Relativistic Quantum Information Forum - Recent Posts

WG2 Seminar by Youka Kaku

Albert Roura — Sat, 18 Jul 2026 08:19:00 +0000

Date and time: 16 July 2026, 15:00 (CEST)

Speaker: Youka Kaku (Stevens Institute of Technology)

Title: Quantumness unique to gravity in quantum clock systems

Abstract: In 2017, several groups proposed testing whether Newtonian gravity can generate quantum entanglement between spatially superposed masses. This idea has attracted considerable attention as a possible low-energy test of the quantum nature of gravity. However, entanglement between spatial superposition states is not unique to gravity; it can also be generated by other interactions, such as the Coulomb interaction. In this talk, we shall discuss a quantum effect that is more specific to gravity. We focus on a quantum clock system, namely a particle with internal energy levels, which reveals various gravitational phenomena arising from mass-energy equivalence. We consider an atomic interferometer for the clock system under an external force generated by a spatially superposed source. As a result, we show that the collapse and revival of the interference visibility depend qualitatively on whether the external force is described by first-quantized Newtonian gravity, Schrödinger-Newton gravity, or the Coulomb interaction. This difference reflects in entanglement between the internal energy states of the clock and the spatial superposition state of the source. We show that this type of entanglement is generated uniquely by the first-quantized Newtonian gravity when the weak equivalence principle is respected.

This talk is based on Phys. Rev. D 106, 126005 (2022).

Video recording: https://youtu.be/f0qfs2Xy0l8

Presentation slides: https://drive.google.com/file/d/1gZu1dd5dQFoLxWrizzkBGbGzwmCZXH46/view?usp=sharing

WG2 Seminar by Torsten Zache

Albert Roura — Thu, 16 Jul 2026 09:58:06 +0000

Date and time: 18 June 2026, 10:00 (CEST)

Speaker: Torsten Zache (Universität Innsbruck)

Title: Non-local mass superpositions and optical clock interferometry in atomic ensemble quantum networks

Abstract: Quantum networks are emerging as powerful platforms for sensing, communication, and fundamental tests of physics. We propose a programmable quantum sensing network based on entangled atomic ensembles, where optical clock qubits realize mass superpositions arising via mass-energy equivalence, as in atom and atom-clock interferometry. Our approach uniquely combines scalability to large atom numbers with minimal control requirements, relying only on collective addressing of internal atomic states. This enables the creation of both non-local and local superpositions with spatial separations beyond those achievable in conventional matter-wave interferometry with single atoms. Starting from Bell-type seed states distributed via photonic channels, collective operations within atomic ensembles coherently build many-body mass superpositions sensitive to gravitational redshift. The resulting architecture implements a non-local Ramsey interferometer, where gravitationally induced phase shifts are imprinted on nonlocal entangled states and are read out through local measurements at the network nodes. Beyond extending the spatial reach of mass superpositions, our scheme establishes a scalable, programmable platform to probe the interface of quantum mechanics and gravity, and offers a new experimental pathway to test atom and atom-clock interferometer proposals, e.g. for probing gravitational dephasing, in a network-based quantum laboratory.

Video recording: https://youtu.be/a6EcHRpZWKs

Presentation slides: https://drive.google.com/file/d/1j-D8AkVk3TQoiK3pmnJFD4EA1QimEorD/view?usp=sharing

WG2 Seminar by Maria Chiara Braidotti

Albert Roura — Thu, 16 Jul 2026 09:54:27 +0000

Date and time: 21 May 2026, 15:00 (CEST)

Speaker: Maria Chiara Braidotti (University of Glasgow)

Title: Exploiting Rotation for the Generation of Quantum Entanglement

Abstract: Rotation, a fundamental form of non-inertial motion, can profoundly influence the behaviour of quantum systems. In this presentation, I will discuss recent advances in understanding how rotational motion affects quantum correlations and entanglement. Building on a novel approach that combines Sagnac interferometry with modern quantum techniques, I will show how rotation can expose, modify, and even generate entanglement between particles that are initially uncorrelated. These results open new perspectives on the interplay between motion and quantum information, while laying the groundwork for future applications in quantum sensing and relativistic quantum technologies.

WG2 Seminar by Jonathan Oppenheim

Albert Roura — Wed, 03 Jun 2026 12:00:26 +0000

Date and time: 16 April 2026, 15:00 (CEST)

Speaker: Jonathan Oppenheim (University College London)

Title: Decoherence vs diffusion: testing the quantum nature of gravity

Abstract: We consider two interacting systems when one is treated classically while the other remains quantum. The most general form of such dynamics can be derived and has implications for the foundations of quantum theory, and for the problem of understanding gravity when spacetime is treated as fundamentally or effectively classically. In particular, we construct a consistent classical-quantum description of general relativity coupled to quantum field theory. The pure gravity theory is formally renormalisable. If any system is treated as fundamentally classical, the dynamics necessarily results in decoherence of quantum systems, and a breakdown in predictability in classical phase space. Nonetheless the quantum state remains pure conditioned on the classical trajectory. We prove that a trade-off between the rate of decoherence and the degree of diffusion induced in the classical system is a general feature of all classical-quantum dynamics. Applying the trade-off to general relativity enables us to experimentally test the nature of spacetime. Bounds on decoherence rates arising from interferometry experiments, combined with precision acceleration measurements, squeezes the theory from both sides and can be used to rule out theories in which spacetime is described classically.

