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Fluctuations in the electron system of a superconductor exposed to a photon flux
We report on fluctuations in the electron system, Cooper pairs and
quasiparticles, of a superconducting aluminium film. The superconductor is
exposed to pair-breaking photons (1.54 THz), which are coupled through an
antenna. The change in the complex conductivity of the superconductor upon a
change in the quasiparticle number is read out by a microwave resonator. A
large range in radiation power can be chosen by carefully filtering the
radiation from a blackbody source. We identify two regimes. At high radiation
power, fluctuations in the electron system caused by the random arrival rate of
the photons are resolved, giving a straightforward measure of the optical
efficiency (48%). At low radiation power fluctuations are dominated by excess
quasiparticles, the number of which is measured through their recombination
lifetime
Reduced frequency noise in superconducting resonators
We report a reduction of the frequency noise in coplanar waveguide
superconducting resonators. The reduction of 7 dB is achieved by removing the
exposed dielectric substrate surface from the region with high electric fields
and by using NbTiN. In a model-analysis the surface of NbTiN is found to be a
negligible source of noise, experimentally supported by a comparison with NbTiN
on SiOx resonators. The reduction is additive to decreasing the noise by
widening the resonators.Comment: 4 pages, 4 figure
Microwave-induced excess quasiparticles in superconducting resonators measured through correlated conductivity fluctuations
We have measured the number of quasiparticles and their lifetime in aluminium
superconducting microwave resonators. The number of excess quasiparticles below
160 mK decreases from 72 to 17 m with a 6 dB decrease of the
microwave power. The quasiparticle lifetime increases accordingly from 1.4 to
3.5 ms. These properties of the superconductor were measured through the
spectrum of correlated fluctuations in the quasiparticle system and condensate
of the superconductor, which show up in the resonator amplitude and phase
respectively. Because uncorrelated noise sources vanish, fluctuations in the
superconductor can be studied with a sensitivity close to the vacuum noise
Gait and electromyographic analysis of patients recovering after limb-saving surgery
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Is Quantum Spacetime Foam Unstable?
A very simple wormhole geometry is considered as a model of a mode of
topological fluctutation in Planck-scale spacetime foam. Quantum dynamics of
the hole reduces to quantum mechanics of one variable, throat radius, and
admits a WKB analysis. The hole is quantum-mechanically unstable: It has no
bound states. Wormhole wave functions must eventually leak to large radii. This
suggests that stability considerations along these lines may place strong
constraints on the nature and even the existence of spacetime foam.Comment: 15 page
Number fluctuations of sparse quasiparticles in a superconductor
We have directly measured quasiparticle number fluctuations in a thin film
superconducting Al resonator in thermal equilibrium. The spectrum of these
fluctuations provides a measure of both the density and the lifetime of the
quasiparticles. We observe that the quasiparticle density decreases
exponentially with decreasing temperature, as theoretically predicted, but
saturates below 160 mK to 25-55 per cubic micron. We show that this saturation
is consistent with the measured saturation in the quasiparticle lifetime, which
also explains similar observations in qubit decoherence times
Hawking radiation without black hole entropy
In this Letter I point out that Hawking radiation is a purely kinematic
effect that is generic to Lorentzian geometries. Hawking radiation arises for
any test field on any Lorentzian geometry containing an event horizon
regardless of whether or not the Lorentzian geometry satisfies the dynamical
Einstein equations of general relativity. On the other hand, the classical laws
of black hole mechanics are intrinsically linked to the Einstein equations of
general relativity (or their perturbative extension into either semiclassical
quantum gravity or string-inspired scenarios). In particular, the laws of black
hole thermodynamics, and the identification of the entropy of a black hole with
its area, are inextricably linked with the dynamical equations satisfied by the
Lorentzian geometry: entropy is proportional to area (plus corrections) if and
only if the dynamical equations are the Einstein equations (plus corrections).
It is quite possible to have Hawking radiation occur in physical situations in
which the laws of black hole mechanics do not apply, and in situations in which
the notion of black hole entropy does not even make any sense. This observation
has important implications for any derivation of black hole entropy that seeks
to deduce black hole entropy from the Hawking radiation.Comment: Uses ReV_TeX 3.0; Five pages in two-column forma
Anomalous response of superconducting titanium nitride resonators to terahertz radiation
We present an experimental study of KIDs fabricated of atomic layer deposited
TiN films, and characterized at radiation frequencies of ~GHz. The
responsivity to radiation is measured and found to increase with increasing
radiation powers, opposite to what is expected from theory and observed for
hybrid niobium titanium nitride / aluminium (NbTiN/Al) and all-aluminium
(all-Al) KIDs. The noise is found to be independent of the level of the
radiation power. The noise equivalent power (NEP) improves with higher
radiation powers, also opposite to what is observed and well understood for
hybrid NbTiN/Al and all-Al KIDs. We suggest that an inhomogeneous state of
these disordered superconductors should be used to explain these observations
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