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 `$\lambda$-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

The Roles of Causal Propagator in Relativistic Quantum Information

This thesis focuses on the roles of causal propagator — the expectation value of the field commutator — in relativistic quantum information. Consider two observers Alice and Bob, each carrying a two-level quantum system (“qubit”) interacting with a common relativistic quantum field environment in spacetime. Since the interaction occurs for a finite duration and has a finite spatial extent, we can think of Alice and Bob’s qubits as being essentially localized in spacetime. We say that Alice and Bob can signal or communicate to one another via the field if the protocol they use depends on the field commutator evaluated around Alice’s and Bob’s interaction regions. The main tool we use to deal with signaling issues is the so-called Unruh-DeWitt particle detector model. The main content of the thesis is organized into four chapters (Chapter 4-7). Chapter 4 and 5 are concerned with the entanglement harvesting protocol, where Alice and Bob attempt to extract vacuum entanglement from the relativistic environment by locally coupling their qubits to the field. In Chapter 6 we revisit a problem involving entanglement dynamics of two qubits subjected to Unruh acceleration and ask whether the existing results are perturbatively reliable. Finally, in Chapter 7 we discuss a slightly different but related topic that we call modest holography, a form of metric reconstruction in asymptotically flat spacetimes that relies on a bulk-to-boundary correspondence between the correlators of two different quantum field theories. We will see that the causal propagator plays different but essential roles in all of these subjects, possibly in ways that are more important than what it may seem at first sight. There is a sense in which one can argue that what makes relativistic quantum information relativistic is indeed the signaling component: it is about the causal structure of spacetime, and spacetime curvature provides the “details”