# Weekly Papers on Quantum Foundations (51)

Exact Gravity Duals for Simple Quantum Circuits. (arXiv:2112.12158v1 [hep-th])

Holographic complexity proposals have sparked interest in quantifying the cost of state preparation in quantum field theories and its possible dual gravitational manifestations. The most basic ingredient in defining complexity is the notion of a class of circuits that, when acting on a given reference state, all produce a desired target state. In the present work we build on studies of circuits performing local conformal transformations in general two-dimensional conformal field theories and construct the exact gravity dual to such circuits. In our approach to holographic complexity, the gravity dual to the optimal circuit is the one that minimizes an externally chosen cost assigned to each circuit. Our results provide a basis for studying exact gravity duals to circuit costs from first principles.

Analogue gravitational lensing in optical Bose-Einstein condensates. (arXiv:2112.06235v3 [quant-ph] UPDATED)

We consider acoustic propagation of phonons in the presence of a non-rotating vortex with radial flow in a Bose-Einstein condensate (BEC) of photons. Since the vortex can be used to simulate a static acoustic black hole, the phonon would experience a considerable spacetime curvature at appreciable distance from the vortex core. The trajectory of the phonons is bended after passing by the vortex, which can be used as a simulation of gravitational lensing for phonons in a photonic BEC.

Comments on the Aharonov-Bohm Effect. (arXiv:2112.11171v2 [quant-ph] UPDATED)

In the original setting of the Aharonov-Bohm, the gauge invariant physical longitudinal mode of the vector potential, which is written by the gauge invariant physical current $(-e)\bar{\psi}{\boldsymbol \gamma} \psi$, gives the desired contribution to the Aharonov-Bohm effect. While the scalar mode of the vector potential, which changes under the gauge transformation so that it is the unphysical mode, give no contribution to the Aharonov-Bohm effect. Then Aharonov-Bohm effect really occurs by the physical longitudinal mode in the original Aharonov-Bohm’s setting. In the setting of Tonomura {\it et al.}, where the magnet is shielded with the superconducting material, not only the magnetic field but also the longitudinal mode of the vector potential become massive by the Meissner effect. Then not only the magnetic field but also the physical longitudinal mode does not come out to the region where the electron travels. In such setting, only the scalar mode of the vector potential exists in the region where the electron travels, but there is no contribution to the Aharonov-Bohm effect from that mode. Then, theoretically, the Aharonov-Bohm effect does not occur in the Tonomura {\it et al.}’s setting. In the quantum theory, the electron is treated as the wave, and the longitudinal mode give the change of the phase, which gives the Aharonov-Bohm effect. In the classical theory, the electron is treated as the particle, and the only existing longitudinal mode gives the change of the angular momentum. For the particle, there is no concept of the phase, so that there is no Aharonov-Bohm effect.

Maxwell’s Demon must remain sebservient to Clausius’s statement. (arXiv:2112.12300v1 [physics.class-ph])

Authors: P.D. Gujrati

Using classical thermodynamics, we argue that Maxwell’s demon loses its battle against Clausius as any temperature difference or other thermodynamic forces it creates is immediately compensated by spontaneous counterbalancing flows that bring about equilibration by slower particles in principle. Being constrained by these spontaneously generated equilibration processes in which he actively but unwittingly participates, the demon is incapable of destroying equilibrium and violating the second law. In fact, our investigation shows that he is unintentionally designed to support it, and does not alter the temperature.

Precision tests of fundamental physics with η and η′ mesons

Publication date: 26 January 2022

Source: Physics Reports, Volume 945

Author(s): Liping Gan, Bastian Kubis, Emilie Passemar, Sean Tulin

Emergent times in holographic duality. (arXiv:2112.12156v1 [hep-th])

Authors: Samuel LeutheusserHong Liu

In holographic duality an eternal AdS black hole is described by two copies of the boundary CFT in the thermal field double state. In this paper we provide explicit constructions in the boundary theory of infalling time evolutions which can take bulk observers behind the horizon. The constructions also help to illuminate the boundary emergence of the black hole horizons, the interiors, and the associated causal structure. A key element is the emergence, in the large $N$ limit of the boundary theory, of a type III$_1$ von Neumann algebraic structure from the type I boundary operator algebra and the half-sided modular translation structure associated with it.

