6,329 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
On tuning a reactive silencer by varying the position of an internal membrane
A mode-matching method is used to investigate the performance of a two-dimensional, modified reactive silencer. The modification takes the form of a membrane which is attached to the internal walls of the expansion chamber parallel to the axis of the inlet/outlet ducts. The height of the membrane above the level of the inlet/outlet ducts can be varied and, by this means, the device is tuned. It is shown that the stopband produced by the silencer can be broadened and/or shifted depending upon the height to which the membrane is raised. Attention is focused on the efficiency of the device at low-frequencies - the regime where dissipative silencers are usually least effective. The potential use of the device as a component in a hybrid silencer for heating ventilation and air-conditioning (HVAC) ducting systems is discussed
Commonalities in the psychological factors associated with problem gambling and internet dependence
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
Entangled Light in Moving Frames
We calculate the entanglement between a pair of polarization-entangled photon
beams as a function of the reference frame, in a fully relativistic framework.
We find the transformation law for helicity basis states and show that, while
it is frequency independent, a Lorentz transformation on a momentum-helicity
eigenstate produces a momentum-dependent phase. This phase leads to changes in
the reduced polarization density matrix, such that entanglement is either
decreased or increased, depending on the boost direction, the rapidity, and the
spread of the beam.Comment: 4 pages and 3 figures. Minor corrections, footnote on optimal basis
state
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
The Vortex Phase Qubit: Generating Arbitrary, Counter-Rotating, Coherent Superpositions in Bose-Einstein Condensates via Optical Angular Momentum Beams
We propose a scheme for generation of arbitrary coherent superposition of
vortex states in Bose-Einstein condensates (BEC) using the orbital angular
momentum (OAM) states of light. We devise a scheme to generate coherent
superpositions of two counter-rotating OAM states of light using known
experimental techniques. We show that a specially designed Raman scheme allows
transfer of the optical vortex superposition state onto an initially
non-rotating BEC. This creates an arbitrary and coherent superposition of a
vortex and anti-vortex pair in the BEC. The ideas presented here could be
extended to generate entangled vortex states, design memories for the OAM
states of light, and perform other quantum information tasks. Applications to
inertial sensing are also discussed.Comment: 4 pages, 4 figures, Revtex4, to be submitted to Phys. Rev. Let
Do Perceptions of Ballot Secrecy Influence Turnout? Results from a Field Experiment
Although the secret ballot has long been secured as a legal matter in the United States, formal secrecy protections are not equivalent to convincing citizens that they may vote privately and without fear of reprisal. We present survey evidence that those who have not previously voted are particularly likely to voice doubts about the secrecy of the voting process. We then report results from a field experiment where we provided registered voters with information about ballot secrecy protections prior to the 2010 general election. We find that these letters increased turnout for registered citizens without records of previous turnout, but did not appear to influence the behavior of citizens who had previously voted. These results suggest that although the secret ballot is a long-standing institution in the United States, providing basic information about ballot secrecy can affect the decision to participate to an important degree.
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