8th Intensive Lecture by Quantum Gravity Gatherings
Quantum reference frames and their applications in high-energy physics
24-26 September 2025 @ RIKEN, Wako, Japan
In the eighth event of the intensive lecture series by the Quantum Gravity Gatherings (QGG) study group at RIKEN iTHEMS, our distinguished speaker, Prof. Philipp Höhn from the Okinawa Institute of Science and Technology (OIST), will deliver a three-day lecture series on the theme of Quantum Reference Frames.
In standard textbook scenarios, reference frames serve only to define different descriptions of a given physical problem to which they are external. However, many situations in physics, such as those in gauge theories, gravity, or quantum communication, lack any meaningful external frame. This raises the question of how to describe physics in the absence of such a frame. The answer lies in adopting internal reference frames, leading to a relational perspective and embodying the idea that all reference frames are associated with physical systems. In quantum theory, these are known as quantum reference frames (QRFs). They are quantum subsystems that transform under a symmetry group and may exist in superposition, become entangled, or evolve dynamically. One ultimate goal is to formulate a quantum relativity principle that, in an appropriate context, may reveal the quantum structure of spacetime, in analogy with how classical frame relations encode classical spacetime structures. While relational ideas have long been part of gravitational theory, QRFs introduce a new operational paradigm that is highly relevant to high-energy physics. In particular, the construction of gauge-invariant observables, whether through dressing methods or gauge fixing, relies fundamentally on internal frames. This insight provides a foundation for a novel notion of quantum covariance in both the foundations of quantum theory and high-energy physics. These lectures, organized in the style of QGG, have been thoughtfully designed to offer a pedagogical review of the subject while highlighting its essential and unique computational techniques. Through these lectures, attendees will gain a concrete understanding of QRFs and its potential application to their own research and studies.
The lectures will be presented in a blackboard-style format (in English, no online broadcasting), which aims to foster a more interactive experience, encouraging active participation and in-depth Q&A discussions. In addition, we will also arrange a special session where any interested participant will have the opportunity to give a short presentation on a topic of their choice. We believe that the casual and dynamic atmosphere of this event will help spark interactions among all the participants, especially graduate students and early-career researchers, and serve as a catalyst for collaborative learning where they can engage with one another and enthusiastically share ideas.
Participants may also be interested in the QRF 2025 workshop organized by Prof. Höhn's group, Qubits and Spacetime Unit in OIST. This lecture series is being held as a joint event to complement and enrich the workshop experience.
Registration
Registration Form- The registration deadline for all participants is September 7th. However, if the number of registered participants exceeds the capacity of the event room, we may need to close registration prior to the specified deadline.
- Participants are strongly encouraged to give a short talk on any topic of their choice (self-introduction, research, review of some works, what they want to research in the future, etc.). This interactive session is designed to foster a lively and engaging environment, facilitating increased communication and knowledge exchange among all participants.
- We can partially support the expenses for junior participants. Note that
- Giving a short talk is mandatory for those receiving support and for students attending the banquet.
- We have a limited number of accommodations available for participants residing in remote areas. Please note that the check-in date is fixed for September 23rd and the check-out date is set for September 26th, and these dates cannot be changed.
Intensive Talk
Prof. Philipp Höhn (OIST)
(Prof. Höhn's Webpage)
Abstract:
Quantum reference frames (QRFs) are a universal tool for dealing with symmetries in quantum systems. Roughly speaking, they are internal subsystems that transform in some non-trivial way under the symmetry group of interest and constitute the means for describing quantum systems from the inside
in purely relational terms. QRFs are thus crucial for describing and extracting physics whenever no external reference frame for the symmetry group is available. This is in particular the case when the symmetries are gauge, as in gauge theory and gravity, where QRFs arise whenever building physical observables. The choice of internal QRF is typically non-unique, giving rise to a novel quantum form of covariance of physical properties under QRF transformations. This lecture series will explore this novel perspective in detail with a specific emphasis on applications in high-energy physics and gravity.
I will begin by introducing QRFs in mechanical setups and explain how they give rise to quantum structures of covariance that mimic those underlying special relativity. I will explain how this leads to subsystem relativity, the insight that different QRF decompose the total system in different ways into gauge-invariant subsystems, and how this leads to the QRF dependence of correlations, entropies, and thermal properties. We will then explore how relational dynamics in Hamiltonian constrained systems and the infamous "problem of time" can be addressed with clocks identified as temporal QRFs. In transitioning to the field theory setting, we will first consider hybrid scenarios, where QRFs are quantum mechanical, but the remaining degrees of freedom are quantum fields including gravitons. I will explain how this encompasses the recent discussion of "observers", generalized entropies, and gravitational von Neumann algebras by Witten et al. and how subsystem relativity leads to the conclusion that gravitational entanglement entropies are observer dependent. We will then discuss the classical analog of QRFs in gauge theory and gravity and how they can be used to build gauge-invariant relational observables and to describe local subsystems. This will connect with discussions on edge and soft modes in the literature, the former of which turn out to be QRFs as well. This has bearing on entanglement entropies in gauge theories, which I will describe on the lattice, providing a novel relational construction that overcomes the challenges faced by previous constructions, which yielded non-distillable contributions to the entropy and can be recovered as the intersection of "all QRF perspectives". Finally, I will describe how the classical discussion of dynamical reference frames can be used to build a manifestly gauge-invariant path integral formulation that opens up novel relational perspectives on effective actions and the renormalization group in gravitational contexts, which is typically plagued by a lack of manifest diffeomorphism-invariance. I will conclude with open questions and challenges in the field.
Program
September 2410:15 - 10:30 Registration and reception
10:30 - 12:00 Lecture 1
12:00 - 13:30 Lunch
13:30 - 15:00 Lecture 2
15:00 - 16:00 Coffee break
16:00 - 17:00 Lecture 3
17:10 - 18:10 Short talk session
18:20 - 21.00 Banquet
September 25
10:30 - 12:00 Lecture 4
12:00 - 13:30 Lunch
13:30 - 15:00 Lecture 5
15:00 - 16:00 Coffee break
16:00 - 17:00 Lecture 6
17:10 - 18:10 Short talk session
September 26
10:30 - 12:00 Lecture 7
12:00 - 13:30 Lunch
13:30 - 15:00 Lecture 8
15:00 - 16:00 Coffee break
16:00 - 17:00 Lecture 9 & Closing
Place
435-437 Seminar Room, 4F of Main Research Building, RIKEN Wako CampusOrganizers
Che-Yu Chen (iTHEMS)Yuta Hamada (KEK)
Puttarak Jai-akson (iTHEMS)
Christy Kelly (iTHEMS)
Ryo Namba (iTHEMS)
Toshifumi Noumi (The University of Tokyo)
Naritaka Oshita (Kyoto Hakubi/iTHEMS)
Sotaro Sugishita (Kyoto University/iTHEMS)
Yuki Yokokura (iTHEMS)
Contact: Puttarak Jai-akson puttarak.jai-akson@riken.jp / Yuki Yokokura yuki.yokokura@riken.jp
Sponsors
