770 research outputs found
Optimizing inhomogeneous spin ensembles for quantum memory
We propose a general method to maximize the fidelity of writing, storage and
reading of quantum information (QI) in a spectrally inhomogeneous spin ensemble
used as quantum memory. The method is based on preselecting the optimal
spectral portion of the ensemble by a judiciously designed pulse. It allows
drastic improvement of quantum memory realized by spin ensembles that store QI
from a resonator or an optical beam.Comment: Corrected m
Anomalous latent heat in non-equilibrium phase transitions
We study first-order phase transitions in a two-temperature system, where due
to the time-scale separation all the basic thermodynamical quantities (free
energy, entropy, etc) are well-defined. The sign of the latent heat is found to
be counterintuitive: it is positive when going from the phase where the
temperatures and the entropy are higher to the one where these quantities are
lower. The effect exists only out of equilibrium and requires conflicting
interactions. It is displayed on a lattice gas model of ferromagnetically
interacting spin-1/2 particles.Comment: 4 pages, 2 figure
Dynamics and evaporation of defects in Mott-insulating clusters of boson pairs
Repulsively bound pairs of particles in a lattice governed by the
Bose-Hubbard model can form stable incompressible clusters of dimers
corresponding to finite-size n=2 Mott insulators. Here we study the dynamics of
hole defects in such clusters corresponding to unpaired particles which can
resonantly tunnel out of the cluster into the lattice vacuum. Due to bosonic
statistics, the unpaired particles have different effective mass inside and
outside the cluster, and "evaporation" of hole defects from the cluster
boundaries is possible only when their quasi-momenta are within a certain
transmission range. We show that quasi-thermalization of hole defects occurs in
the presence of catalyzing particle defects which thereby purify the Mott
insulating clusters. We study the dynamics of one-dimensional system using
analytical techniques and numerically exact t-DMRG simulations. We derive an
effective strong-interaction model that enables simulations of the system
dynamics for much longer times. We also discuss a more general case of two
bosonic species which reduces to the fermionic Hubbard model in the strong
interaction limit.Comment: 12 pages, 10 figures, minor update
Negative conductivity and anomalous screening in two-dimensional electron systems subjected to microwave radiation
A 2D electron system in a quantized magnetic field can be driven by microwave
radiation into a non-equilibrium state with strong magnetooscillations of the
dissipative conductivity. We demonstrate that in such system a negative
conductivity can coexist with a positive diffusion coefficient. In a finite
system, solution of coupled electrostatic and linear transport problems shows
that the diffusion can stabilize a state with negative conductivity.
Specifically, this happens when the system size is smaller than the absolute
value of the non-equilibrium screening length that diverges at the point where
the conductivity changes sign. We predict that a negative resistance can be
measured in such a state. Further, for a non-zero difference between the work
functions of two contacts, we explore the distribution of the electrostatic
potential and of the electron density in the sample. We show that in the
diffusion-stabilized regime of negative conductivity the system splits into two
regions with opposite directions of electric field. This effect is a precursor
of the domain structure that has been predicted to emerge spontaneously in the
microwave-induced zero-resistance states.Comment: 8 pages, 4 figure
Extraordinary electron transmission through a periodic array of quantum dots
We study electron transmission through a periodic array of quantum dots (QD)
sandwiched between doped semiconductor leads. When the Fermi wavelength of
tunneling electron exceeds the array lattice constant, the off-resonant per QD
conductance is enhanced by several orders of magnitude relative to the
single-QD conductance. The physical mechanism of the enhancement is
delocalization of a small fraction of system eigenstates caused by coherent
coupling of QDs via the electron continuum in the leads.Comment: 4 pages, 3 figure
Tunable photonic band gaps with coherently driven atoms in optical lattices
Optical lattice loaded with cold atoms can exhibit a tunable photonic band
gap for a weak probe field under the conditions of electromagnetically induced
transparency. This system possesses a number of advantageous properties,
including reduced relaxation of Raman coherence and the associated probe
absorption, and simultaneous enhancement of the index modulation and the
resulting reflectivity of the medium. This flexible system has a potential to
serve as a testbed of various designs for the linear and nonlinear photonic
band gap materials at a very low light level and can be employed for realizing
deterministic entanglement between weak quantum fields
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