16,223 research outputs found
Identification of the Sequence of Steps Intrinsic to Spheromak Formation
A planar coaxial electrostatic helicity source is used for studying the relaxation process intrinsic to spheromak formation Experimental observations reveal that spheromak formation involves: (1) breakdown and creation of a number of distinct, arched, filamentary, plasma-filled flux loops that span from cathode to anode gas nozzles, (2) merging of these loops to form a central column, (3) jet-like expansion of the central column, (4) kink instability of the central column, (5) conversion of toroidal flux to poloidal flux by the kink instability. Steps 1 and 3 indicate that spheromak formation involves an MHD pumping of plasma from the gas nozzles into the magnetic flux tube linking the nozzles. In order to measure this pumping, the gas puffing system has been modified to permit simultaneous injection of different gas species into the two ends of the flux tube linking the wall. Gated CCD cameras with narrow-band optical filters are used to track the pumped flows
The information about the state of a charge qubit gained by a weakly coupled quantum point contact
We analyze the information that one can learn about the state of a quantum
two-level system, i.e. a qubit, when probed weakly by a nearby detector. We
consider the general case where the qubit Hamiltonian and the qubit's operator
probed by the detector do not commute. Because the qubit's state keeps evolving
while being probed and the measurement data is mixed with a detector-related
background noise, one might expect the detector to fail in this case. We show,
however, that under suitable conditions and by proper analysis of the
measurement data useful information about the initial state of the qubit can be
extracted. Our approach complements the usual master-equation and
quantum-trajectory approaches, which describe the evolution of the qubit's
quantum state during the measurement process but do not keep track of the
acquired measurement information.Comment: 5 pages, 3 figures; Published in the proceedings of the Nobel
Symposium 141: Qubits for Future Quantum Informatio
Weak and strong measurement of a qubit using a switching-based detector
We analyze the operation of a switching-based detector that probes a qubit's
observable that does not commute with the qubit's Hamiltonian, leading to a
nontrivial interplay between the measurement and free-qubit dynamics. In order
to obtain analytic results and develop intuitive understanding of the different
possible regimes of operation, we use a theoretical model where the detector is
a quantum two-level system that is constantly monitored by a macroscopic
system. We analyze how to interpret the outcome of the measurement and how the
state of the qubit evolves while it is being measured. We find that the answers
to the above questions depend on the relation between the different parameters
in the problem. In addition to the traditional strong-measurement regime, we
identify a number of regimes associated with weak qubit-detector coupling. An
incoherent detector whose switching time is measurable with high accuracy can
provide high-fidelity information, but the measurement basis is determined only
upon switching of the detector. An incoherent detector whose switching time can
be known only with low accuracy provides a measurement in the qubit's energy
eigenbasis with reduced measurement fidelity. A coherent detector measures the
qubit in its energy eigenbasis and, under certain conditions, can provide
high-fidelity information.Comment: 20 pages (two-column), 6 figure
Dynamical instability and domain formation in a spin-1 Bose condensate
We interpret the recently observed spatial domain formation in spin-1 atomic
condensates as a result of dynamical instability. Within the mean field theory,
a homogeneous condensate is dynamically unstable (stable) for ferromagnetic
(antiferromagnetic) atomic interactions. We find this dynamical instability
naturally leads to spontaneous domain formation as observed in several recent
experiments for condensates with rather small numbers of atoms. For trapped
condensates, our numerical simulations compare quantitatively to the
experimental results, thus largely confirming the physical insight from our
analysis of the homogeneous case.Comment: RevTex4, 4 pages with 3 color eps figure, to appear in Phys. Rev.
Let
Self-consistent models of triaxial galaxies in MOND gravity
The Bekenstein-Milgrom gravity theory with a modified Poisson equation is
tested here for the existence of triaxial equilibrium solutions. Using the
non-negative least square method, we show that self-consistent triaxial
galaxies exist for baryonic models with a mild density cusp . Self-consistency is achieved for a wide range of central
concentrations, , representing
low-to-high surface brightness galaxies. Our results demonstrate for the first
time that the orbit superposition technique is fruitful for constructing galaxy
models beyond Newtonian gravity, and triaxial cuspy galaxies might exist
without the help of Cold dark Matter.Comment: 19 pages, 1 table, 7 figures, Accepted for publication in Ap
Soft Mode Dynamics Above and Below the Burns Temperature in the Relaxor Pb(Mg_1/3Nb_2/3)O_3
We report neutron inelastic scattering measurements of the lowest-energy
transverse optic (TO) phonon branch in the relaxor Pb(Mg_1/3Nb_2/3)O_3 from 400
to 1100 K. Far above the Burns temperature T_d ~ 620 K we observe well-defined
propagating TO modes at all wave vectors q, and a zone center TO mode that
softens in a manner consistent with that of a ferroelectric soft mode. Below
T_d the zone center TO mode is overdamped. This damping extends up to, but not
above, the waterfall wave vector q_wf, which is a measure of the average size
of the PNR.Comment: 4 pages, 4 figures; modified discussion of Fig. 3, shortened
captions, added reference, corrected typos, accepted by Phys. Rev. Let
Topological quantum phase transition in an extended Kitaev spin model
We study the quantum phase transition between Abelian and non-Abelian phases
in an extended Kitaev spin model on the honeycomb lattice, where the periodic
boundary condition is applied by placing the lattice on a torus. Our analytical
results show that this spin model exhibits a continuous quantum phase
transition. Also, we reveal the relationship between bipartite entanglement and
the ground-state energy. Our approach directly shows that both the entanglement
and the ground-state energy can be used to characterize the topological quantum
phase transition in the extended Kitaev spin model.Comment: 9 Pages, 4 figure
Confidence and Backaction in the Quantum Filter Equation
We study the confidence and backaction of state reconstruction based on a
continuous weak measurement and the quantum filter equation. As a physical
example we use the traditional model of a double quantum dot being continuously
monitored by a quantum point contact. We examine the confidence of the estimate
of a state constructed from the measurement record, and the effect of
backaction of that measurement on that state. Finally, in the case of general
measurements we show that using the relative entropy as a measure of confidence
allows us to define the lower bound on the confidence as a type of quantum
discord.Comment: 9 pages, 6 figure
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