2,799 research outputs found
A loophole to the universal photon spectrum in electromagnetic cascades: application to the "cosmological lithium problem"
The standard theory of electromagnetic cascades onto a photon background
predicts a quasi-universal shape for the resulting non-thermal photon spectrum.
This has been applied to very disparate fields, including non-thermal big bang
nucleosynthesis (BBN). However, once the energy of the injected photons falls
below the pair-production threshold the spectral shape is very different, a
fact that has been overlooked in past literature. This loophole may have
important phenomenological consequences, since it generically alters the BBN
bounds on non-thermal relics: for instance it allows to re-open the possibility
of purely electromagnetic solutions to the so-called "cosmological lithium
problem", which were thought to be excluded by other cosmological constraints.
We show this with a proof-of-principle example and a simple particle physics
model, compared with previous literature.Comment: 5 pages, 2 figures, typos corrected; matches version published in
PRL. (Version 1 of this article was submitted to arxiv on Jan. 8th, kept on
hold by arxiv moderators due to unspecified classification doubts for almost
one month.
Decoding Schemes for Foliated Sparse Quantum Error Correcting Codes
Foliated quantum codes are a resource for fault-tolerant measurement-based
quantum error correction for quantum repeaters and for quantum computation.
They represent a general approach to integrating a range of possible quantum
error correcting codes into larger fault-tolerant networks. Here we present an
efficient heuristic decoding scheme for foliated quantum codes, based on
message passing between primal and dual code 'sheets'. We test this decoder on
two different families of sparse quantum error correcting code: turbo codes and
bicycle codes, and show reasonably high numerical performance thresholds. We
also present a construction schedule for building such code states.Comment: 23 pages, 15 figures, accepted for publication in Phys. Rev.
Fast Decoders for Topological Quantum Codes
We present a family of algorithms, combining real-space renormalization
methods and belief propagation, to estimate the free energy of a topologically
ordered system in the presence of defects. Such an algorithm is needed to
preserve the quantum information stored in the ground space of a topologically
ordered system and to decode topological error-correcting codes. For a system
of linear size L, our algorithm runs in time log L compared to L^6 needed for
the minimum-weight perfect matching algorithm previously used in this context
and achieves a higher depolarizing error threshold.Comment: 4 pages, 4 figure
Magic State Distillation with Low Space Overhead and Optimal Asymptotic Input Count
We present an infinite family of protocols to distill magic states for
-gates that has a low space overhead and uses an asymptotic number of input
magic states to achieve a given target error that is conjectured to be optimal.
The space overhead, defined as the ratio between the physical qubits to the
number of output magic states, is asymptotically constant, while both the
number of input magic states used per output state and the -gate depth of
the circuit scale linearly in the logarithm of the target error (up to
). Unlike other distillation protocols, this protocol
achieves this performance without concatenation and the input magic states are
injected at various steps in the circuit rather than all at the start of the
circuit. The protocol can be modified to distill magic states for other gates
at the third level of the Clifford hierarchy, with the same asymptotic
performance. The protocol relies on the construction of weakly self-dual CSS
codes with many logical qubits and large distance, allowing us to implement
control-SWAPs on multiple qubits. We call this code the "inner code". The
control-SWAPs are then used to measure properties of the magic state and detect
errors, using another code that we call the "outer code". Alternatively, we use
weakly-self dual CSS codes which implement controlled Hadamards for the inner
code, reducing circuit depth. We present several specific small examples of
this protocol.Comment: 39 pages, (v2) renamed "odd" and "even" weakly self-dual CSS codes of
(v1) to "normal" and "hyperbolic" codes, respectively. (v3) published in
Quantu
Dark Matter annihilations in halos and high-redshift sources of reionization of the universe
It is well known that annihilations in the homogeneous fluid of dark matter
(DM) can leave imprints in the cosmic microwave background (CMB) anisotropy
power spectrum. However, the relevance of DM annihilations in halos for
cosmological observables is still subject to debate, with previous works
reaching different conclusions on this point. Also, all previous studies used a
single type of parameterization for the astrophysical reionization, and
included no astrophysical source for the heating of the intergalactic medium.