Quantum Gravity at Low Energies. (arXiv:2112.12235v1 [gr-qc])

Authors: David Wallace

I provide a conceptually-focused presentation of low-energy quantum gravity’ (LEQG), the effective quantum field theory obtained from general relativity and which provides a well-defined theory of quantum gravity at energies well below the Planck scale. I emphasize the extent to which some such theory is required by the abundant observational evidence in astrophysics and cosmology for situations which require a simultaneous treatment of quantum-mechanical and gravitational effects, \emph{contra} the often-heard claim that all observed phenomena can be accounted for either by classical gravity or by non-gravitational quantum mechanics, and I give a detailed account of the way in which a treatment of the theory as fluctuations on a classical background emerges as an approximation to the underlying theory rather than being put in by hand. I discuss the search for a Planck-scale quantum-gravity theory from the perspective of LEQG and give an introduction to the Cosmological Constant problem as it arises within LEQG.

A Measurement of the Cosmic Expansion Within our Lifetime. (arXiv:2112.12599v1 [gr-qc])

Authors: Fulvio Melia

The most exciting future observation in cosmology will feature a monitoring of the cosmic expansion in real time, unlike anything that has ever been attempted before. This campaign will uncover crucial physical properties of the various constituents in the Universe, and perhaps answer a simpler question concerning whether or not the cosmic expansion is even accelerating. An unambiguous yes/no response to this query will significantly impact cosmology, of course, but also the standard model of particle physics. Here, we discuss — in a straightforward way — how to understand the so-called redshift drift’ sought by this campaign, and why its measurement will help us refine the standard-model parameters if the answer is yes.’ A no’ answer, on the other hand, could be more revolutionary, in the sense that it might provide a resolution of several long-standing problems and inconsistencies in our current cosmological models. An outcome of zero redshift drift, for example, would obviate the need for a cosmological constant and render inflation completely redundant.

Paradoxes before the paradox: surface gravity and the information loss problem. (arXiv:2109.13939v2 [gr-qc] UPDATED)

The information loss paradox is widely regarded as one of the biggest open problems in theoretical physics. Several classical and quantum features must be present to enable its formulation. First, an event horizon is needed to justify the objective status of tracing out degrees of freedom inside the black hole. Second, evaporation must be completed (or nearly completed) in finite time according to a distant observer, and thus the formation of the black hole should also occur in finite time. In spherical symmetry these requirements constrain the possible metrics strongly enough to obtain a unique black hole formation scenario and match their parameters with the semiclassical results. However, the two principal generalizations of surface gravity, the quantity that determines the Hawking temperature, do not agree with each other on the dynamic background. Neither can correspond to the emission of nearly-thermal radiation. We infer from this that the information loss problem cannot be posed in its standard form.

The black hole information puzzle and the quantum de Finetti theorem. (arXiv:2110.14653v2 [hep-th] UPDATED)

Authors: Renato RennerJinzhao Wang

The black hole information puzzle arises from a discrepancy between conclusions drawn from general relativity and quantum theory about the nature of the radiation emitted by a black hole. According to Hawking’s original argument, the radiation is thermal and its entropy thus increases monotonically as the black hole evaporates. Conversely, due to the reversibility of time evolution according to quantum theory, the radiation entropy should start to decrease after a certain time, as predicted by the Page curve. This decrease has been confirmed by new calculations based on the replica trick, which also exhibit its geometrical origin: spacetime wormholes that form between the replicas. Here we analyse the discrepancy between these and Hawking’s original conclusions from a quantum information theory viewpoint, using in particular the quantum de Finetti theorem. The theorem implies the existence of extra information, $W$, which is neither part of the black hole nor the radiation, but plays the role of a reference. The entropy obtained via the replica trick can then be identified to be the entropy $S(R|W)$ of the radiation conditioned on the reference $W$, whereas Hawking’s original result corresponds to the non-conditional entropy $S(R)$. The entropy $S(R|W)$, which mathematically is an ensemble average, gains an operational meaning in an experiment with $N$ independently prepared black holes: For large $N$, it equals the normalised entropy of their joint radiation, $S(R_1 \cdots R_N)/N$. The discrepancy between this entropy and $S(R)$ implies that the black holes are correlated. The replica wormholes may thus be interpreted as the geometrical representation of this correlation. Our results also suggest a many-black-hole extension of the widely used random unitary model, which we support with non-trivial checks.

