77,976 research outputs found
On the entanglement of a quantum field with a dispersive medium
In this Letter we study the entanglement of a quantum radiation field
interacting with a dielectric medium. In particular, we describe the quantum
mixed state of a field interacting with a dielectric through plasma and Drude
models and show that these generate very different entanglement behavior, as
manifested in the entanglement entropy of the field. We also present a formula
for a "Casimir" entanglement entropy, i.e., the distance dependence of the
field entropy. Finally, we study a toy model of the interaction between two
plates. In this model, the field entanglement entropy is divergent; however, as
in the Casimir effect, its distance-dependent part is finite, and the field
matter entanglement is reduced when the objects are far.Comment: Final published PRL versio
Cooperative effects in Josephson junctions in a cavity in the strong coupling regime
We analyze the behavior of systems of two and three qubits made by Josephson
junctions, treated in the two level approximation, driven by a radiation mode
in a cavity. The regime we consider is a strong coupling one recently
experimentally reached for a single junction. Rabi oscillations are obtained
with the frequency proportional to integer order Bessel functions in the limit
of a large photon number, similarly to the case of the single qubit. A
selection rule is derived for the appearance of Rabi oscillations. A quantum
amplifier built with a large number of Josephson junctions in a cavity in the
strong coupling regime is also described.Comment: 9 pages, no figures. Version accepted for publication in Physical
Review
Limits on entanglement in rotationally-invariant scattering of spin systems
This paper investigates the dynamical generation of entanglement in
scattering systems, in particular two spin systems that interact via
rotationally-invariant scattering. The spin degrees of freedom of the in-states
are assumed to be in unentangled, pure states, as defined by the entropy of
entanglement. Because of the restriction of rotationally-symmetric
interactions, perfectly-entangling S-matrices, i.e. those that lead to a
maximally entangled out-state, only exist for a certain class of separable
in-states. Using Clebsch-Gordan coefficients for the rotation group, the
scattering phases that determine the S-matrix are determined for the case of
spin systems with , 1, and 3/2.Comment: 6 pages, no figures; v.2: sections added, edited for clarity,
conclusions and calculation unchanged, typos corrected; v.3: new abstrct,
revised first two sections, added reference
Superradiance induced topological vortex phase in a Bose-Einstein condensate
We investigate theoretically a topological vortex phase transition induced by
a superradiant phase transition in an atomic Bose-Einstein condensate driven by
a Laguerre-Gaussian optical mode. We show that superradiant radiation can
either carry zero angular momentum, or be in a rotating Laguerre-Gaussian mode
with angular momentum. The conditions leading to these two regimes are
determined in terms of the width for the pump laser and the condensate size for
the limiting cases where the recoil energy is both much smaller and larger than
the atomic interaction energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
A novel method for the injection and manipulation of magnetic charge states in nanostructures
Realising the promise of next-generation magnetic nanotechnologies is
contingent on the development of novel methods for controlling magnetic states
at the nanoscale. There is currently demand for simple and flexible techniques
to access exotic magnetisation states without convoluted fabrication and
application processes. 360 degree domain walls (metastable twists in
magnetisation separating two domains with parallel magnetisation) are one such
state, which is currently of great interest in data storage and magnonics.
Here, we demonstrate a straightforward and powerful process whereby a moving
magnetic charge, provided experimentally by a magnetic force microscope tip,
can write and manipulate magnetic charge states in ferromagnetic nanowires. The
method is applicable to a wide range of nanowire architectures with
considerable benefits over existing techniques. We confirm the method's
efficacy via the injection and spatial manipulation of 360 degree domain walls
in Py and Co nanowires. Experimental results are supported by micromagnetic
simulations of the tip-nanowire interaction.Comment: in Scientific Reports (2016
Giant electrocaloric effect around T
We use molecular dynamics with a first-principles-based shell model potential
to study the electrocaloric effect (ECE) in lithium niobate, LiNbO, and
find a giant electrocaloric effect along a line passing through the
ferroelectric transition. With applied electric field, a line of maximum ECE
passes through the zero field ferroelectric transition, continuing along a
Widom line at high temperatures with increasing field, and along the
instability that leads to homogeneous ferroelectric switching below with
an applied field antiparallel to the spontaneous polarization. This line is
defined as the minimum in the inverse capacitance under applied electric field.
