
measurements bring a disturbance to the system being measured. We consider two
observers who initially assign identical mixed-state density operators to a
two-state quantum system. The question we address is to what extent one
observer can, by measurement, increase the purity of his density operator
without affecting the purity of the other observer's. If there were no
restrictions on the first observer's measurements, then he could carry this out
trivially by measuring the initial density operator's eigenbasis. If, however,
the allowed measurements are those of finite strength---i.e., those
measurements strictly within the interior of the convex set of all
measurements---then the issue becomes significantly more complex. We find that
for a large class of such measurements the first observer's purity increases
the most precisely when there is some loss of purity for the second observer.
More generally the tradeoff between the two purities, when it exists, forms a
monotonic relation. This tradeoff has potential application to quantum state
control and feedback.Comment: 15 pages, revtex3, 3 eps figure
Schnabl's L_0 Operator in the Continuous Basis
Following Schnabl's analytic solution to string field theory, we calculate
the operators for a scalar field in the
continuous basis. We find an explicit and simple expression for them
that further simplifies for their sum, which is block diagonal in this basis.
We generalize this result for the bosonized ghost sector, verify their
commutation relation and relate our expressions to wedge state representations.Comment: 1+16 pages. JHEP style. Typos correcte
Pooling quantum states obtained by indirect measurements
We consider the pooling of quantum states when Alice and Bob both have one
part of a tripartite system and, on the basis of measurements on their
respective parts, each infers a quantum state for the third part S. We denote
the conditioned states which Alice and Bob assign to S by alpha and beta
respectively, while the unconditioned state of S is rho. The state assigned by
an overseer, who has all the data available to Alice and Bob, is omega. The
pooler is told only alpha, beta, and rho. We show that for certain classes of
tripartite states, this information is enough for her to reconstruct omega by
the formula omega \propto alpha rho^{-1} beta. Specifically, we identify two
classes of states for which this pooling formula works: (i) all pure states for
which the rank of rho is equal to the product of the ranks of the states of
Alice's and Bob's subsystems; (ii) all mixtures of tripartite product states
that are mutually orthogonal on S.Comment: Corrected a mistake regarding the scope of our original result. This
version to be published in Phys. Rev. A. 6 pages, 1 figur
Character Formulae and Partition Functions in Higher Dimensional Conformal Field Theory
A discussion of character formulae for positive energy unitary irreducible
representations of the the conformal group is given, employing Verma modules
and Weyl group reflections. Product formulae for various conformal group
representations are found. These include generalisations of those found by
Flato and Fronsdal for SO(3,2). In even dimensions the products for free
representations split into two types depending on whether the dimension is
divisible by four or not.Comment: 43 pages, uses harvmac,version 2 2 references added, minor typos
correcte
Oscillating Casimir force between impurities in one-dimensional Fermi liquids
We study the interaction of two localized impurities in a repulsive
one-dimensional Fermi liquid via bosonization. In a previous paper [Phys. Rev.
A 72, 023616 (2005)], it was shown that at distances much larger than the
interparticle spacing the impurities interact through a Casimir-type force
mediated by the zero sound phonons of the underlying quantum liquid. Here we
extend these results and show that the strength and sign of this Casimir
interaction depend sensitively on the impurities separation. These oscillations
in the Casimir interaction have the same period as Friedel oscillations. Their
maxima correspond to tunneling resonances tuned by the impurities separation.Comment: This paper is a continuation of Phys. Rev. A 72, 023616 (2005). v2:
two appendix adde
Quantum State Disturbance vs. Information Gain: Uncertainty Relations for Quantum Information
When an observer wants to identify a quantum state, which is known to be one
of a given set of non-orthogonal states, the act of observation causes a
disturbance to that state. We investigate the tradeoff between the information
gain and that disturbance. This issue has important applications in quantum
cryptography. The optimal detection method, for a given tolerated disturbance,
is explicitly found in the case of two equiprobable non-orthogonal pure states.Comment: 20 pages, standard LaTeX, four png figures (also available from the
authors: [email protected] and [email protected]
Nonlinear viscoelasticity of metastable complex fluids
Many metastable complex fluids such as colloidal glasses and gels show
distinct nonlinear viscoelasticity with increasing oscillatory-strain
amplitude; the storage modulus decreases monotonically as the strain amplitude
increases whereas the loss modulus has a distinct peak before it decreases at
larger strains. We present a qualitative argument to explain this ubiquitous
behavior and use mode coupling theory (MCT) to confirm it. We compare
theoretical predictions to the measured nonlinear viscoelasticity in a dense
hard sphere colloidal suspensions; reasonable agreement is obtained. The
argument given here can be used to obtain new information about linear
viscoelasticity of metastable complex fluids from nonlinear strain
measurements.Comment: 7 pages, 3 figures, accepted for publication in Europhys. Let
The Lie Algebraic Significance of Symmetric Informationally Complete Measurements
Examples of symmetric informationally complete positive operator valued
measures (SIC-POVMs) have been constructed in every dimension less than or
equal to 67. However, it remains an open question whether they exist in all
finite dimensions. A SIC-POVM is usually thought of as a highly symmetric
structure in quantum state space. However, its elements can equally well be
regarded as a basis for the Lie algebra gl(d,C). In this paper we examine the
resulting structure constants, which are calculated from the traces of the
triple products of the SIC-POVM elements and which, it turns out, characterize
the SIC-POVM up to unitary equivalence. We show that the structure constants
have numerous remarkable properties. In particular we show that the existence
of a SIC-POVM in dimension d is equivalent to the existence of a certain
structure in the adjoint representation of gl(d,C). We hope that transforming
the problem in this way, from a question about quantum state space to a
question about Lie algebras, may help to make the existence problem tractable.Comment: 56 page
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