1,154 research outputs found
Crossed-boson exchange contribution and Bethe-Salpeter equation
The contribution to the binding energy of a two-body system due to the
crossed two-boson exchange contribution is calculated, using the Bethe-Salpeter
equation. This is done for distinguishable, scalar particles interacting via
the exchange of scalar massive bosons. The sensitivity of the results to the
off-shell behavior of the operator accounting for this contribution is
discussed. Large corrections to the Bethe-Salpeter results in the ladder
approximation are found. For neutral scalar bosons, the mass obtained for the
two-body system is close to what has been calculated with various forms of the
instantaneous approximation, including the standard non-relativistic approach.
The specific character of this result is demonstrated by a calculation
involving charged bosons, which evidences a quite different pattern. Our
results explain for some part those obtained by Nieuwenhuis and Tjon on a
different basis. Some discrepancy appears with increasing coupling constants,
suggesting the existence of sizeable contributions involving more than
two-boson exchanges.Comment: 13 pages, 5 .eps figures, submitted to 'Few Body Systems
Bounds to unitary evolution
Upper and lower bounds are established for the survival probability
of a quantum state, in terms of the energy moments
. Introducing a cut-off in the energy generally
enables considerable improvement in these bounds and allows the method to be
used where the exact energy moments do not exist.Comment: 5 pages, 8 figure
The fundamental limit on the rate of quantum dynamics: the unified bound is tight
The question of how fast a quantum state can evolve has attracted a
considerable attention in connection with quantum measurement, metrology, and
information processing. Since only orthogonal states can be unambiguously
distinguished, a transition from a state to an orthogonal one can be taken as
the elementary step of a computational process. Therefore, such a transition
can be interpreted as the operation of "flipping a qubit", and the number of
orthogonal states visited by the system per unit time can be viewed as the
maximum rate of operation.
A lower bound on the orthogonalization time, based on the energy spread
DeltaE, was found by Mandelstam and Tamm. Another bound, based on the average
energy E, was established by Margolus and Levitin. The bounds coincide, and can
be exactly attained by certain initial states if DeltaE=E; however, the problem
remained open of what the situation is otherwise.
Here we consider the unified bound that takes into account both DeltaE and E.
We prove that there exist no initial states that saturate the bound if DeltaE
is not equal to E. However, the bound remains tight: for any given values of
DeltaE and E, there exists a one-parameter family of initial states that can
approach the bound arbitrarily close when the parameter approaches its limit
value. The relation between the largest energy level, the average energy, and
the orthogonalization time is also discussed. These results establish the
fundamental quantum limit on the rate of operation of any
information-processing system.Comment: 4 pages 1 PS figure Late
A generalization of Margolus-Levitin bound
The Margolus-Levitin lower bound on minimal time required for a state to be
transformed into an orthogonal state is generalized. It is shown that for some
initial states new bound is stronger than the Margolus-Levitin one.Comment: 6 pages, no figures; some comments added; final version accepted for
publication in Phys. Rev.
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
Ultra-short solitons and kinetic effects in nonlinear metamaterials
We present a stability analysis of a modified nonlinear Schroedinger equation
describing the propagation of ultra-short pulses in negative refractive index
media. Moreover, using methods of quantum statistics, we derive a kinetic
equation for the pulses, making it possible to analyze and describe partial
coherence in metamaterials. It is shown that a novel short pulse soliton, which
is found analytically, can propagate in the medium.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
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
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
Monopole Condensation in full QCD using the Schroedinger Functional
We use a lattice thermal partition functional to study Abelian monopole
condensation in full QCD with staggered fermions. We present
preliminary results on and lattices.Comment: Lattice2002(topology). 3 pages, 3 figure
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