We investigate the effects of pressure, temperature and applied electric field
on the ECE. The behavior we observe in LiNbO should generally apply to
ferroelectrics; we therefore suggest that the operating temperature for
refrigeration and energy scavenging applications should be above the
ferroelectric transition region to obtain large electrocaloric response. We
find a relationship among , the Widom line and homogeneous switching that
should be universal among ferroelectrics, relaxors, multiferroics, and the same
behavior should be found under applied magnetic fields in ferromagnets.Comment: 5 page
Molecular orbital calculations of two-electron states for P donor solid-state spin qubits
We theoretically study the Hilbert space structure of two neighbouring P
donor electrons in silicon-based quantum computer architectures. To use
electron spins as qubits, a crucial condition is the isolation of the electron
spins from their environment, including the electronic orbital degrees of
freedom. We provide detailed electronic structure calculations of both the
single donor electron wave function and the two-electron pair wave function. We
adopted a molecular orbital method for the two-electron problem, forming a
basis with the calculated single donor electron orbitals. Our two-electron
basis contains many singlet and triplet orbital excited states, in addition to
the two simple ground state singlet and triplet orbitals usually used in the
Heitler-London approximation to describe the two-electron donor pair wave
function. We determined the excitation spectrum of the two-donor system, and
study its dependence on strain, lattice position and inter donor separation.
This allows us to determine how isolated the ground state singlet and triplet
orbitals are from the rest of the excited state Hilbert space. In addition to
calculating the energy spectrum, we are also able to evaluate the exchange
coupling between the two donor electrons, and the double occupancy probability
that both electrons will reside on the same P donor. These two quantities are
very important for logical operations in solid-state quantum computing devices,
as a large exchange coupling achieves faster gating times, whilst the magnitude
of the double occupancy probability can affect the error rate.Comment: 15 pages (2-column
Electron vortex beams in a magnetic field: A new twist on Landau levels and Aharonov-Bohm states
We examine the propagation of the recently-discovered electron vortex beams
in a longitudinal magnetic field. We consider both the Aharonov-Bohm
configuration with a single flux line and the Landau case of a uniform magnetic
field. While stationary Aharonov-Bohm modes represent Bessel beams with flux-
and vortex-dependent probability distributions, stationary Landau states
manifest themselves as non-diffracting Laguerre-Gaussian beams. Furthermore,
the Landau-state beams possess field- and vortex-dependent phases: (i) the
Zeeman phase from coupling the quantized angular momentum to the magnetic field
and (ii) the Gouy phase, known from optical Laguerre-Gaussian beams.
Remarkably, together these phases determine the structure of Landau energy
levels. This unified Zeeman-Landau-Gouy phase manifests itself in a nontrivial
evolution of images formed by various superpositions of modes. We demonstrate
that, depending on the chosen superposition, the image can rotate in a magnetic
field with either (i) Larmor, (ii) cyclotron (double-Larmor), or (iii) zero
frequency. At the same time, its centroid always follows the classical
cyclotron trajectory, in agreement with the Ehrenfest theorem. Remarkably, the
non-rotating superpositions reproduce stable multi-vortex configurations that
appear in rotating superfluids. Our results open up an avenue for the direct
electron-microscopy observation of fundamental properties of free quantum
electron states in magnetic fields.Comment: 21 pages, 10 figures, 1 table, to appear in Phys. Rev.
Charge Transfer in Partition Theory
The recently proposed Partition Theory (PT) [J.Phys.Chem.A 111, 2229 (2007)]
is illustrated on a simple one-dimensional model of a heteronuclear diatomic
molecule. It is shown that a sharp definition for the charge of molecular
fragments emerges from PT, and that the ensuing population analysis can be used
to study how charge redistributes during dissociation and the implications of
that redistribution for the dipole moment. Interpreting small differences
between the isolated parts' ionization potentials as due to environmental
inhomogeneities, we gain insight into how electron localization takes place in
H2+ as the molecule dissociates. Furthermore, by studying the preservation of
the shapes of the parts as different parameters of the model are varied, we
address the issue of transferability of the parts. We find good transferability
within the chemically meaningful parameter regime, raising hopes that PT will
prove useful in chemical applications.Comment: 12 pages, 16 figure
Revivals of Coherence in Chaotic Atom-Optics Billiards
We investigate the coherence properties of thermal atoms confined in optical
dipole traps where the underlying classical dynamics is chaotic. A perturbative
expression derived for the coherence of the echo scheme of [Andersen et. al.,
Phys. Rev. Lett. 90, 023001 (2003)] shows it is a function of the survival
probability or fidelity of eigenstates of the motion of the atoms in the trap.
The echo coherence and the survival probability display "system specific"
features, even when the underlying classical dynamics is chaotic. In
particular, partial revivals in the echo signal and the survival probability
are found for a small shift of the potential. Next, a "semi-classical"
expression for the averaged echo signal is presented and used to calculate the
echo signal for atoms in a light sheet wedge billiard. Revivals in the echo
coherence are found in this system, indicating they may be a generic feature of
dipole traps
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