684 research outputs found
Entropy, majorization and thermodynamics in general probabilistic theories
In this note we lay some groundwork for the resource theory of thermodynamics
in general probabilistic theories (GPTs). We consider theories satisfying a
purely convex abstraction of the spectral decomposition of density matrices:
that every state has a decomposition, with unique probabilities, into perfectly
distinguishable pure states. The spectral entropy, and analogues using other
Schur-concave functions, can be defined as the entropy of these probabilities.
We describe additional conditions under which the outcome probabilities of a
fine-grained measurement are majorized by those for a spectral measurement, and
therefore the "spectral entropy" is the measurement entropy (and therefore
concave). These conditions are (1) projectivity, which abstracts aspects of the
Lueders-von Neumann projection postulate in quantum theory, in particular that
every face of the state space is the positive part of the image of a certain
kind of projection operator called a filter; and (2) symmetry of transition
probabilities. The conjunction of these, as shown earlier by Araki, is
equivalent to a strong geometric property of the unnormalized state cone known
as perfection: that there is an inner product according to which every face of
the cone, including the cone itself, is self-dual. Using some assumptions about
the thermodynamic cost of certain processes that are partially motivated by our
postulates, especially projectivity, we extend von Neumann's argument that the
thermodynamic entropy of a quantum system is its spectral entropy to
generalized probabilistic systems satisfying spectrality.Comment: In Proceedings QPL 2015, arXiv:1511.0118
Computational irreducibility and compatibilism: towards a formalization
If our actions are determined by the laws of nature, can we meaningfully
claim to possess free will? Compatibilists argue that the answer is yes, and
that free will is compatible with complete determinism. Previously, it has been
suggested that the notion of computational irreducibility can shed light on
this relation: it implies that there cannot in general be "shortcuts" to the
decisions of agents, explaining why deterministic agents often appear to have
free will. In this paper, we introduce a variant of computational
irreducibility that intends to capture more accurately aspects of actual (as
opposed to apparent) free will: computational sourcehood, i.e. the phenomenon
that the successful prediction of a process' outputs must typically involve an
almost-exact representation of the relevant features of that process,
regardless of the time it takes to arrive at the prediction. We conjecture that
many processes have this property, and we study different possibilities for how
to formalize this conjecture in terms of universal Turing machines. While we
are not able to settle the conjecture, we give several results and
constructions that shed light on the quest for its correct formulation.Comment: 15 pages, 1 figur
Internal quantum reference frames for finite Abelian groups
Employing internal quantum systems as reference frames is a crucial concept
in quantum gravity, gauge theories and quantum foundations whenever external
relata are unavailable. In this work, we give a comprehensive and
self-contained treatment of such quantum reference frames (QRFs) for the case
when the underlying configuration space is a finite Abelian group,
significantly extending our previous work (Quantum 5, 530 (2021)). The
simplicity of this setup admits a fully rigorous quantum information-theoretic
analysis, while maintaining sufficient structure for exploring many of the
conceptual and structural questions also pertinent to more complicated setups.
We exploit this to derive several important structures of constraint
quantization with quantum information-theoretic methods and to reveal the
relation between different approaches to QRF covariance. In particular, we
characterize the "physical Hilbert space" -- the arena of the
"perspective-neutral" approach -- as the maximal subspace that admits
frame-independent descriptions of purifications of states. We then demonstrate
the kinematical equivalence and, surprising, dynamical inequivalence of the
"perspective-neutral" and the "alignability" approach to QRFs. While the former
admits unitaries generating transitions between arbitrary subsystem relations,
the latter, remarkably, admits no such dynamics when requiring
symmetry-preservation. We illustrate these findings by example of interacting
discrete particles, including how dynamics can be described "relative to one of
the subsystems".Comment: 22 pages, 1 figure. V2: close to published versio
The HI content of Early-Type Galaxies from the ALFALFA survey I. Catalogued HI sources in the Virgo cluster
Aims: We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is
covering 17% of the sky at 21 cm, to study the HI content of Early-Type
galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the
hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to
understand its origin and fate, and to relate it to the formation and evolution
history of these objects. Methods: This paper deals with the first part of our
study, which is devoted to the 8-16 deg. declination strip in the Virgo
cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have
defined an optical sample of 939 ETG, 457 of which are brighter than the VCC
completeness limit at B_T=18.0. We have correlated this optical sample with the
catalogue of detected HI sources from ALFALFA. Results: Out of the 389 ETG from
the VCC with B_T<=18.0, outside the 1 deg. region of poor HI detection around
M87, and corrected for background contamination of VCC galaxies without a known
radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness
limit of 3.5 and 7.6 x 10^7 Mo of HI for dwarf and giant ETG, respectively. In
addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection
rate is lower than previously claimed. The majority of the detected ETG appear
to have peculiar morphology and to be located near the edges of the Virgo
cluster. Conclusions: Our preliminary conclusion is that cluster ETG contain
very little neutral gas, with the exceptions of a few peculiar dwarf galaxies
at the edge of the ETG classification and of very few larger ETG, where the
cold gas could have a recent external origin.Comment: Accepted for publication in Astronomy and Astrophysics; 6 pages, 3
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