1,048 research outputs found
Quantum Zeno subspaces and dynamical superselection rules
The quantum Zeno evolution of a quantum system takes place in a proper
subspace of the total Hilbert space. The physical and mathematical features of
the "Zeno subspaces" depend on the measuring apparatus: when this is included
in the quantum description, the Zeno effect becomes a mere consequence of the
dynamics and, remarkably, can be cast in terms of an adiabatic theorem, with a
dynamical superselection rule. We look at several examples and focus on quantum
computation and decoherence-free subspaces.Comment: 35 pages, 5 figure
Three different manifestations of the quantum Zeno effect
Three different manifestations of the quantum Zeno effect are discussed,
compared and shown to be physically equivalent. We look at frequent projective
measurements, frequent unitary "kicks" and strong continuous coupling. In all
these cases, the Hilbert space of the system splits into invariant "Zeno"
subspaces, among which any transition is hindered.Comment: 16 pages, 4 figure
Berry phase due to quantum measurements
The usual, "static" version of the quantum Zeno effect consists in the
hindrance of the evolution of a quantum systems due to repeated measurements.
There is however a "dynamic" version of the same phenomenon, first discussed by
von Neumann in 1932 and subsequently explored by Aharonov and Anandan, in which
a system is forced to follow a given trajectory. A Berry phase appears if such
a trajectory is a closed loop in the projective Hilbert space. A specific
example involving neutron spin is considered and a similar situation with
photon polarization is investigated.Comment: 6 pages, 2 figures. Contribution to the Sixth Central-European
Workshop on Quantum Optics, Chudobin near Olomouc, Czech Republic, April-May
199
A Brief History of the GKLS Equation
We reconstruct the chain of events, intuitions and ideas that led to the
formulation of the Gorini, Kossakowski, Lindblad and Sudarshan equation.Comment: Based on a talk given by D.C. at the 48th Symposium on Mathematical
Physics "Gorini-Kossakowski-Lindblad-Sudarshan Master Equation - 40 Years
After" (Toru\'n, June 10-12, 2016). To be published in the special volume of
OSI
Quantum Zeno dynamics: mathematical and physical aspects
If frequent measurements ascertain whether a quantum system is still in its
initial state, transitions to other states are hindered and the quantum Zeno
effect takes place. However, in its broader formulation, the quantum Zeno
effect does not necessarily freeze everything. On the contrary, for frequent
projections onto a multidimensional subspace, the system can evolve away from
its initial state, although it remains in the subspace defined by the
measurement. The continuing time evolution within the projected "quantum Zeno
subspace" is called "quantum Zeno dynamics:" for instance, if the measurements
ascertain whether a quantum particle is in a given spatial region, the
evolution is unitary and the generator of the Zeno dynamics is the Hamiltonian
with hard-wall (Dirichlet) boundary conditions. We discuss the physical and
mathematical aspects of this evolution, highlighting the open mathematical
problems. We then analyze some alternative strategies to obtain a Zeno dynamics
and show that they are physically equivalent.Comment: 52 pages, 10 figure
Modifying the lifetime of an unstable system by an intense electromagnetic field
We study the temporal behavior of a three-level system (such as an atom or a
molecule), initially prepared in an excited state, bathed in a laser field
tuned at the transition frequency of the other level. We analyze the dependence
of the lifetime of the initial state on the intensity of the laser field. The
phenomenon we discuss is related to both electromagnetic induced transparency
and quantum Zeno effect.Comment: 10 pages, 3 figures. Contribution to Sixth Central-European Workshop
on Quantum Optics, Chudobin near Olomouc, Czech Republic, April-May 199
Unstable systems and quantum Zeno phenomena in quantum field theory
We analyze the Zeno phenomenon in quantum field theory. The decay of an
unstable system can be modified by changing the time interval between
successive measurements (or by varying the coupling to an external system that
plays the role of measuring apparatus). We speak of quantum Zeno effect if the
decay is slowed and of inverse quantum Zeno (or Heraclitus) effect if it is
accelerated. The analysis of the transition between these two regimes requires
close scrutiny of the features of the interaction Hamiltonian. We look in
detail at quantum field theoretical models of the Lee type.Comment: 25 pages, 6 figure
Kick and fix: the roots of quantum control
When two operators and do not commute, the calculation of the
exponential operator is a difficult and crucial problem. The
applications are vast and diversified: to name but a few examples, quantum
evolutions, product formulas, quantum control, Zeno effect. The latter are of
great interest in quantum applications and quantum technologies. We present
here a historical survey of results and techniques, and discuss differences and
similarities. We also highlight the link with the strong coupling regime, via
the adiabatic theorem, and contend that the "pulsed" and "continuous"
formulations differ only in the order by which two limits are taken, and are
but two faces of the same coin.Comment: 6 page
Wigner function and coherence properties of cold and thermal neutrons
We analyze the coherence properties of a cold or a thermal neutron by
utilizing the Wigner quasidistribution function. We look in particular at a
recent experiment performed by Badurek {\em et al.}, in which a polarized
neutron crosses a magnetic field that is orthogonal to its spin, producing
highly non-classical states. The quantal coherence is extremely sensitive to
the field fluctuation at high neutron momenta. A "decoherence parameter" is
introduced in order to get quantitative estimates of the losses of coherence.Comment: 6 pages, 3 figures. Contribution to the Sixth Central-European
Workshop on Quantum Optics, Chudobin near Olomouc, Czech Republic, April-May
199
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