5 research outputs found
On optimum Hamiltonians for state transformations
For a prescribed pair of quantum states |psi_I> and |psi_F> we establish an
elementary derivation of the optimum Hamiltonian, under constraints on its
eigenvalues, that generates the unitary transformation |psi_I> --> |psi_F> in
the shortest duration. The derivation is geometric in character and does not
rely on variational calculus.Comment: 5 page
An alternative to the conventional micro-canonical ensemble
Usual approach to the foundations of quantum statistical physics is based on
conventional micro-canonical ensemble as a starting point for deriving
Boltzmann-Gibbs (BG) equilibrium. It leaves, however, a number of conceptual
and practical questions unanswered. Here we discuss these questions, thereby
motivating the study of a natural alternative known as Quantum Micro-Canonical
(QMC) ensemble. We present a detailed numerical study of the properties of the
QMC ensemble for finite quantum systems revealing a good agreement with the
existing analytical results for large quantum systems. We also propose the way
to introduce analytical corrections accounting for finite-size effects. With
the above corrections, the agreement between the analytical and the numerical
results becomes very accurate. The QMC ensemble leads to an unconventional kind
of equilibrium, which may be realizable after strong perturbations in small
isolated quantum systems having large number of levels. We demonstrate that the
variance of energy fluctuations can be used to discriminate the QMC equilibrium
from the BG equilibrium. We further suggest that the reason, why BG equilibrium
commonly occurs in nature rather than the QMC-type equilibrium, has something
to do with the notion of quantum collapse.Comment: 25 pages, 6 figure
A generalized quantum microcanonical ensemble
We discuss a generalized quantum microcanonical ensemble. It describes
isolated systems that are not necessarily in an eigenstate of the Hamilton
operator. Statistical averages are obtained by a combination of a time average
and a maximum entropy argument to resolve the lack of knowledge about initial
conditions. As a result, statistical averages of linear observables coincide
with values obtained in the canonical ensemble. Non-canonical averages can be
obtained by taking into account conserved quantities which are non-linear
functions of the microstate.Comment: improved version, new titl