In this work, we revisit these problems. When standard approaches are adopted,
we find that the ionization fraction does exhibit a very particular (and
potentially constraining) pattern, but the currently measurable optical depth
to reionization is left almost unchanged: In agreement with the most of the
previous literature, for plausible halo models we find that the modification of
the signal with respect to the one coming from annihilations in the smooth
background is tiny, below cosmic variance within currently allowed parameter
space. However, if different and probably more realistic treatments of the
astrophysical sources of reionization and heating are adopted, a more
pronounced effect of the DM annihilation in halos is possible. We thus conclude
that within currently adopted baseline models the impact of the virialised DM
structures cannot be uncovered by CMB power spectra measurements, but a larger
impact is possible if peculiar models are invoked for the redshift evolution of
the DM annihilation signal or different assumptions are made for the
astrophysical contributions. A better understanding (both theoretical and
observational) of the reionization and temperature history of the universe,
notably via the 21 cm signal, seems the most promising way for using halo
formation as a tool in DM searches, improving over the sensitivity of current
cosmological probes.Comment: 30 pages, 11 figures. v2: extended version (notably astrophysical
source effects significantly expanded), references added, main conclusions
unchanged. Matches version accepted by JCA
A fresh look at linear cosmological constraints on a decaying dark matter component
We consider a cosmological model in which a fraction of the Dark Matter
(DM) is allowed to decay in an invisible relativistic component, and compute
the resulting constraints on both the decay width (or inverse lifetime)
and from purely gravitational arguments. We report a full
derivation of the Boltzmann hierarchy, correcting a mistake in previous
literature, and compute the impact of the decay --as a function of the
lifetime-- on the CMB and matter power spectra. From CMB only, we obtain that
no more than 3.8 % of the DM could have decayed in the time between
recombination and today (all bounds quoted at 95 % CL). We also comment on the
important application of this bound to the case where primordial black holes
constitute DM, a scenario notoriously difficult to constrain. For lifetimes
longer than the age of the Universe, the bounds can be cast as Gyr. For the first time, we also checked that
degeneracies with massive neutrinos are broken when information from the large
scale structure is used. Even secondary effects like CMB lensing suffice to
this purpose. Decaying DM models have been invoked to solve a possible tension
between low redshift astronomical measurements of and and the ones inferred by Planck. We reassess this claim finding that with
the most recent BAO, HST and data extracted from the CFHT survey,
the tension is only slightly reduced despite the two additional free
parameters, loosening the bound to Gyr.
The bound however improves to Gyr if only
data consistent with the CMB are included. This highlights the importance of
establishing whether the tension is due to real physical effects or unaccounted
systematics, for settling the reach of achievable constraints on decaying DM.Comment: 30p, 11 figures, comments welcom
Quantum simulation of time-dependent Hamiltonians and the convenient illusion of Hilbert space
We consider the manifold of all quantum many-body states that can be
generated by arbitrary time-dependent local Hamiltonians in a time that scales
polynomially in the system size, and show that it occupies an exponentially
small volume in Hilbert space. This implies that the overwhelming majority of
states in Hilbert space are not physical as they can only be produced after an
exponentially long time. We establish this fact by making use of a
time-dependent generalization of the Suzuki-Trotter expansion, followed by a
counting argument. This also demonstrates that a computational model based on
arbitrarily rapidly changing Hamiltonians is no more powerful than the standard
quantum circuit model.Comment: Presented at QIP 201
Steam bottoming cycle for an adiabatic diesel engine
Steam bottoming cycles using adiabatic diesel engine exhaust heat which projected substantial performance and economic benefits for long haul trucks were studied. Steam cycle and system component variables, system cost, size and performance were analyzed. An 811 K/6.90 MPa state of the art reciprocating expander steam system with a monotube boiler and radiator core condenser was selected for preliminary design. The costs of the diesel with bottoming system (TC/B) and a NASA specified turbocompound adiabatic diesel with aftercooling with the same total output were compared, the annual fuel savings less the added maintenance cost was determined to cover the increase initial cost of the TC/B system in a payback period of 2.3 years. Steam bottoming system freeze protection strategies were developed, technological advances required for improved system reliability are considered and the cost and performance of advanced systes are evaluated
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