Observables in Quantum Gravity: from Theory to Experiment

12 Jan 2025, 17:00 → 17 Jan 2025, 13:00 US/Mountain

Aspen Center for Physics

Observables in Quantum Gravity: from Theory to Experiment

Aspen Center for Physics

January 12-17, 2025

The fundamental ingredients of quantum gravity remain mysterious despite decades of attention. Many approaches seek to identify nonperturbative effects associated with a UV-complete description, including black hole evaporation, the quantum nature of spacetime singularities, and the emergence of spacetime from microscopic degrees of freedom.  However, far less attention has been paid to what an experimentalist in a lab would actually measure—even in principle—in the presence of quantum gravity effects.  The proposed conference aims to address this deficiency, confronting head-on the questions of what are the observables in quantum gravity, and how are they defined, computed, and measured in experiment.  This conference seeks to convene members of many different communities working on different aspects of these questions, identifying pipelines between formal ideas and experiment, and critically assessing longstanding proposals. We aim to incorporate a diverse array of approaches, from algebras of observables, dressing, and relational observables to measurable systems including gravitational wave memory, cosmology, black holes, quantum simulation, and tabletop tests.  A successful conference would compare and contrast approaches to observables in quantum gravity, connect researchers working on the same topics but in disparate communities, and galvanize progress by spelling out concrete problems for experimentalists and theorists to study in a language widely understood across communities.

List of speakers:

• Trond Anderson
• Markus Aspelmeyer
• Batoul Banihashemi
• Eugenia Colafranceschi
• Daine Danielson
• Ben Freivogel
• Elliott Gesteau
• Steve Giddings
• Daniel Green
• Philipp Hoehn
• Jonathan Jeffrey
• Molly Kaplan
• Bryce Kobrin
• Hong Liu
• Andrew Lucas
• Julian Sonner
• Brian Swingle
• Jake Taylor
• Mark Van Raamsdonk
• Sander Vermeulen
• Jordan Wilson-Gerow
• Kathryn Zurek

Organizers:

Daniel Carney (LBNL) 
Thomas Faulkner (UIUC)
Cynthia Keeler (Arizona State University)
Nima Lashkari (Purdue University)
Allic Sivaramakrishnan (Caltech)
Antony Speranza (University of Amsterdam)

Sunday 12 January

17:00 Welcome Reception

Monday 13 January

1 Opening Remarks

Speaker: Antony Speranza

2 Hong Liu - Emergence of spacetime in holography

Speaker: Hong Liu

09:30 Morning break

3 Molly Kaplan - De Sitter quantum gravity and the emergence of local algebras

Quantum theories of gravity are generally expected to have some degree of nonlocality, with familiar local physics emerging only in a particular limit. Perturbative quantum gravity around backgrounds with isometries and compact Cauchy slices provides an interesting laboratory in which this emergence can be explored. In this context, the remaining isometries are gauge symmetries and, as a result, gauge-invariant observables cannot be localized. Instead, local physics can arise only through certain relational constructions. In this talk, we explore such issues for perturbative quantum gravity around de Sitter space. In particular, we describe a class of gauge-invariant observables which, under appropriate conditions, provide good approximations to certain algebras of local fields. Our results suggest that, near any minimal hypersphere in dS, this approximation can be accurate only over regions in which the corresponding global time coordinate spans an interval of order ln G^(-1). In contrast, however, we find that the approximation can be accurate over arbitrarily large regions of global dS so long as those regions are located far to the future or past of such a minimal sphere. This talk is based on the paper arXiv:2410.00111 with Donald Marolf, Xuyang Yu, and Ying Zhao.

Speaker: Molly Kaplan

4 Steve Giddings - Quantum gravity observables: observation, algebras, and mathematical structure

Speaker: Steve Giddings

11:30 Midday break

5 Bryce Kobrin and Trond Anderson - Towards new applications with hybrid analog-digital quantum simulators

Quantum simulators offer the potential to explore correlated quantum phenomena with greater flexibility and control than found in the natural world. In this two-part talk, we present an overview of recent theoretical and experimental advances in quantum simulation. We first discuss the development of microscopic holographic models and efforts to probe gravitational-like dynamics via many-body quantum teleportation in the so-called traversable wormhole circuit. We outline connections between many-body teleportation and operator growth, identify distinct mechanisms that enable teleportation, and comment on recent experiments that realize the teleportation protocol in digital quantum processors. In the second part, we present the recent development of a hybrid analog-digital quantum simulator comprising 69 superconducting qubits which supports both universal quantum gates and high-fidelity analog evolution. We compare the performance of the hybrid simulator to a digital quantum processor and demonstrate beyond-classical performance in cross-entropy benchmarking experiments. Leveraging the great versatility of this hybrid platform, we also probe thermalization dynamics, energy transport, and critical phenomena in the XY-model, revealing a breakdown of the Kibble-Zurek mechanism. Taken together, our discussion highlights the possibilities and challenges of simulating interacting quantum systems, including dynamical phenomena inspired by quantum theories of gravity.

