The challenge of quantum gravity is to build a theory valid over all the scales down to the minuscule Planck scale, which explains how the known physics of general relativity and quantum (field) theory emerge from it. This deep challenge is the main impetus of the research efforts of the Qubits and Spacetime Unit.

There are many approaches to quantum gravity, and, rather than adhering to a specific one, we pursue somewhat of an "approach-independent approach". This means that our focus lies on generic challenges, which many approaches to quantum gravity will have to face up to and which have implications and relevance also in other areas of physics. Thus, we focus less on Planck scale physics and more on effective descriptions in quantum gravity that are closer to the known physics. The idea is to develop concepts, tools and methodology, which will be useful for various approaches to quantum gravity and beyond, e.g., for classical gravity, gauge theories, quantum information and the foundations of quantum theory, but we also import ingredients from the latter areas. Our research therefore resides at the interface of a broad spectrum of topics in fundamental physics and this is also reflected in the composition of our Unit. In particular, we host and have hosted representatives from different approaches to quantum gravity, as well as from quantum foundations and quantum information theory.

A main tool which we have been developing recently is that of quantum reference frames. These are internal frames associated with quantum subsystems and appear in any quantum system subject to a symmetry principle. As such, they are relevant in all of the above mentioned areas, as well as in quantum thermodynamics. While we have explored them in all of these contexts, we are especially interested in their application to gauge theories and gravity, whose physical content they define via dressed/relational observables. For example, we are interested in finite regions and subsystems in gauge theory and gravity, where quantum frames help to define and regulate entanglement entropies. Quantum frames further give rise to a novel quantum covariance of physical properties and laws that we study. However, we also research on other topics, including spacetime thermodynamics and quantum error correcting codes in high-energy physics.

Faculty

Philipp Höhn

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Some recent recorded overview talks:

Philipp Höhn, "Relational entanglement entropies in gauge theory and gravity", 29 October 2025, Extreme Universe 2025, workshop @ YITP Kyoto (video)
Philipp Höhn, Intensive lectures series on "Quantum reference frames and their application in high-energy physics", 9 lectures, 24-26 September, Quantum Gravity Gatherings, RIKEN iTHEMS (recorded lectures)
Philipp Höhn, "Gravitational algebras, entropies, and quantum reference frames", 25 April 2025, RQI COST Action WG3 Seminar (video)
Philipp Höhn, "Linking quantum error correction and gauge theories with quantum reference frames", 21 Mar 2025, Asia Pacific Quantum Error Correction Seminars from NU Singapore (video)
Philipp Höhn, "Gravitational algebras, entropies, and quantum reference frames", 18 Feb 2025, International LQG Seminar (video)
Philipp Höhn, "Quantum frames, subsystems and gravity", 9 Nov 2023, gravity seminar @ KITP/UC Santa Barbara (video)
Philipp Höhn, "Quantum frames and subsystems", 13 Sep 2023, Extreme Universe -YITP Workshop on Holography, Gravity and Quantum Information, workshop @ YITP Kyoto (video)
Philipp Höhn, "Complementarity identities from an informational reconstruction", Aug 1 2023, The Quantum Reconstruction Program and Beyond, workshop @ U Graz (video)
Philipp Höhn, "Observables and dynamical frames in gravity", 11 July 2023, QG2023, conference @ U Nijmegen, (video)

New articles:

Gonçalo Araújo-Regado, Ayngaran Thavanesan, and Aron Wall, "Holographic cosmology at finite time", arXiv:2511.04511
Sergio Aguilar-Gutierrez, "Cosmological entanglement entropy and edge modes from double-scaled SYK & its connection with Krylov complexity", arXiv:2511.03779
Sergio Aguilar-Gutierrez, "Evolution With(out) Time: Relational Time & BPS Complexity Growth in $\mathcal{N}=2$ Double-Scaled SYK", arXiv:2510.11777
Tom Ladstätter and Luca Marchetti, "Interacting scalar field cosmology from full quantum gravity", arXiv:2508.16194
Julian De Vuyst, Philipp A. Höhn and Artur Tsobanjan, "On the relation between perspective-neutral, algebraic, and effective quantum reference frames", arXiv:2507.14131
Gonçalo Araujo-Regado, Philipp A. Höhn and Francesco Sartini, "Relational entanglement entropies and quantum reference frames in gauge theories", arXiv:2506.23459
Sergio Aguilar-Gutierrez, "Symmetry Sectors in Chord Space and Relational Holography in the DSSYK", arXiv:2506.21447
Sergio Aguilar-Gutierrez and Jiuci Xu, "Geometry of chord intertwiner, multiple shocks and switchback in double-scaled SYK", arXiv:2506.19013
Sergio Aguilar-Gutierrez, Yichao Fu, Kuntal Pal and Klaas Parmentier, "Quasinormal modes and complexity in saddle-dominated SU(N) spin systems", arXiv:2506.05458
Sergio Aguilar-Gutierrez, Hugo Camargo, Viktor Jahnke, Keun-Young Kim and Mitsuhiro Nishida, "Krylov operator complexity in holographic CFTs: Smeared boundary construction and the dual proper radial momentum", arXiv:2506:03273
Sergio Aguilar-Gutierrez, "From chords to dynamical wormholes with matter: Towards a bulk double-scaled (SYK) algebra", arXiv:2505.22716
Stefan Eccles, "Why ETH? On thermalization and locality", arXiv:2502.04784
Julian De Vuyst, Stefan Eccles, Philipp A. Höhn and Josh Kirklin, "Crossed products and quantum reference frames: on the observer-dependence of gravitational entropy", JHEP 07 (2025) 063, arXiv:2412.15502
Sylvain Carrozza, Aidan Chatwin-Davies, Philipp A. Höhn and Fabio M. Mele, "A correspondence between quantum error correcting codes and quantum reference frames", arXiv:2412.15317
Gonçalo Araujo-Regado, Philipp A. Höhn, Francesco Sartini and Bilyana Tomova, "Soft edges: the many links between soft and edge modes", JHEP 07 (2025) 180, arXiv:2412.14548
Luca Marchetti and Edward Wilson-Ewing, "Relational observables in group field theory", arXiv:2412.14622
Luca Marchetti, Hassan Mehmood and Viqar Husain, "An exactly soluble group field theory", arXiv:2412.09851
Julian De Vuyst, Stefan Eccles, Philipp A. Höhn and Josh Kirklin, "Linearization (in)stabilities and crossed products", JHEP 05 (2025) 211, arXiv:2411.19931
Jibril Ben Achour, et al, "Quantum gravity, hydrodynamics and emergent cosmology: a collection of perspectives", Gen Relativ Gravit 57, 2 (2025), arXiv:2411.12628
Sergio Aguilar-Gutierrez, "$T^2$ deformation in the double-scaled SYK model: Stretched horizon thermodynamics", arXiv:2410.18303
Sergio Aguilar-Gutierrez, Andrew Svesko and Manus Visser, " $T\bar{T}$ deformations from AdS₂ to dS₂", JHEP 01 (2025) 120, arXiv:2410.18257
Leonardo Chataignier, Philipp A. Höhn, Maximilian Lock and Fabio M. Mele, "Relational dynamics with periodic clocks", arXiv:2409.06479
Julian De Vuyst, Stefan Eccles, Philipp A. Höhn and Josh Kirklin, "Gravitational entropy is observer-dependent", JHEP 07 (2025) 146, arXiv:2405.00114
Elena Cáceres, Stefan Eccles, Jason Pollack and Sarah Racz, "Generic ETH: eigenstate thermalization beyond the microcanonical", arXiv:2403.05197