171 research outputs found
Local rectification of heat flux
We present a chain-of-atoms model where heat is rectified, with different
fluxes from the hot to the cold baths located at the chain boundaries when the
temperature bias is reversed. The chain is homogeneous except for boundary
effects and a local modification of the interactions at one site, the
"impurity". The rectification mechanism is due here to the localized impurity,
the only asymmetrical element of the structure, apart from the externally
imposed temperature bias, and does not rely on putting in contact different
materials or other known mechanisms such as grading or long-range interactions.
The effect survives if all interaction forces are linear except the ones for
the impurity.Comment: 5 pages, 5 figure
Stabilization of solitons in PT models with supersymmetry by periodic management
We introduce a system based on dual-core nonlinear waveguides with the
balanced gain and loss acting separately in the cores. The system features a
"supersymmetry" when the gain and loss are equal to the inter-core coupling.
This system admits a variety of exact solutions (we focus on solitons), which
are subject to a specific subexponential instability. We demonstrate that the
application of a "management", in the form of periodic simultaneous switch of
the sign of the gain, loss, and inter-coupling, effectively stabilizes
solitons, without destroying the supersymmetry. The management turns the
solitons into attractors, for which an attraction basin is identified. The
initial amplitude asymmetry and phase mismatch between the components
transforms the solitons into quasi-stable breathers.Comment: In press EPL 201
Dynamics of higher-order solitons in regular and PT-symmetric nonlinear couplers
Dynamics of symmetric and antisymmetric 2-solitons and 3-solitons is studied
in the model of the nonlinear dual-core coupler and its PT-symmetric version.
Regions of the convergence of the injected perturbed symmetric and
antisymmetric N-solitons into symmetric and asymmetric quasi-solitons are
found. In the PT-symmetric system, with the balanced gain and loss acting in
the two cores, borders of the stability against the blowup are identified.
Notably, in all the cases the stability regions are larger for antisymmetric
2-soliton inputs than for their symmetric counterparts, on the contrary to
previously known results for fundamental solitons (N=1). Dynamical regimes
(switching) are also studied for the 2-soliton injected into a single core of
the coupler. In particular, a region of splitting of the input into a pair of
symmetric solitons is found, which is explained as a manifestation of the
resonance between the vibrations of the 2-soliton and oscillations of energy
between the two cores in the coupler.Comment: To appear in EPL journa
Atom cooling by non-adiabatic expansion
Motivated by the recent discovery that a reflecting wall moving with a
square-root in time trajectory behaves as a universal stopper of classical
particles regardless of their initial velocities, we compare linear in time and
square-root in time expansions of a box to achieve efficient atom cooling. For
the quantum single-atom wavefunctions studied the square-root in time expansion
presents important advantages: asymptotically it leads to zero average energy
whereas any linear in time (constant box-wall velocity) expansion leaves a
non-zero residual energy, except in the limit of an infinitely slow expansion.
For finite final times and box lengths we set a number of bounds and cooling
principles which again confirm the superior performance of the square-root in
time expansion, even more clearly for increasing excitation of the initial
state. Breakdown of adiabaticity is generally fatal for cooling with the linear
expansion but not so with the square-root expansion.Comment: 4 pages, 4 figure
Transitionless quantum drivings for the harmonic oscillator
Two methods to change a quantum harmonic oscillator frequency without
transitions in a finite time are described and compared. The first method, a
transitionless-tracking algorithm, makes use of a generalized harmonic
oscillator and a non-local potential. The second method, based on engineering
an invariant of motion, only modifies the harmonic frequency in time, keeping
the potential local at all times.Comment: 11 pages, 1 figure. Submitted for publicatio
Weak Measurements in Non-Hermitian Systems
"Weak measurements" -- involving a weak unitary interaction between a quantum
system and a meter followed by a projective measurement -- are investigated
when the system has a non-Hermitian Hamiltonian. We show in particular how the
standard definition of the "weak value" of an observable must be modified.
These studies are undertaken in the context of bound state scattering theory, a
non-Hermitian formalism for which the Hilbert spaces involved are unambiguously
defined and the metric operators can be explicitly computed. Numerical examples
are given for a model system
Fast atomic transport without vibrational heating
We use the dynamical invariants associated with the Hamiltonian of an atom in
a one dimensional moving trap to inverse engineer the trap motion and perform
fast atomic transport without final vibrational heating. The atom is driven
non-adiabatically through a shortcut to the result of adiabatic, slow trap
motion. For harmonic potentials this only requires designing appropriate trap
trajectories, whereas perfect transport in anharmonic traps may be achieved by
applying an extra field to compensate the forces in the rest frame of the trap.
The results can be extended to atom stopping or launching. The limitations due
to geometrical constraints, energies and accelerations involved are analyzed,
as well as the relation to previous approaches (based on classical trajectories
or "fast-forward" and "bang-bang" methods) which can be integrated in the
invariant-based framework.Comment: 10 pages, 5 figure
Self-dual Spectral Singularities and Coherent Perfect Absorbing Lasers without PT-symmetry
A PT-symmetric optically active medium that lases at the threshold gain also
acts as a complete perfect absorber at the laser wavelength. This is because
spectral singularities of PT-symmetric complex potentials are always
accompanied by their time-reversal dual. We investigate the significance of
PT-symmetry for the appearance of these self-dual spectral singularities. In
particular, using a realistic optical system we show that self-dual spectral
singularities can emerge also for non-PT-symmetric configurations. This
signifies the existence of non-PT-symmetric CPA-lasers.Comment: 11 pages, 3 figures, 1 table, accepted for publication in J. Phys.
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