We are holding a local conference on Relativistic Quantum Information (RQI), which will take place in Stockholm (Stockholm University & NORDITA) on Friday the 17th of November 2023, 13:00-19:30 CET. The conference will be in-person (venue Albano 3: 6228 - Mega) and will also be broadcast online via the ISRQI YouTube channel.
The RQI Circuit is a new initiative by the International Society for Relativistic Quantum Information (ISRQI). The purpose of the Circuit is to promote research in RQI being conducted at selected universities, as well as to provide students and young researchers the opportunity to present their work to the global RQI community, especially those who could not attend the RQI-N conference earlier this year.
The RQI Circuit Stockholm will start at 2pm CET and will be broadcast in the ISRQI YouTube Channel. The local organizers are Germain Tobar, Evan Gale, and Vasileios Fragkos.
One of the key ways in which quantum mechanics differs from general relativity is that it requires a fixed background reference frame for spacetime. In fact, this appears to be one of the main conceptual obstacles to uniting the two theories. Additionally, a combination of the two theories is expected to yield `indefinite' causal structures. In this paper, we present a background-independent formulation of the process matrix formalism---a form of quantum mechanics that allows for indefinite causal structure---while retaining operationally well-defined measurement statistics. We do this by imposing that the probabilities arising in the formalism---which we ascribe to measurement outcomes across the points of a discrete spacetime---be invariant under permutations of spacetime points. We find (a) that one still obtains nontrivial, indefinite causal structures with background independence, (b) that we lose the idea of local operations in distinct laboratories, but can recover it by encoding a reference frame into the physical states of our system, and (c) that permutation invariance imposes surprising symmetry constraints that, although formally similar to a superselection rule, cannot be interpreted as such.
Maybe instead of quantizing relativistic field theories, which we argue to be problematic both conceptually and formally, we could try to relativize the quantum mechanical framework itself? I wish to advertise a research program constituting a direction in the general landscape of the so-called Quantum Reference Frames. The broad idea is to treat reference frames as both physical systems and quantum-mechanical objects, aiming to reformulate the quantum mechanical formalism to account for relational degrees of freedom. Our new approach is well-supported mathematically and is heavily influenced by quantum measurement theory, making it more operationally oriented than other similar attempts. In this talk, I will outline the guiding principles of our research, introduce the main elements of our proposed framework, update on our current progress and mention potential future research directions.
The mathematical treatment of the interaction between matter and light, especially in relativistic scenarios, is challenging. Even fundamental models, such as the Unruh-DeWitt detector model, present significant obstacles when seeking to treat exactly detector responses, communication scenarios, or entanglement extraction processes.
In many cases, perturbation theory allows for analytic derivations of fascinating effects. They give rise to the question of what happens beyond perturbation theory? In other cases, even perturbative calculations require advanced numerics, for example, in the neighborhood of black holes.
These challenges motivated some recent works which I will outline in my talk. In particular, I will focus on [1], in which we employ star-to-chain transformations to non-perturbatively and numerically exactly treat, for example, response and radiation emission in the Unruh effect.
Furthermore, I review works on entanglement extraction and signaling in Schwarzschild spacetime [2,3], which were enabled by advanced numerical Green function methods.
Time permitting, I will touch upon connections to recent and ongoing works concerning the entanglement structure of Gaussian states.
[1] R. H. Jonsson and J. Knörzer, “Chain-mapping methods for relativistic light-matter interactions.” arXiv, Jun. 19, 2023. doi: 10.48550/arXiv.2306.11136.
[2] J. G. A. Caribé, R. H. Jonsson, M. Casals, A. Kempf, and E. Martín-Martínez, “Lensing of vacuum entanglement near Schwarzschild black holes,” Phys. Rev. D, vol. 108, no. 2, p. 025016, Jul. 2023, doi: 10.1103/PhysRevD.108.025016.