Speakers: Bryce Kobrin, Trond Anderson

17:30 Evening Break

6 Brian Swingle - Statistical Probes of the Black Hole Interior

From the outside, black holes can be viewed as rapidly thermalizing quantum many-body systems. The interior of the black hole is more mysterious. We discuss concrete exterior observables which are precisely related to the interior geometry and allow us to probe its growth. The twist is that these observables are statistical and involve many identical copies of the black hole. We also discuss how these results fit into the "quantum gravity in the lab" program.

Speaker: Brian Swingle

20:00 Happy hour - Mi Chola restaurant

Tuesday 14 January

7 Jake Taylor - Pushing the experimental limits for testing quantum aspects of gravity

Speaker: Jake Taylor

09:30 Morning Break

8 Markus Aspelmeyer - Quantum sources of gravity in table-top experiments: status & challenges

Speaker: Markus Aspelmeyer

9 Sander Vermeulen - Photon Counting Interferometry to Detect Geontropic Space-Time Fluctuations with GQuEST

The GQuEST experiment uses tabletop-scale Michelson laser interferometers to probe for fluctuations in space-time. We present a new experimental technique, called photon counting interferometry’, which yields sensitivity beyond the interferometric standard quantum limit. We will implement this technique to search for individual photons produced by hypotheticalgeontropic’ space-time fluctuations, as proposed by Verlinde & Zurek. Our photon-counting readout design uses optical cavities to filter out quantum shot noise, a noise source that limits gravitational-wave detectors such as LIGO. Photon-counting readout fundamentally outperforms quantum squeezing, and is the optimal quantum metrology scheme to measure stochastic signals, such as space-time fluctuations. The GQuEST experiment thus enables a practicable search for signatures of quantum gravity in a laboratory-scale experiment.

11:30 Midday Break

10 Mark Van Raamsdonk - Cosmology from holography

I will describe how the standard tools of holography might be used to define microscopic models of big-bang cosmology. I discuss local'' models where a bubble of the cosmological spacetime is embedded in an asymptotically AdS spacetime, andglobal'' models where the full spacetime is encoded in some auxiliary degrees of freedom. While the gravitational effective field theories arising from holographic models have negative cosmological constant, generic cosmological solutions have scalar fields evolving on cosmological time scales that can give late time acceleration. If a model of this type describes nature, a generic prediction is a decreasing dark energy. We present evidence that such a decrease is suggested by supernova and BAO data.

Speaker: Mark Van Raamsdonk

11 Batoul Banihashemi - Gravitational ensembles with conformal boundaries

We study thermodynamics of gravitational systems with conformal boundary conditions, where the conformal class of the boundary metric and the trace of the extrinsic curvature K are held fixed. In the high temperature limit the series of subextensive terms in the free energy are compared to predictions from thermal effective field theory. In all considered cases there is agreement in the structure of the high temperature expansion. Interestingly, the first subextensive correction to the free energy is negative, violating a conjectured bound on this coefficient in quantum field theory. We interpret this as a signal that gravity does not fully decouple in the putative boundary dual. Ensembles with negative K are tied to solutions with cosmic-type horizons, where the system boundary is smaller than the horizon. In some cases, these solutions dominate the phase diagram and are necessary for consistency with thermal effective field theory.

Speaker: Batoul Banihashemi

12 Poster Session

Wednesday 15 January

13 Ben Freivogel - Quantum Widths of the Black Hole Horizon

How large are quantum fluctuations in the location of the black hole horizon? I will describe ongoing work addressing this question, sometimes called the `quantum width' of the black hole horizon. I will connect to recent work by Bousso-Penington and Parikh-Pereira, as well as older work of Marolf, which define different quantum widths for the black hole horizon.

Speaker: Ben Freivogel

09:30 Morning Break

14 Jordan Wilson-Gerow - Decoherence by warm horizons

Recently Danielson, Satishchandran, and Wald (DSW) have shown that quantum superpositions held outside of Killing horizons will decohere at a steady rate. This occurs because of the inevitable radiation of soft photons (gravitons), which imprint an electromagnetic (gravitational) ``which-path'' memory onto the horizon. Rather than appealing to this global description, an experimenter ought to also have a local description for the cause of decoherence. One might intuitively guess that this is just the bombardment of Hawking/Unruh radiation on the system, however simple calculations challenge this idea -- the same superposition held in a finite temperature inertial laboratory does not decohere at the DSW rate. In this work we provide a local description of the decoherence by mapping the DSW set-up onto a worldline-localized model resembling an Unruh-DeWitt particle detector. We present an interpretation in terms of random local forces which do not sufficiently self-average over long times. Using the Rindler horizon as a concrete example we clarify the crucial role of temperature, and show that the Unruh effect is the only quantum mechanical effect underlying these random forces. A general lesson is that for an environment which induces Ohmic friction on the central system (as one gets from the classical Abraham-Lorentz-Dirac force, in an accelerating frame) the fluctuation-dissipation theorem implies that when this environment is at finite temperature it will cause steady decoherence on the central system. Our results agree with DSW and provide a complementary local perspective.

