949 research outputs found
Optimal Quantum Clocks
A quantum clock must satisfy two basic constraints. The first is a bound on
the time resolution of the clock given by the difference between its maximum
and minimum energy eigenvalues. The second follows from Holevo's bound on how
much classical information can be encoded in a quantum system. We show that
asymptotically, as the dimension of the Hilbert space of the clock tends to
infinity, both constraints can be satisfied simultaneously. The experimental
realization of such an optimal quantum clock using trapped ions is discussed.Comment: 4 pages, revtex, 1 figure, revision contains some new result
The casuality and/or energy-momentum conservation constraints on QCD amplitudes in small x regime
The causality and/or the energy-momentum constraints on the amplitudes of
high energy processes are generalized to QCD. The constraints imply that the
energetic parton may experience at most one inelastic collision only and that
the number of the constituents in the light cone wave function of the
projectile is increasing with the collision energy and the atomic number.Comment: 24 pages,8 figures. The paper is streamlined, some references are
changed and misprints are eliminate
Speed limits for quantum gates in multi-qubit systems
We use analytical and numerical calculations to obtain speed limits for
various unitary quantum operations in multiqubit systems under typical
experimental conditions. The operations that we consider include single-, two-,
and three-qubit gates, as well as quantum-state transfer in a chain of qubits.
We find in particular that simple methods for implementing two-qubit gates
generally provide the fastest possible implementations of these gates. We also
find that the three-qubit Toffoli gate time varies greatly depending on the
type of interactions and the system's geometry, taking only slightly longer
than a two-qubit controlled-NOT (CNOT) gate for a triangle geometry. The speed
limit for quantum-state transfer across a qubit chain is set by the maximum
spin-wave speed in the chain.Comment: 7 pages (two-column), 2 figures, 2 table
Product Integral Formalism and Non-Abelian Stokes Theorem
We make use of the properties of product integrals to obtain a surface
product integral representation for the Wilson loop operator. The result can be
interpreted as the non-abelian version of Stokes' theorem.Comment: Latex; condensed version of hep-th/9903221, to appear in Jour. Math.
Phy
The role of entanglement in dynamical evolution
Entanglement or entanglement generating interactions permit to achieve the
maximum allowed speed in the dynamical evolution of a composite system, when
the energy resources are distributed among subsystems. The cases of
pre-existing entanglement and of entanglement-building interactions are
separately addressed. The role of classical correlations is also discussed.Comment: 5 pages, 1 figure. Revised versio
Configurations of Handles and the Classification of Divergences in the String Partition Function
The divergences that arise in the regularized partition function for closed
bosonic string theory in flat space lead to three types of perturbation series
expansions, distinguished by their genus dependence. This classification of
infinities can be traced to geometrical characteristics of the string
worldsheet. Some categories of divergences may be eliminated in string theories
formulated on compact manifolds.Comment: 24 pages, DAMTP-R/94/1
Electrons as quasi-bosons in magnetic white dwarfs
A white dwarf star achieves its equilibrium from the balancing of the
gravitational compression against the Fermi degeneracy pressure of the electron
gas. In field theory there are examples (e.g. the monopole-charge system) where
a strong magnetic field can transform a boson into a fermion or a fermion into
a boson. In some condensed matter systems (e.g. fractional quantum Hall
systems) a strong magnetic field can transform electrons into effective
fermions, or effective anyons. Based on these examples we investigate the
possibility that the strong magnetic fields of some white dwarfs may transform
some fraction of the electrons into effective bosons. This could have
consequences for the structure of highly magnetized white dwarfs. It would
alter the mass-radius relationship, and in certain instances one could envision
a scenario where a white dwarf below the Chandrasekhar limit could nevertheless
collapse into a neutron star due to a weakening of the electron degeneracy
pressure. In addition the transformation of electrons into effective bosons
could result in the electrons Bose condensing, which could speed up the cooling
rate of white dwarfs.Comment: 10 pages. To be published IJMP
Shear and bulk viscosities for pure glue matter
Shear and bulk viscosities are calculated in a quasiparticle
model within a relaxation time approximation for pure gluon matter. Below
the confined sector is described within a quasiparticle glueball model.
Particular attention is paid to behavior of the shear and bulk viscosities near
. The constructed equation of state reproduces the first-order phase
transition for the glue matter. It is shown that with this equation of state it
is possible to describe the temperature dependence of the shear viscosity to
entropy ratio and the bulk viscosity to entropy ratio in
reasonable agreement with available lattice data but absolute values of the
ratio underestimate the upper limits of this ratio in the lattice
measurements typically by an order of magnitude.Comment: 8 pages, 4 figures; the published versio
Fundamental Limits on the Speed of Evolution of Quantum States
This paper reports on some new inequalities of
Margolus-Levitin-Mandelstam-Tamm-type involving the speed of quantum evolution
between two orthogonal pure states. The clear determinant of the qualitative
behavior of this time scale is the statistics of the energy spectrum. An
often-overlooked correspondence between the real-time behavior of a quantum
system and the statistical mechanics of a transformed (imaginary-time)
thermodynamic system appears promising as a source of qualitative insights into
the quantum dynamics.Comment: 6 pages, 1 eps figur
Permutation combinatorics of worldsheet moduli space
52 pages, 21 figures52 pages, 21 figures; minor corrections, "On the" dropped from title, matches published version52 pages, 21 figures; minor corrections, "On the" dropped from title, matches published versio
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