The research of Barrio-RQI is in the confluence of General Relativity, Quantum Theory, Information Theory and Thermodynamics. Most of the research that we do falls under the title Relativistic Quantum Information (RQI). RQI is concerned with placing quantum information theory within the yet more fundamental framework of Quantum Field Theory in curved spacetime. RQI pulls together concepts and ideas from special relativity, quantum optics, general relativity, quantum communication and quantum computation. From this still relatively safe ground, explorations are then made into black hole physics, cosmology and into various approaches of quantum gravity. Vice versa, we are also very interested in applications of the newfound techniques, such as new ways to control entanglement, in the more traditional application areas of quantum information theory: quantum communication, quantum simulation, quantum computing and quantum metrology. You can find below a more detailed overview of some of the topics that we’re currently interested in, along with a few highlighted papers for each topic. For a full list of our published papers, click here.
Contents
QFT and GR: Informational and Foundational Aspects
Foundations of quantum theory are most commonly discussed in the context of non relativistic quantum mechanics. Part of the reason is that systems of infinite degrees of freedom like relativistic quantum field theories are mathematically involved and hard to interpret operationally. Clarifying the conceptual and mathematical basis of QFT one can address foundational questions (like locality/non-locality/measurement problem/transmission of information) in the yet more fundamental framework of quantum field theory on curved spacetime.
This is a broad field of research in our group. We carry out research ranging from the entanglement in cosmology to the Unruh (and Anti-Unruh!) effects. Hawking radiation and the black hole information loss problem (usually called familiarly the Black hole Information paradox) is also within our scope. Issues of localization (Seriously! What is a photon and where does it live!?) and how to measure relativistic quantum systems (forget projective measurements) and operational approaches to different takes on quantum gravity. The propagation of information in quantum field theory is also a very active research avenue of the group: Factoring in relativity allows types of communication that are not possible classically or in non-relativistic quantum theory. In the same fashion, Quantum Field Theory and General Relativity allow for extreme spacetime solutions such as wormholes and Alcubierre warp drives. We study the fundamental limits of these solutions.
It is difficult to give a quick summary, but you can read about some of these aspects in Phys.org and in 1510.04701, 1701.03805, 1607.05287, 1502.07749, 1605.03973, 1606.06292 among many others!
Entanglement Harvesting
Can spacelike separated atoms get entangled interacting just with the vacuum?
Yes! Since the 80’s it’s been known that the ground state of a quantum field contains entanglement between different regions of spacetime, and, remarkably, this entanglement persists even for regions that are spacelike separated. This presence of this entanglement has often been used in attempts to answer fundamental problems in modern theoretical physics, such as the black hole information loss problem. However, even the seemingly simple question of how much entanglement is present in a quantum field is quite non-trivial to answer from a fundamental level.
More recently, in the field of RQI, this question has been answered using a more operational approach: instead of asking “how entangled are two regions of a quantum field?”, we instead ask “given two initially unentangled qubits living in different regions of spacetime, how entangled are they following their local interactions with the field?”. Besides the fact that the latter question is much more accessible, it’s also the more natural question to ask from an experimentalist’s perspective – after all we can only probe a quantum field by coupling a detector (atom, qubit, etc.) to it.
In the same way that a combine harvests grains from a farmer’s field, we say that two detectors harvest entanglement from a quantum field. (OK, not really in the same way, but that’s where the name comes from.) To better understand the fundamental questions surrounding entanglement in quantum fields, as well as to move towards an experimental implementation of an entanglement harvesting protocol (which might prove useful in generating entanglement for quantum information purposes).
To get a flavour of what we do on entanglement harvesting field check this video and these papers : 1506.03081, 1507.02688, 1704.08263, 1803.11214
Relativistic Quantum Optics
Hold on! What do you mean by relativistic? Isn’t quantum optics about photons?
Precisely!
The word ‘relativistic’ emphasizes the contrast to quantum optics where often approximations, such as the rotating-wave approximation or the single-mode approximation are taken which happen to break the covariance of the theory, and even allow for superluminal signalling violating thus causality. In relativistic quantum optics we are interested in studying the interplay of light and matter under special or general relativistic considerations — that is to say without breaking the covariance of the physical model. It is then natural to ask questions such as: How do relativistic effects change the physics in systems of light-matter interactions, and how to describe these interactions under general changes of reference frames? How valid are approximations in quantum optics that break the Lorentz covariance of the theory? We are also concerned with how, for example, a realistic qubit can be modelled, which is important in quantum information processing in order to make the right predictions.
You get an introductory overview into RQO with these papers: 1803.01867, 1509.07864, 1710.06875.
Rapid Repeated Interactions
The study of quantum systems interacting with an unknown environment, open quantum dynamics, is relevant to a variety of different pursuits from designing quantum technologies and computers to the understanding quantum thermodynamics. Collision Models of open quantum dynamics considers environments that are composed of numerous independent ancillas which interact with (bombard) the system one at a time, in succession. Recently, powerful tools have been developed to analyze the non-unitary dynamics induced in the system when this bombardment is sufficiently rapid (1605.04302). Applications have been found in implementing quantum control (1506.06749), purification (1611.07530), entanglement harvesting (in progress) and in understanding the mechanism behind thermal contact (1805.11118) and friction (in progress).
Quantum Thermodynamics
Thermodynamics is an incredibly useful theory in the classical world—with just a few variables, we can describe complex systems and design tools like engines. Extending this theory to the quantum regime is, however, nontrivial in a number of ways. Variables which are simple to define for classical systems, like work and heat, acquire a whole new dimension for quantum systems. These new dimensions bring wildly expanded tools with them. Quantum correlations, for instance, can store more work than classical correlations, and, when you add relativity into the picture, can be used to bias energy basis measurements to “teleport” energy from one system to another through a quantum field.
A comprehensive overview of the field can be found in 1505.07835. Contributions from our group include the creation of negative energy densities using quantum energy teleportation (1701.03805), employing correlations to improve cooling methods (1703.03816), and showing that fluctuations in work cost are important to consider in protocols to generate correlations (1805.11106).
Mathematical Methods
The study of Quantum Field Theory, Relativity and Quantum Information often requires the development of mathematical methods and tools that make the formal treatment of the problem attainable. Examples of these can be found in the Gaussian formalism for quantum mechanics (1709.07891,1902.02239,1212.1973).