9,215 research outputs found
Hawking Radiation as Tunneling
We present a short and direct derivation of Hawking radiation as a tunneling
process, based on particles in a dynamical geometry. The imaginary part of the
action for the classically forbidden process is related to the Boltzmann factor
for emission at the Hawking temperature. Because the derivation respects
conservation laws, the exact spectrum is not precisely thermal. We compare and
contrast the problem of spontaneous emission of charged particles from a
charged conductor.Comment: LaTeX, 10 pages; v2. journal version, added section on relation of
black hole radiation to electric charge emission from a charged conducting
sphere; v3. restored cut referenc
Chiral black hole in three-dimensional gravitational Chern-Simons
A chiral black hole can be defined from the three-dimensional pure
gravitational Chern-Simons action as an independent gravitational theory. The
third order derivative of the Cotton tensor gives a dimensional constant which
plays a role of the cosmological constant. The handedness of angular momentum
depends on the signature of the Chern-Simons coefficient. Even in the massless
black hole which corresponds to the static black hole, it has a nonvanishing
angular momentum. We also study statistical entropy and thermodynamic
stability.Comment: 6 pages, a reference added, minor changes to introductio
Interaction cost of non-local gates
We introduce the interaction cost of a non-local gate as the minimal time of
interaction required to perform the gate when assisting the process with fast
local unitaries. This cost, of interest both in the areas of quantum control
and quantum information, depends on the specific interaction, and allows to
compare in an operationally meaningful manner any two non-local gates. In the
case of a two-qubit system, an analytical expression for the interaction cost
of any unitary operation given any coupling Hamiltonian is obtained. One gate
may be more time-consuming than another for any possible interaction. This
defines a partial order structure in the set of non-local gates, that compares
their degree of non-locality. We analytically characterize this partial order
in a region of the set of two-qubit gates.Comment: revtex, 4 pages, no pictures, typos corrected, small changes in
nomenclatur
Coulomb Drag as a Probe of the Nature of Compressible States in a Magnetic Field
Magneto-drag reveals the nature of compressible states and the underlying
interplay of disorder and interactions. At \nu=3/2 a clear T^{4/3} dependence
is observed, which signifies the metallic nature of the N=0 Landau level. In
contrast, drag in higher Landau levels reveals an additional contribution,
which anomalously grows with decreasing T before turning to zero following a
thermal activation law. The anomalous drag is discussed in terms of
electron-hole asymmetry arising from disorder and localization, and the
crossover to normal drag at high fields as due to screening of disorder.Comment: 5 pages, 4 figure
Causal structures and the classification of higher order quantum computations
Quantum operations are the most widely used tool in the theory of quantum
information processing, representing elementary transformations of quantum
states that are composed to form complex quantum circuits. The class of quantum
transformations can be extended by including transformations on quantum
operations, and transformations thereof, and so on up to the construction of a
potentially infinite hierarchy of transformations. In the last decade, a
sub-hierarchy, known as quantum combs, was exhaustively studied, and
characterised as the most general class of transformations that can be achieved
by quantum circuits with open slots hosting variable input elements, to form a
complete output quantum circuit. The theory of quantum combs proved to be
successful for the optimisation of information processing tasks otherwise
untreatable. In more recent years the study of maps from combs to combs has
increased, thanks to interesting examples showing how this next order of maps
requires entanglement of the causal order of operations with the state of a
control quantum system, or, even more radically, superpositions of alternate
causal orderings. Some of these non-circuital transformations are known to be
achievable and have even been achieved experimentally, and were proved to
provide some computational advantage in various information-processing tasks
with respect to quantum combs. Here we provide a formal language to form all
possible types of transformations, and use it to prove general structure
theorems for transformations in the hierarchy. We then provide a mathematical
characterisation of the set of maps from combs to combs, hinting at a route for
the complete characterisation of maps in the hierarchy. The classification is
strictly related to the way in which the maps manipulate the causal structure
of input circuits.Comment: 12 pages, revtex styl
Stabilized hot electron bolometer heterodyne receiver at 2.5 THz
We report on a method to stabilize a hot electron bolometer (HEB) mixer at 2.5 THz. The technique utilizes feedback control of the local oscillator (LO) laser power by means of a swing-arm actuator placed in the optical beam path. We demonstrate that this technique yields a factor of 50 improvement in the spectroscopic Allan variance time which is shown to be over 30 s in a 12 MHz noise fluctuation bandwidth. Furthermore, broadband signal direct detection effects may be minimized by this technique. The technique is versatile and can be applied to practically any local oscillator at any frequency
Characterization of tomographically faithful states in terms of their Wigner function
A bipartite quantum state is tomographically faithful when it can be used as
an input of a quantum operation acting on one of the two quantum systems, such
that the joint output state carries a complete information about the operation
itself. Tomographically faithful states are a necessary ingredient for
tomography of quantum operations and for complete quantum calibration of
measuring apparatuses. In this paper we provide a complete classification of
such states for continuous variables in terms of the Wigner function of the
state. For two-mode Gaussian states faithfulness simply resorts to correlation
between the modes.Comment: 9 pages. IOPAMS style. Some improvement
Evolution equation of entanglement for general bipartite systems
We explore how entanglement of a general bipartite system evolves when one
subsystem undergoes the action of an arbitrary noisy channel. It is found that
the dynamics of entanglement for general bipartite systems under the influence
of such channel is determined by the channel's action on the maximally
entangled state, which includes as a special case the results for two-qubit
systems [Nature Physics 4, 99 (2008)]. In particular, for multi-qubit or
qubit-qudit systems, we get a general factorization law for evolution equation
of entanglement with one qubit being subject to a noisy channel. Our results
can help the experimental characterization of entanglement dynamics.Comment: 4 pages, 1 figur
Activated Transport in the individual Layers that form the =1 Exciton Condensate
We observe the total filling factor =1 quantum Hall state in a
bilayer two-dimensional electron system with virtually no tunnelling. We find
thermally activated transport in the balanced system with a monotonic increase
of the activation energy with decreasing below 1.65. In the
imbalanced system we find activated transport in each of the layers separately,
yet the activation energies show a striking asymmetry around the balance point.
This implies that the gap to charge-excitations in the {\em individual} layers
is substantially different for positive and negative imbalance.Comment: 4 pages. 4 figure
Quantum fields in gravity
We give a brief description of some compelling connections between general
relativity and thermodynamics through i) the semi-classical tunnelling
method(s) and ii) the field-theoretical modelling of Unruh-DeWitt detectors. In
both approaches it is possible to interpret some quantities in a
thermodynamical frame.Comment: 4 pages, no figures, contribution to the proceedings of the
conference "Relativity and Gravitation - 100 years after Einstein in Prague
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