Speaker: Jordan Wilson-Gerow

15 Daine Danielson - Horizon Algebras and Soft Quantum Information

We consider a thought experiment in which an experimenter, Alice, creates a spatial superposition of a charged (or massive) particle in the vicinity of a Killing horizon. This gives rise to a superposition of soft electromagnetic (or gravitational) field configurations on the black hole horizon, in a direct analog to the memory effect at null infinity. We use the quantum fidelity to compute the distinguishability of the resulting states of the black hole interior. The analysis of this—which has historically been obstructed by infrared divergences—can be carried out by extending the algebra of observables beyond the usual algebra of compactly supported observables, to include unitary algebra elements that produce noncompact field configurations on the horizon. In fact, we find that certain quantum information theoretic quantities are generically easier to compute in this enlarged algebra, and that analogous “horizon algebras” arise on de Sitter horizons and at null infinity. After Alice has held her superposition open for a given proper time, we compute the distinguishability of the resulting interior states, according to any spacelike observer Bob (or family of observers) allowed to carry out optimal measurements in the black hole interior. The resulting distinguishability is shown to be precisely equal to the decoherence of Alice’s superposition in the exterior.

Speaker: Daine Danielson

11:30 Midday Break

16 Public Lecture - Dan Carney - 100 Years of Quantum Space-time: from Theory to Experiment

Speaker: Dan Carney

Thursday 16 January

17 Philipp Hoehn - Gravitational algebras, entropies, and quantum reference frames

I will explain how the recent discussion about observers and crossed product algebras in perturbative quantum gravity is related to quantum reference frames and how this leads to an observer-dependence of gravitational entropies. (Based on 2405.00114; 2412.15502; 2411.19931)

Speaker: Philipp Hoehn

09:30 Morning Break

18 Eugenia Colafranceschi - Constructing observables in quantum gravity: an axiomatic path integral approach

Speaker: Eugenia Colafranceschi

19 Elliott Gesteau - Limits of observables and the emergence of causality

Speaker: Elliott Gesteau

11:30 Midday Break

20 Daniel Green - Cosmological Observables and their Quantum Origins

The maps of the Universe made by cosmic surveys are classical, both in principle and in practice. This obscures the quantum origin of structure our Universe from inflation but simplifies the physical interpretation of the observables. I will review recent progress in connecting observable quantities measured in current and future surveys to deeper principles about the structure of physical laws. I will also discuss how the quantities relevant to cosmological correlators manifest themselves in other observables in cosmological and asymptotically flat backgrounds.

Speaker: Daniel Green

17:15 Evening Break

21 Kathryn Zurek - The quantum mechanics of a causal diamond

Speaker: Kathryn Zurek

19:00 Banquet - Aspen Meadows

Friday 17 January

22 Andrew Lucas - The reconstruction map in JT gravity

An open question in AdS/CFT is how to reconstruct semiclassical bulk operators precisely enough that non-perturbative quantum effects can be computed. We propose a set of physically-motivated requirements for this reconstruction map, and we explicitly construct a map satisfying these requirements in JT gravity. Intuitively, our map is found by canonically quantizing a canonically-transformed classical JT Hamiltonian in a box (rather than on a line). The canonical transformation is chosen to ensure the correct spectrum of the fundamental quantum theory. We then obtain analytical predictions for the dynamics of the wormhole length, including its quantum fluctuations, by using our reconstruction map along with techniques from quantum ergodicity theory. The random matrix statistics of non-perturbative JT implies that the average wormhole length is non-monotonic in time, which provides a non-trivial prediction of our formalism.

Speaker: Andrew Lucas

09:30 Midday Break

23 Julian Sonner - Quantum simulating holographic duality

Holographic duality posits that quantum gravity under the assumption of certain boundary conditions is described by strongly interacting quantum matter. In the context of a theory-experiment consortium (Lausanne-Geneva-Trento-Zürich), we aim to quantum simulate the arguably simplest instance of such a holographic many-body phase by using ultra cold Fermions in a high-finesse optical cavity. In this talk I will describe our recent work, which aims to synthetically realise quenched disorder quantum many-body systems, including the so-called Dirac-SYK (Sachdev-Ye-Kitaev) class of models in single-mode optical cavities.

Speaker: Julian Sonner

24 Jonathan Jeffrey - Towards Holography with Cold Atoms in a Cavity

Probing the holographic principle on a physical platform requires tailored interactions between boundary degrees of freedom to produce a gravitational bulk. Our group has demonstrated programmable nonlocal interactions and control of the entanglement between clusters of cold atoms, and probed the emergence of a treelike bulk geometry inspired by p-adic AdS/CFT. Here, we discuss future directions to study holography in our cavity system. The proposed scheme is to engineer MERA-inspired couplings, whereupon bulk measurement induces a boundary state with power-law correlations whose continuum limit is conjectured to be dual to AdS. We describe routes to bulk reconstruction by computing the entanglement entropies of the boundary state.

Speaker: Jonathan Jeffrey