323 research outputs found
Measurement-induced phase transitions in sparse nonlocal scramblers
Measurement-induced phase transitions arise due to a competition between the scrambling of quantum information in a many-body system and local measurements. In this work we investigate these transitions in different classes of fast scramblers, systems that scramble quantum information as quickly as is conjectured to be possible – on a timescale proportional to the logarithm of the system size. In particular, we consider sets of deterministic sparse couplings that naturally interpolate between local circuits that slowly scramble information and highly nonlocal circuits that achieve the fast-scrambling limit. We find that circuits featuring sparse nonlocal interactions are able to withstand substantially higher rates of local measurement than circuits with only local interactions, even at comparable gate depths. We also study the quantum error-correcting codes that support the volume-law entangled phase and find that our maximally nonlocal circuits yield codes with nearly extensive contiguous code distance. Use of these sparse, deterministic circuits opens pathways towards the design of noise-resilient quantum circuits and error correcting codes in current and future quantum devices with minimum gate numbers
What is the Temperature Dependence of the Casimir Effect?
There has been recent criticism of our approach to the Casimir force between
real metallic surfaces at finite temperature, saying it is in conflict with the
third law of thermodynamics and in contradiction with experiment. We show that
these claims are unwarranted, and that our approach has strong theoretical
support, while the experimental situation is still unclear.Comment: 6 pages, REVTeX, final revision includes two new references and
related discussio
On the Temperature Dependence of the Casimir Effect
The temperature dependence of the Casimir force between a real metallic plate
and a metallic sphere is analyzed on the basis of optical data concerning the
dispersion relation of metals such as gold and copper. Realistic permittivities
imply, together with basic thermodynamic considerations, that the transverse
electric zero mode does not contribute. This results in observable differences
with the conventional prediction, which does not take this physical requirement
into account. The results are shown to be consistent with the third law of
thermodynamics, as well as being consistent with current experiments. However,
the predicted temperature dependence should be detectable in future
experiments. The inadequacies of approaches based on {\it ad hoc} assumptions,
such as the plasma dispersion relation and the use of surface impedance without
transverse momentum dependence, are discussed.Comment: 14 pages, 3 eps figures, revtex4. New version includes clarifications
and new reference. Accepted for publication in Phys. Rev.
Copper Alloy-Impregnated Carbon-Carbon Hybrid Composites for Electronic Packaging Applications
Porous carbon-carbon preforms, based on three-dimensional networks of PAN (Polyacrylonitrile)-based carbon fibers and various volume fractions of chemical vapor-deposited (CVD) carbon, were impregnated by oxygen-free, high-conductivity (OFHC) Cu, Cu-6Si-0.9Cr, and Cu-0.3Si-0.3Cr (wt pct) alloys by pressure infiltration casting. The obtained composites were characterized for their coefficient of thermal expansion (CTE) and thermal conductivity (K) along the through-thickness and two in-plane directions. One composite, with a 28 vol pct Cu-0.3Si-0.3Cr alloy, showed outstanding potential for thermal management applications in electronic applications. This composite exhibited approximately isotropic thermal expansion properties (CTE = 4 to 6.5 ppm/K) and thermal conductivities (k greater than or equal to 260 W/m K)
Calculation of the Casimir Force between Similar and Dissimilar Metal Plates at Finite Temperature
The Casimir pressure is calculated between parallel metal plates, containing
the materials Au, Cu, or Al. Our motivation for making this calculation is the
need of comparing theoretical predictions, based on the Lifshitz formula, with
experiments that are becoming gradually more accurate. In particular, the
finite temperature correction is considered, in view of the recent discussion
in the literature on this point. A special attention is given to the case where
the difference between the Casimir pressures at two different temperatures,
T=300 K and T=350 K, is involved. This seems to be a case that will be
experimentally attainable in the near future, and it will be a critical test of
the temperature correction.Comment: 23 latex pages, 12 figures. Introductory section expanded, 4 new
references. To appear in J. Phys. A: Math. Ge
Thermal corrections to the Casimir effect
The Casimir effect, reflecting quantum vacuum fluctuations in the
electromagnetic field in a region with material boundaries, has been studied
both theoretically and experimentally since 1948. The forces between dielectric
and metallic surfaces both plane and curved have been measured at the 10 to 1
percent level in a variety of room-temperature experiments, and remarkable
agreement with the zero-temperature theory has been achieved. In fitting the
data various corrections due to surface roughness, patch potentials, curvature,
and temperature have been incorporated. It is the latter that is the subject of
the present article. We point out that, in fact, no temperature dependence has
yet been detected, and that the experimental situation is still too fluid to
permit conclusions about thermal corrections to the Casimir effect.
