474 research outputs found
Characterisation of the dynamical quantum state of a zero temperature Bose-Einstein condensate
We describe the quantum state of a Bose-Einstein condensate at zero
temperature. By evaluating the Q-function we show that the ground state of
Bose-Einstein condensate under the Hartree approximation is squeezed. We find
that multimode Schroedinger cat states are generated as the condensate evolves
in a ballistic expansion.Comment: 13 pages, 6 figure
An Improved Neutron Electric Dipole Moment Experiment
A new measurement of the neutron EDM, using Ramsey's method of separated
oscillatory fields, is in preparation at the new high intensity source of
ultra-cold neutrons (UCN) at the Paul Scherrer Institute, Villigen, Switzerland
(PSI). The existence of a non-zero nEDM would violate both parity and time
reversal symmetry and, given the CPT theorem, might lead to a discovery of new
CP violating mechanisms. Already the current upper limit for the nEDM
(|d_n|<2.9E-26 e.cm) constrains some extensions of the Standard Model.
The new experiment aims at a two orders of magnitude reduction of the
experimental uncertainty, to be achieved mainly by (1) the higher UCN flux
provided by the new PSI source, (2) better magnetic field control with improved
magnetometry and (3) a double chamber configuration with opposite electric
field directions.
The first stage of the experiment will use an upgrade of the RAL/Sussex/ILL
group's apparatus (which has produced the current best result) moved from
Institut Laue-Langevin to PSI. The final accuracy will be achieved in a further
step with a new spectrometer, presently in the design phase.Comment: Flavor Physics & CP Violation Conference, Taipei, 200
Formation of a molecular Bose-Einstein condensate and an entangled atomic gas by Feshbach resonance
Processes of association in an atomic Bose-Einstein condensate, and
dissociation of the resulting molecular condensate, due to Feshbach resonance
in a time-dependent magnetic field, are analyzed incorporating non-mean-field
quantum corrections and inelastic collisions. Calculations for the Na atomic
condensate demonstrate that there exist optimal conditions under which about
80% of the atomic population can be converted to a relatively long-lived
molecular condensate (with lifetimes of 10 ms and more). Entangled atoms in
two-mode squeezed states (with noise reduction of about 30 dB) may also be
formed by molecular dissociation. A gas of atoms in squeezed or entangled
states can have applications in quantum computing, communications, and
measurements.Comment: LaTeX, 5 pages with 4 figures, uses REVTeX
Early Universe Quantum Processes in BEC Collapse Experiments
We show that in the collapse of a Bose-Einstein condensate (BEC) {For an
excellent introduction to BEC theory, see C. Pethick and H. Smith,
Bose-Einstein condensation in dilute gases (Cambridge University Press,
Cambridge, England, 2002)} certain processes involved and mechanisms at work
share a common origin with corresponding quantum field processes in the early
universe such as particle creation, structure formation and spinodal
instability. Phenomena associated with the controlled BEC collapse observed in
the experiment of Donley et al E. Donley et. al., Nature 412, 295 (2001)(they
call it `Bose-Nova', see also J. Chin, J. Vogels and W. Ketterle, Phys. Rev.
Lett. 90, 160405 (2003)) such as the appearance of bursts and jets can be
explained as a consequence of the squeezing and amplification of quantum
fluctuations above the condensate by the dynamics of the condensate. Using the
physical insight gained in depicting these cosmological processes, our analysis
of the changing amplitude and particle contents of quantum excitations in these
BEC dynamics provides excellent quantitative fits with the experimental data on
the scaling behavior of the collapse time and the amount of particles emitted
in the jets. Because of the coherence properties of BEC and the high degree of
control and measurement precision in atomic and optical systems, we see great
potential in the design of tabletop experiments for testing out general ideas
and specific (quantum field) processes in the early universe, thus opening up
the possibility for implementing `laboratory cosmology'.Comment: 7 pages, 3 figures. Invited Talk presented at the Peyresq Meetings of
Gravitation and Cosmology, 200
Evolutionary Toggling of Vpx/Vpr Specificity Results in Divergent Recognition of the Restriction Factor SAMHD1
SAMHD1 is a host restriction factor that blocks the ability of lentiviruses such as HIV-1 to undergo reverse transcription in myeloid cells and resting T-cells. This restriction is alleviated by expression of the lentiviral accessory proteins Vpx and Vpr (Vpx/Vpr), which target SAMHD1 for proteasome-mediated degradation. However, the precise determinants within SAMHD1 for recognition by Vpx/Vpr remain unclear. Here we show that evolution of Vpx/Vpr in primate lentiviruses has caused the interface between SAMHD1 and Vpx/Vpr to alter during primate lentiviral evolution. Using multiple HIV-2 and SIV Vpx proteins, we show that Vpx from the HIV-2 and SIVmac lineage, but not Vpx from the SIVmnd2 and SIVrcm lineage, require the C-terminus of SAMHD1 for interaction, ubiquitylation, and degradation. On the other hand, the N-terminus of SAMHD1 governs interactions with Vpx from SIVmnd2 and SIVrcm, but has little effect on Vpx from HIV-2 and SIVmac. Furthermore, we show here that this difference in SAMHD1 recognition is evolutionarily dynamic, with the importance of the N- and C-terminus for interaction of SAMHD1 with Vpx and Vpr toggling during lentiviral evolution. We present a model to explain how the head-to-tail conformation of SAMHD1 proteins favors toggling of the interaction sites by Vpx/Vpr during this virus-host arms race. Such drastic functional divergence within a lentiviral protein highlights a novel plasticity in the evolutionary dynamics of viral antagonists for restriction factors during lentiviral adaptation to its hosts. © 2013 Fregoso et al
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