3,534 research outputs found
Two qubits can be entangled in two distinct temperature regions
We have found that for a wide range of two-qubit Hamiltonians the
canonical-ensemble thermal state is entangled in two distinct temperature
regions. In most cases the ground state is entangled; however we have also
found an example where the ground state is separable and there are still two
regions. This demonstrates that the qualitative behavior of entanglement with
temperature can be much more complicated than might otherwise have been
expected; it is not simply determined by the entanglement of the ground state,
even for the simple case of two qubits. Furthermore, we prove a finite bound on
the number of possible entangled regions for two qubits, thus showing that
arbitrarily many transitions from entanglement to separability are not
possible. We also provide an elementary proof that the spectrum of the thermal
state at a lower temperature majorizes that at a higher temperature, for any
Hamiltonian, and use this result to show that only one entangled region is
possible for the special case of Hamiltonians without magnetic fields.Comment: 6 pages, 4 figures, many new result
Mobilization of the platinum group elements by low-temperature fluids: Implications for mineralization and the iridium controversy
Geochemical investigations on the widely dispersed Late Proterozoic Acraman impact ejecta horizon and its host marine shales in the Adelaide Geosyncline provide strong evidence for low-temperature mobilization of the platinum group elements (PGE), including Ir. The ejecta horizon was formed when the middle Proterozoic dacitic volcanics in the Gawler Ranges, central South Australia, were impacted by a very large (ca. 4 km) meteorite. The resulting structure, now represented by Lake Acraman, is Australia's largest meteorite impact structure. Debris from the impact was blasted for many hundreds of kilometers, some falling into the shallow sea of the Adelaide Geosyncline, some 300 km to the east of the impact site
Quantum Technology: The Second Quantum Revolution
We are currently in the midst of a second quantum revolution. The first
quantum revolution gave us new rules that govern physical reality. The second
quantum revolution will take these rules and use them to develop new
technologies. In this review we discuss the principles upon which quantum
technology is based and the tools required to develop it. We discuss a number
of examples of research programs that could deliver quantum technologies in
coming decades including; quantum information technology, quantum
electromechanical systems, coherent quantum electronics, quantum optics and
coherent matter technology.Comment: 24 pages and 6 figure
Cold Atom Physics Using Ultra-Thin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions
The strong evanescent field around ultra-thin unclad optical fibers bears a
high potential for detecting, trapping, and manipulating cold atoms.
Introducing such a fiber into a cold atom cloud, we investigate the interaction
of a small number of cold Caesium atoms with the guided fiber mode and with the
fiber surface. Using high resolution spectroscopy, we observe and analyze
light-induced dipole forces, van der Waals interaction, and a significant
enhancement of the spontaneous emission rate of the atoms. The latter can be
assigned to the modification of the vacuum modes by the fiber.Comment: 4 pages, 4 figure
Quantum reflection of atoms from a solid surface at normal incidence
We observed quantum reflection of ultracold atoms from the attractive
potential of a solid surface. Extremely dilute Bose-Einstein condensates of
^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak
gravito-magnetic trap were normally incident on a silicon surface. Reflection
probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s.
The velocity dependence agrees qualitatively with the prediction for quantum
reflection from the attractive Casimir-Polder potential. Atoms confined in a
harmonic trap divided in half by a solid surface exhibited extended lifetime
due to quantum reflection from the surface, implying a reflection probability
above 50 %.Comment: To appear in Phys. Rev. Lett. (December 2004)5 pages, 4 figure
Entanglement of indistinguishable particles in condensed matter physics
The concept of entanglement in systems where the particles are
indistinguishable has been the subject of much recent interest and controversy.
In this paper we study the notion of entanglement of particles introduced by
Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)] in several specific
physical systems, including some that occur in condensed matter physics. The
entanglement of particles is relevant when the identical particles are
itinerant and so not distinguished by their position as in spin models. We show
that entanglement of particles can behave differently to other approaches that
have been used previously, such as entanglement of modes (occupation-number
entanglement) and the entanglement in the two-spin reduced density matrix. We
argue that the entanglement of particles is what could actually be measured in
most experimental scenarios and thus its physical significance is clear. This
suggests entanglement of particles may be useful in connecting theoretical and
experimental studies of entanglement in condensed matter systems.Comment: 13 pages, 6 figures, comments welcome, published version (minor
changes, added references
Effect of an atom on a quantum guided field in a weakly driven fiber-Bragg-grating cavity
We study the interaction of an atom with a quantum guided field in a weakly
driven fiber-Bragg-grating (FBG) cavity. We present an effective Hamiltonian
and derive the density-matrix equations for the combined atom-cavity system. We
calculate the mean photon number, the second-order photon correlation function,
and the atomic excited-state population. We show that, due to the confinement
of the guided cavity field in the fiber cross-section plane and in the space
between the FBG mirrors, the presence of the atom in the FBG cavity can
significantly affect the mean photon number and the photon statistics even
though the cavity finesse is moderate, the cavity is long, and the probe field
is weak.Comment: Accepted for Phys. Rev.
Continuation Sheaves in Dynamics: Sheaf Cohomology and Bifurcation
Continuation of algebraic structures in families of dynamical systems is
described using category theory, sheaves, and lattice algebras. Well-known
concepts in dynamics, such as attractors or invariant sets, are formulated as
functors on appropriate categories of dynamical systems mapping to categories
of lattices, posets, rings or abelian groups. Sheaves are constructed from such
functors, which encode data about the continuation of structure as system
parameters vary. Similarly, morphisms for the sheaves in question arise from
natural transformations. This framework is applied to a variety of lattice
algebras and ring structures associated to dynamical systems, whose algebraic
properties carry over to their respective sheaves. Furthermore, the cohomology
of these sheaves are algebraic invariants which contain information about
bifurcations of the parametrized systems
On the Squeezed Number States and their Phase Space Representations
We compute the photon number distribution, the Q distribution function and
the wave functions in the momentum and position representation for a single
mode squeezed number state using generating functions which allow to obtain any
matrix element in the squeezed number state representation from the matrix
elements in the squeezed coherent state representation. For highly squeezed
number states we discuss the previously unnoted oscillations which appear in
the Q function. We also note that these oscillations can be related to the
photon-number distribution oscillations and to the momentum representation of
the wave function.Comment: 16 pages, 9 figure
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