Theoretically, there are subtle issues concerning thermodynamics and
electrodynamics which have resulted in disparate predictions concerning the
nature of these corrections. However, a general consensus has seemed to emerge
that suggests that the temperature correction to the Casimir effect is
relatively large, and should be observable in future experiments involving
surfaces separated at the few micrometer scale.Comment: 21 pages, 9 eps figures, uses iopart.cls. Final version to be
published in New Journal of Physics, contains Conclusion and clarified
remark
Casimir Force on Real Materials - the Slab and Cavity Geometry
We analyse the potential of the geometry of a slab in a planar cavity for the
purpose of Casimir force experiments. The force and its dependence on
temperature, material properties and finite slab thickness are investigated
both analytically and numerically for slab and walls made of aluminium and
teflon FEP respectively. We conclude that such a setup is ideal for
measurements of the temperature dependence of the Casimir force. By numerical
calculation it is shown that temperature effects are dramatically larger for
dielectrics, suggesting that a dielectric such as teflon FEP whose properties
vary little within a moderate temperature range, should be considered for
experimental purposes. We finally discuss the subtle but fundamental matter of
the various Green's two-point function approaches present in the literature and
show how they are different formulations describing the same phenomenon.Comment: 24 pages, 11 figures; expanded discussion, one appendix added, 1 new
figure and 10 new references. To appear in J. Phys. A: Math. Theo
Casimir forces and non-Newtonian gravitation
The search for non-relativistic deviations from Newtonian gravitation can
lead to new phenomena signalling the unification of gravity with the other
fundamental interactions. Various recent theoretical frameworks indicate a
possible window for non-Newtonian forces with gravitational coupling strength
in the micrometre range. The major expected background in the same range is
attributable to the Casimir force or variants of it if dielectric materials,
rather than conducting ones, are considered. Here we review the measurements of
the Casimir force performed so far in the micrometre range and how they
determine constraints on non-Newtonian gravitation, also discussing the
dominant sources of false signals. We also propose a geometry-independent
parameterization of all data in terms of the measurement of the constant c. Any
Casimir force measurement should lead, once all corrections are taken into
account, to a determination of the constant c which, in order to assess the
accuracy of the measurement, can be compared with its more precise value known
through microscopic measurements. Although the last decade of experiments has
resulted in solid demonstrations of the Casimir force, the situation is not
conclusive with respect to being able to discover new physics. Future
experiments and novel phenomenological analysis will be necessary to discover
non-Newtonian forces or to push the window for their possible existence into
regions of the parameter space which theoretically appear unnatural.Comment: Also available at http://www.iop.org/EJ/abstract/1367-2630/8/10/23
Treelike interactions and fast scrambling with cold atoms
We propose an experimentally realizable quantum spin model that exhibits fast scrambling, based on nonlocal interactions that couple sites whose separation is a power of 2. By controlling the relative strengths of deterministic, nonrandom couplings, we can continuously tune from the linear geometry of a nearest-neighbor spin chain to an ultrametric geometry in which the effective distance between spins is governed by their positions on a tree graph. The transition in geometry can be observed in quench dynamics, and is furthermore manifest in calculations of the entanglement entropy. Between the linear and treelike regimes, we find a peak in entanglement and exponentially fast spreading of quantum information across the system. Our proposed implementation, harnessing photon-mediated interactions among cold atoms in an optical cavity, offers a test case for experimentally observing the emergent geometry of a quantum many-body system
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