Video recording: https://youtu.be/lvd60KjTH0g

Presentation slides: https://drive.google.com/file/d/1bQysFpH7_4QeCDcFYjxBYNp0pTKkBaXz/view?usp=sharing

Questions from the conference Observers and Causality in Quantum Gravity - 2026

Fabio Costa — Sat, 25 Apr 2026 14:15:38 +0000

At the recent conference Observers and Causality in Quantum Gravity, In Bratislava, there have been several interesting discussions on notions such as events, observers, reference frame, causality, as understood in different fields and by different people. We invite everyone from the RQI community to contribute to and extend this discussion here on the Forum. You can start by filling this survey.

Past WG2 seminars (video recording + slides)

Albert Roura — Thu, 23 Apr 2026 10:19:54 +0000

This thread will collect detailed information about past seminars, including links to the video recording and the presentation slides when available.

Upcoming WG2 Online Seminars

navdeeparya — Thu, 23 Apr 2026 08:59:44 +0000

Updates about the upcoming online seminars of the Working Group 2.

RE: Upcoming WG1 online Seminars

Vilasini — Wed, 08 Apr 2026 10:08:44 +0000

Hello everyone,

Hope you are doing well! The details of the next WG1 seminar are given below, please note the speaker update since the previous announcement. Due to a scheduling conflict, Maria Papageorgiou's talk has been postponed to a later date, and we are pleased to welcome Lin-Qing Chen as tomorrow's speaker.

Date and time: Thursday, 9th April, 15:00 CEST

Speaker: Lin-Qing Chen (IQOQI Vienna)

Title: Quantum coordinate transformations via matter fields

Abstract: A central challenge in quantum gravity is to formulate a theory without presupposing a background spacetime. Such a description requires relational observables and a genuinely quantum generalisation of diffeomorphisms. Here we associate coordinate systems with sets of four dynamical scalar fields, whose stress-energy tensors contribute to the constraints of linearised quantum gravity. Such quantum coordinate fields (QCFs) extend the notion of quantum reference frames and enable a relational description of other physical systems. By generalising the perspective-neutral construction of quantum reference frames, we show that relational, gauge invariant observables admit a description in each QCF perspective, and derive consistent transformations between perspectives. The resulting unitary maps implement local quantum coordinate changes, yielding quantum linearized diffeomorphisms beyond superpositions of classical ones. We further discuss the operational aspects of relational observables, the conceptual distinction between QCFs and quantum reference frames, and identify the difference between QCFs transformations and the superposition of classical diffeomorphisms.

Zoom link: https://york-ac-uk.zoom.us/j/95152863408?pwd=oKL9LKGpsOpUm1Ofxsj93SJLrgtns4.1

Hope to see you there! As you may know, the seminars are recorded and available on the RQI COST YouTube channel: https://www.youtube.com/@RQICOST

On behalf of:
Kasia Rejzner (Professor of Mathematics, University of York, WG1 leader)
V. Vilasini (Research Faculty in Quantum Information Theory, Inria, Université Grenoble Alpes, WG1 co-leader)

Conceptual Foundations of QFT Workshop, Vienna, 1-3 June 2026

Maria Papageorgiou — Mon, 02 Feb 2026 13:17:26 +0000

Registration is now open until February 28

https://www.iqoqi-vienna.at/news-events/conceptual-foundations-of-qft

Multi-time quantum process tomography on a superconducting qubit

Christina_giarmatzi — Thu, 22 Jan 2026 00:25:26 +0000

Current quantum technologies are at the cusp of becoming useful, but still face formidable obstacles such as noise. Noise severely limits the ability to scale quantum devices to the point that they would offer an advantage over classical devices. To understand the sources of noise it is necessary to fully characterise the quantum processes occurring across many time steps; only this would reveal any time-correlated noise called non-Markovian. Previous efforts have attempted such a characterisation but obtained only a limited reconstruction of such multi-time processes. In this work, we fully characterise a multi-time quantum process on superconducting hardware using in-house and cloud-based quantum processors. We achieve this by employing sequential measure-and-prepare operations combined with post-processing. Employing a recently developed formalism for multi-time processes, we detect general multi-time correlated noise. We also detect quantum correlated noise which demonstrates that part of the noise originates from quantum sources, such as physically nearby qubits on the chip.

Multi-Time Quantum Process Tomography On A Superconducting Qubit, Christina Giarmatzi, Tyler Jones, Alexei Gilchrist, Prasanna Pakkiam, Arkady Fedorov, Fabio Costa, Quantum 9, 1952 (2025). https://quantum-journal.org/papers/q-2025-12-18-1952/