333 research outputs found
An instability criterion for a finite amplitude localized disturbance in a shear flow of electrically conducting fluids
The stability of shear flows of electrically conducting fluids, with respect
to finite amplitude three-dimensional localized disturbances is considered. The
time evolution of the fluid impulse integral, characterizing such disturbances,
for the case of low magnetic Reynolds number is obtained by integrating
analytically the vorticity equation. Analysis of the resulted equation reveals
a new instability criterion.Comment: 10 pages in LaTex, no figures, accepted in Phys. Fluid
The SQUID Handbook
This two-volume handbook offers a comprehensive and well coordinated presentation of SQUIDs (Superconducting Quantum Interference Devices), including device fundamentals, design, technology, system construction and multiple applications. It is intended to bridge the gap between fundamentals and applications, and will be a valuable textbook reference for graduate students and for professionals engaged in SQUID research and engineering. It will also be of use to specialists in multiple fields of practical SQUID applications, from human brain research and heart diagnostics to airplane and nuclea
Measuring Which-Path Information with Coupled Electronic Mach-Zehnder Interferometers
We theoretically investigate a generalized "which-path" measurement on an
electronic Mach-Zehnder Interferometer (MZI) implemented via Coulomb coupling
to a second electronic MZI acting as a detector. The use of contextual values,
or generalized eigenvalues, enables the precise construction of which-path
operator averages that are valid for any measurement strength from the
available drain currents. The form of the contextual values provides direct
physical insight about the measurement being performed, providing information
about the correlation strength between system and detector, the measurement
inefficiency, and the proper background removal. We find that the detector
interferometer must display maximal wave-like behavior to optimally measure the
particle-like which-path information in the system interferometer,
demonstrating wave-particle complementarity between the system and detector. We
also find that the degree of quantum erasure that can be achieved by
conditioning on a specific detector drain is directly related to the ambiguity
of the measurement. Finally, conditioning the which-path averages on a
particular system drain using the zero frequency cross-correlations produces
conditioned averages that can become anomalously large due to quantum
interference; the weak coupling limit of these conditioned averages can produce
both weak values and detector-dependent semi-weak values.Comment: 17 pages, 12 figures, published version including appendi
Optomechanical cooling of levitated spheres with doubly-resonant fields
Optomechanical cooling of levitated dielectric particles represents a
promising new approach in the quest to cool small mechanical resonators towards
their quantum ground state. We investigate two-mode cooling of levitated
nanospheres in a self-trapping regime. We identify a rich structure of split
sidebands (by a mechanism unrelated to usual strong-coupling effects) and
strong cooling even when one mode is blue detuned. We show the best regimes
occur when both optical fields cooperatively cool and trap the nanosphere,
where cooling rates are over an order of magnitude faster compared to
corresponding single-sideband cooling rates.Comment: 8 Pages, 7 figure
Simultaneous readout of two charge qubits
We consider a system of two solid state charge qubits, coupled to a single
read-out device, consisting of a single-electron transistor (SET). The
conductance of each tunnel junction is influenced by its neighboring qubit, and
thus the current through the transistor is determined by the qubits' state. The
full counting statistics of the electrons passing the transistor is calculated,
and we discuss qubit dephasing, as well as the quantum efficiency of the
readout. The current measurement is then compared to readout using real-time
detection of the SET island's charge state. For the latter method we show that
the quantum efficiency is always unity. Comparing the two methods a simple
geometrical interpretation of the quantum efficiency of the current measurement
appears. Finally, we note that full quantum efficiency in some cases can be
achieved measuring the average charge of the SET island, in addition to the
average current.Comment: 11 pages with 5 figure
Propagation of Light in the Field of Stationary and Radiative Gravitational Multipoles
Extremely high precision of near-future radio/optical interferometric
observatories like SKA, Gaia, SIM and the unparalleled sensitivity of LIGO/LISA
gravitational-wave detectors demands more deep theoretical treatment of
relativistic effects in the propagation of electromagnetic signals through
variable gravitational fields of the solar system, oscillating and precessing
neutron stars, coalescing binary systems, exploding supernova, and colliding
galaxies. Especially important for future gravitational-wave observatories is
the problem of propagation of light rays in the field of multipolar
gravitational waves emitted by a localized source of gravitational radiation.
Present paper suggests physically-adequate and consistent mathematical solution
of this problem in the first post-Minkowskian approximation of General
Relativity which accounts for all time-dependent multipole moments of an
isolated astronomical system.Comment: 36 pages, no figure
Cooling of a mirror by radiation pressure
We describe an experiment in which a mirror is cooled by the radiation
pressure of light. A high-finesse optical cavity with a mirror coated on a
mechanical resonator is used as an optomechanical sensor of the Brownian motion
of the mirror. A feedback mechanism controls this motion via the radiation
pressure of a laser beam reflected on the mirror. We have observed either a
cooling or a heating of the mirror, depending on the gain of the feedback loop.Comment: 4 pages, 6 figures, RevTe
Causality, stability and passivity for a mirror in vacuum
The mean force exerted upon a perfect mirror moving in vacuum in a two
dimensional spacetime has the same expression as the radiation reaction force
computed in classical electron theory. It follows that unacceptable runaway
solutions are predicted. We show that this instability problem does not appear
when partially transmitting mirrors are studied. The mechanical impedance
describing the mirror coupled to vacuum radiation pressure is computed
explicitly; recoil is neglected. It is found to be a passive function, so that
stability is ensured. This is connected to the fact that no energy can be
extracted from the vacuum state.Comment: 5 pages, corrected typo in formula
Gravitomagnetic Field of a Rotating Superconductor and of a Rotating Superfluid
The quantization of the extended canonical momentum in quantum materials
including the effects of gravitational drag is applied successively to the case
of a multiply connected rotating superconductor and superfluid. Experiments
carried out on rotating superconductors, based on the quantization of the
magnetic flux in rotating superconductors, lead to a disagreement with the
theoretical predictions derived from the quantization of a canonical momentum
without any gravitomagnetic term. To what extent can these discrepancies be
attributed to the additional gravitomagnetic term of the extended canonical
momentum? This is an open and important question. For the case of multiply
connected rotating neutral superfluids, gravitational drag effects derived from
rotating superconductor data appear to be hidden in the noise of present
experiments according to a first rough analysis
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