[3] R. H. Jonsson, D. Q. Aruquipa, M. Casals, A. Kempf, and E. Martín-Martínez, “Communication through quantum fields near a black hole,” Phys. Rev. D, vol. 101, no. 12, p. 125005, Jun. 2020, doi: 10.1103/PhysRevD.101.125005.
The topic of particle localisation, namely how one defines a position operator or centre of mass in relativistic quantum mechanics, has been a longstanding question in the foundations of quantum mechanics since the mid-20th century. However, despite the length and breadth of study, the localisation problem is still not well understood.
I examine the implications of particle localisation in the Unruh-DeWitt model, which provides a simple model of a two-level system (aka 'particle detector') coupled to a scalar quantum field. By comparing the first- and second-quantised formulations of a detector with a quantised centre of mass, one is naturally led to two distinct notions of localisation. I consider the consequences of these two localisation schemes in the context of spontaneous emission, finding that the two localisations lead to distinguishable physical consequences, which can in principle be tested by future experiments.
Hawking radiation is the proposed thermal black-body radiation of quantum nature emitted from a black hole. One common way to give an account of Hawking radiation is to consider a detector that follows a static trajectory in the vicinity of a black hole and interacts with the quantum field of the radiation. In the present work, we study the Hawking radiation perceived by a detector that follows a quantum superposition of static trajectories in Schwarzschild spacetime, instead of a unique well-defined trajectory. We analyze the quantum state of the detector after the interaction with a massless real scalar field. We find that for certain trajectories and excitation levels, there are non-vanishing coherences in the final state of the detector. We then examine the dependence of these coherences on the trajectories followed by the detector and relate them to the distinguishability of the different possible states in which the field is left after the excitation of the detector. We interpret our results in terms of the spatial distribution and propagation of particles of the quantum field.
Free quantum field theories on spheres can be used to model important aspects of black holes. I describe examples which were initially inspired by holography in Anti-De Sitter space and discuss some thermodynamics. I also sketch techniques to operationally test their behaviour by scattering, or by monitoring how they radiate.
The large mass of optomechanical systems make them ideal for coupling to and detect weak gravitational fields. In addition, the nonlinear dynamics of the systems offer interesting sensing advantages. In my talk, I will outline the research direction of deriving the fundamental sensing limits of these systems and consider some applications, including to searches of modified gravity theories.
In this talk I will give an overview of the work in our group, where we focus on the interface between general relativity and quantum theory at low energies. We study how quantum signatures of gravity can show in table-top experiments, novel phenomena that arise from the interplay of quantum theory and gravity, and a quantum optics approach to physics beyond the Standard Model.
A major goal of modern physics is to understand and test the regime where quantum mechanics and general relativity both play a role. I will discuss why looking at composite particles subject to relativistic effects opens new avenues for conceptual insights into the interface between quantum theory and gravity, for new experiments, and will likely be crucial for next-generation high-precision technologies.
In this talk, I will briefly introduce methods to characterize the irreversibility of time-continuous and weak quantum measurements from a thermodynamic viewpoint. By defining a statistical arrow of time for individual realizations of the measurement process, I will show that measurements are absolutely irreversible, similar to the free expansion of a single gas particle in a box. I will present a cold-atom realization of this idea and conclude by discussing some examples where quantum measurement added noise can be rectified to produce useful work, aid quantum ground state cooling, and fuel the ticks of an autonomous quantum clock.
The quantisation of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyse the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental evidence of the quantisation of gravity.
After more than a 100 yrs of General Relativity, we still argue how we should quantize gravity across all possible energy scales. In this talk, I will highlight some of the unexpected features of gravitational phenomena that might be responsible for the hardness of such a task. In particular, I will argue that quantum field theory might not be the correct framework to embed a non-perturbative theory of quantum gravity; I will also put to question the notion that we can talk about spatially-local subsystems when gravity is on, and finally argue that these hints towards an emergent nature of gravitational physics.
In this talk Magdalena Zych will talk about the career opportunities in Stockholm, and will share important information for researchers who intend to apply for positions in NORDITA and at the University of Stockholm.