Black holes and their horizons in semiclassical and modified theories of gravity. (arXiv:2112.06515v2 [gr-qc] UPDATED)

For distant observers black holes are trapped spacetime domains bounded by apparent horizons. We review properties of the near-horizon geometry emphasizing the consequences of two common implicit assumptions of semiclassical physics. The first is a consequence of the cosmic censorship conjecture, namely that curvature scalars are finite at apparent horizons. The second is that horizons form in finite asymptotic time (i.e. according to distant observers), a property implicitly assumed in conventional descriptions of black hole formation and evaporation. Taking these as the only requirements within the semiclassical framework, we find that in spherical symmetry only two classes of dynamic solutions are admissible, both describing evaporating black holes and expanding white holes. We review their properties and present the implications. The null energy condition is violated in the vicinity of the outer and satisfied in the vicinity of the inner apparent/anti-trapping horizon. Apparent and anti-trapping horizons are timelike surfaces of intermediately singular behavior, which is demonstrated in negative energy density firewalls. These and other properties are also present in axially symmetric solutions. Different generalizations of surface gravity to dynamic spacetimes are discordant and do not match the semiclassical results. We conclude by discussing signatures of these models and implications for the identification of observed ultra-compact objects.

Perceiving Direction in Directionless Time

Farr, Matt (2021) Perceiving Direction in Directionless Time. [Preprint]

Entropy and Reversible Catalysis

Author(s): H. Wilming

I show that nondecreasing entropy provides a necessary and sufficient condition to convert the state of a physical system into a different state by a reversible transformation that acts on the system of interest and a further “catalyst,” whose state has to remain invariant exactly in the transition….

[Phys. Rev. Lett. 127, 260402] Published Thu Dec 23, 2021

Riemannian geometry of resonant optical responses

Nature Physics, Published online: 23 December 2021; doi:10.1038/s41567-021-01465-z

The modern understanding of quantum transport relies on geometric concepts such as the Berry phase. The geometric approach has now been extended to the theory of optical transitions.

Learning Temporal Quantum Tomography

Author(s): Quoc Hoan Tran and Kohei Nakajima

Quantifying and verifying the control level in preparing a quantum state are central challenges in building quantum devices. The quantum state is characterized from experimental measurements, using a procedure known as tomography, which requires a vast number of resources. However, tomography for a …

[Phys. Rev. Lett. 127, 260401] Published Wed Dec 22, 2021

Quantum Fisher Information from Randomized Measurements

Author(s): Aniket Rath, Cyril Branciard, Anna Minguzzi, and Benoît Vermersch

The quantum Fisher information (QFI) is a fundamental quantity of interest in many areas from quantum metrology to quantum information theory. It can in particular be used as a witness to establish the degree of multiparticle entanglement in quantum many-body systems. In this work, we use polynomial…

[Phys. Rev. Lett. 127, 260501] Published Wed Dec 22, 2021

A (strictly) contemporary perspective on trans-Planckian censorship

Schneider, Mike D. (2021) A (strictly) contemporary perspective on trans-Planckian censorship. [Preprint]

A Survey on Uncertainty Relations and Quantum Measurements: Arguments for Lucrative Parsimony in Approaches of Matters

Dumitru, Spiridon (2021) A Survey on Uncertainty Relations and Quantum Measurements: Arguments for Lucrative Parsimony in Approaches of Matters. PROGRESS IN PHYSICS, 17 (1). pp. 38-70. ISSN 1555-5534 (print) 1555-5615 (web)

Wave function realism and three dimensions

Vaidman, Lev (2021) Wave function realism and three dimensions. [Preprint]

Four Attitudes Towards Singularities in the Search for a Theory of Quantum Gravity

Crowther, Karen and De Haro, Sebastian (2021) Four Attitudes Towards Singularities in the Search for a Theory of Quantum Gravity. [Preprint]

The constructivist’s programme and the problem of pregeometry

Linnemann, Niels and Salimkhani, Kian (2021) The constructivist’s programme and the problem of pregeometry. [Preprint]