25 research outputs found
Fuzzy spacetime: fundamental limits of quantum-optical holographic bulk reconstruction
In this Essay we argue that there are fundamental limits imposed by quantum
theory and thermodynamics on spacetime metric reconstruction using localized
quantum-mechanical probes: the "fuzziness" of spacetime that arise from
operational measurement protocols is already present before one reaches the
quantum-gravitational regime. We do this by providing a concrete,
non-perturbative realization of metric reconstruction using quantum-optical
model of particle detectors in relativistic quantum information. The
non-perturbative approach allows us to realize a version of "short-distance
physics corresponds to poor statistics" idea by Kempf, but this occurs way
above the Planck scale. These fundamental limitations can be given a
holographic dual interpretation using bulk-to-boundary correspondence between
scalar correlators in asymptotically flat spacetimes.Comment: 10 pages, no figure. Essay written for the Gravity Research
Foundation 2023 Awards for Essays on Gravitatio
Characterization of non-perturbative qubit channel induced by a quantum field
In this work we provide some characterization of the quantum channel induced
by non-perturbative interaction between a single qubit with a quantized
massless scalar field in arbitrary globally hyperbolic curved spacetimes. The
qubit interacts with the field via Unruh-DeWitt detector model and we consider
two non-perturbative regimes: (i) when the interaction is very rapid,
effectively at a single instant in time (\textit{delta-coupled detector}); and
(ii) when the qubit has degenerate energy level (\textit{gapless detector}). We
organize the results in terms of quantum channels and Weyl algebras of
observables in the algebraic quantum field theory (AQFT). We collect various
quantum information-theoretic results pertaining to these channels, such as
entropy production of the field and the qubit, recoverability of the qubit
channels, and causal propagation of noise due to the interactions. We show that
by treating the displacement and squeezing operations as elements of the Weyl
algebra, we can generalize existing non-perturbative calculations involving the
qubit channels to non-quasifree Gaussian states in curved spacetimes with
little extra effort and provide transparent interpretation of these unitaries
in real space. We also generalize the existing result about cohering and
decohering power of a quantum channel induced by the quantum field to curved
spacetimes in a very compact manner.Comment: 22 pages + 3 pages of references; 3 figures, RevTeX4-2; v3: fixed
citation
Thermodynamics of hairy black holes in Lovelock gravity
We perform a thorough study of the thermodynamic properties of a class of
Lovelock black holes with conformal scalar hair arising from coupling of a real
scalar field to the dimensionally extended Euler densities. We study the
linearized equations of motion of the theory and describe constraints under
which the theory is free from ghosts/tachyons. We then consider, within the
context of black hole chemistry, the thermodynamics of the hairy black holes in
the Gauss-Bonnet and cubic Lovelock theories. We clarify the connection between
isolated critical points and thermodynamic singularities, finding a one
parameter family of these critical points which occur for well-defined
thermodynamic parameters. We also report on a number of novel results,
including `virtual triple points' and the first example of a
`-line'---a line of second order phase transitions---in black hole
thermodynamics.Comment: 62 pages, 30 figures. Minor changes and typos corrected. Updated to
match published versio
Modest holography and bulk reconstruction in asymptotically flat spacetimes
In this work we present a "modest" holographic reconstruction of the bulk
geometry in asymptotically flat spacetime using the two-point correlators of
boundary quantum field theory (QFT) in asymptotically flat spacetime. The
boundary QFT lives on the null boundary of the spacetime, namely null infinity
and/or the Killing horizons. The bulk reconstruction relies on two unrelated
results: (i) there is a bulk-to-boundary type correspondence between free
quantum fields living in the bulk manifold and free quantum fields living on
its null boundary, and (ii) one can construct the metric by making use of the
Hadamard expansion of the field living in the bulk. This holographic
reconstruction is "modest" in that the fields used are non-interacting and not
strong-weak holographic duality in the sense of AdS/CFT, but it works for
generic asymptotically flat spacetime subject to some reasonably mild
conditions.Comment: 19+5 pages, 5 figures; RevTeX4-2; v2: fixed typo and added some minor
clarifications; v3: fixed to match published versio
Aspects of Quantum Field Theory with Boundary Conditions
This thesis has two modest goals. The primary goal is to deliver three results involving
particle detectors interacting with a quantum field in presence of non-trivial boundary
conditions (Dirichlet, Neumann, periodic; dynamical or otherwise). The secondary goal is
to cover some technical, less “interesting” aspects of numerical integration performed in
one of the works discussed in this thesis.
For the primary goal, we will first discuss how particle detector models known as Unruh-
DeWitt model, which mimics essential aspects of light-matter interaction in quantum field
theory (QFT) in general curved spacetimes, can be used to reanalyse the Weak Equivalence
Principle (WEP) involving uniformly accelerating cavity (Dirichlet boundaries). This
complements past literature, expands past results to cover highly non-diagonal field states and clarifies a minor disagreement
with another old result. We will then move on to the problem of zero mode of a
bosonic quantum field in presence of periodic and Neumann boundary conditions and show
that relativistic considerations require careful treatment of zero mode in order to respect
(micro)causality of QFT. We will quantify the amount of causality violation when the
zero mode is ignored. Finally, we will discuss entanglement dynamics between two detectors
coupled to a bosonic field in presence of non-uniformly accelerating mirror (moving
Dirichlet boundary) for several non-trivial mirror trajectories.
For the secondary goal, we aim to briefly summarize some technical difficulties regarding
symbolic and numerical integration encountered in these works. While this is not directly
relevant for the physical results of the papers, explicit discussion seems appropriate and
useful even if concise. In particular, we will discuss, in the context of Unruh-DeWitt model,
a particular way involving Mathematica’s symbolic integration which prove superior in
many settings than simply “plug-in-and-integrate” from textbooks or the literature,
as one might naturally do in the absence of closed-form expressions. This will prove useful
as an explicit reference for future Unruh-DeWitt-related studies when more complicated integrals
of similar nature are encountered