Research interests
Indefinite quantum causality
Causal relations are normally established on the basis of an underlying background against which all events are positioned and which we call ‘space-time’. However, in order to depict the interplay between quantum mechanics and general relativity, modern physics may have to revise this paradigm [1—3]. In particular, in order to fully represent both theories, causal structures may need to be both dynamic (as required by general relativity) and indefinite (due to quantum theory).
Until a few years ago, this assumption seemed impossible to formalise, as both standard quantum mechanics and quantum field theory assume a background space-time and predefined causal relation. The landscape has changed over the past decade, with a large body of work devoted to this avenue of research. The cornerstone of this development is the formulation of extensions of quantum theory that do not presuppose a defined underlying causal structure [4—7]. This has not only proven to be important from the point of view of the foundations of physics, but also opened the way to new opportunities in the processing and transmission of quantum information [8—10]. This topic has been one of the research areas that has most profoundly characterised my past and, in some respects, current research activity.
Credit: V. Saggio & University of Vienna
Arrow of time in quantum physics
If we wish to define the time flow of a physical phenomenon, we must refer to the estimate of the change that this phenomenon has brought about on the physical system subjected to it [11]. Indeed, at their most fundamental level, physical systems generally obey laws that are reversible in time, and thus the evolutions of systems often do not inherently distinguish between forward and backward temporal directions. In the case of classical physics, thermodynamics (and in particular entropy variation) is often used to define the arrow of time of a physical system (often referred to as the ‘thermodynamic arrow of time’ [12]). However, applying the same principles to the quantum-mechanical context leads to results that are rather counterintuitive, due to quantum superposition and entanglement. Such principles, applied to the notion of the thermodynamic arrow of time, imply that quantum mechanics could allow for the superposition of thermodynamic processes that produce opposite variations in entropy [13]. This raises the question of whether this definition is suitable for determining the orientation of the arrow in time in quantum mechanics. The study of this question, both from a physical and philosophical angle, has been among my research interests in recent years.
Integrated photonics technologies
The high competitiveness between the various approaches to quantum information devices has stimulated their rapid development. Integrated photonics is currently among the most favoured areas in this race [14], and the different tools derived from it are of a variety of uses [15]. However, while the development of quantum foundations usually fuels the field of quantum technologies, the benefits of these for the design of new foundational experiments are often realised with some delay. My main experimental research objective is to apply the notions of modern quantum optics and quantum engineering to the analysis of frontier topics of quantum information theory and quantum foundations. My goal is to bring both fields to significant progress. In fact, I believe that not only the study of time and causality in quantum physics will benefit from the results of the experimental realisation of very challenging experiments, but also integrated photonics will benefit from the conjunction of experimental techniques that, though individually state-of-the-art, have rarely been combined in the realisation of such sophisticated circuits.
Credit: J. Frazer & QETLabs
Work in quantum thermodynamics
Work is defined as the product of an applied force and the distance along a particular trajectory. However, this definition cannot be adopted in quantum mechanics, as quantum objects do not always travel along specific trajectories [16]. In quantum mechanics, physicists typically define work as the difference between the final and initial energy of a system (the so-called ‘two-point measurement scheme’) [17]. Yet, this requires measuring the system at two different times, which destroys any coherence that may exist in the initial state. The question of how to provide an operationally accessible definition of quantum work is still open [18], and this has been one of my theoretical research interests in recent years. The reason why it is important to find such a definition stems in part from the development of quantum devices, such as nanoscale motors and refrigerators, in which quantum effects can dominate. In these systems, thermodynamic fluctuations are sometimes so large that assuming thermal equilibrium is no longer adequate. Consequently, the basic thermodynamic properties of these systems, such as entropy and free energy, require new formulations based on appropriate quantum definitions of the work done on (or extracted from) such systems.
References (non-exhaustive)
[1] L. Hardy, Probability theories with dynamic causal structure: A new framework for quantum gravity, arXiv preprint arXiv:gr-qc/0509120v1 (2005).
[2] L. Hardy, Towards quantum gravity: a framework for probabilistic theories with non-fixed causal structure, Journal of Physics A: Mathematical and Theoretical 40, 3081–3099 (2007).
[3] M. Zych, F. Costa, I. Pikovski, & Č. Brukner, Bell’s theorem for temporal order, Nature Communications 10, 3772 (2019).
[4] L. Hardy, Reformulating and reconstructing quantum theory, arXiv preprint arXiv:1104.2066v3 (2011).
[5] G. Chiribella, G. M. D’Ariano, P. Perinotti, & B. Valiron, Quantum computations without definite causal structure, Physical Review A 88 (2), 022318 (2013).
[6] G. Chiribella, Perfect discrimination of no-signalling channels via quantum superposition of causal structures, Physical Review A 86 (4), 040301 (2012).
[7] O. Oreshkov, F. Costa, & Č. Brukner, Quantum correlations with no causal order, Nature Communications 3, 1092 (2012).
[8] A. Feix, M. Araújo, & Č. Brukner, Quantum superposition of the order of parties as a communication resource, Physical Review A 92 (5), 052326 (2015).
[9] P. A. Guérin, A. Feix, M. Araújo, & Č. Brukner, Exponential communication complexity advantage from quantum superposition of the direction of communication, Physical Review Letters 117 (10), 100502 (2016).
[10] D. Ebler, S. Salek, & G. Chiribella, Enhanced communication with the assistance of indefinite causal order, Physical Review Letters 120 (2), 120502 (2018).
[11] J. J. Halliwell, J. Pérez-Mercader, & W. H. Zurek, Physical Origins of Time Asymmetry Paperback, The University Press (1996).
[12] L. Mlodinow & T. A. Brun, Relation between the psychological and thermodynamic arrows of time Physical Review E 89, 052102 (2014).
[13] G. Rubino, G. Manzano & Č. Brukner, Quantum superposition of thermodynamic evolutions with opposing time’s arrows, Communications Physics 4, 251 (2021).
[14] T. Rudolph, Why I am optimistic about the silicon-photonic route to quantum computing, arXiv preprint arXiv:1607.08535 (2016).
[15] J. Wang, F. Sciarrino, A. Laing & M. G. Thompson, Integrated photonic quantum technologies, Nature Photonics 14, 273–284 (2020).
[16] M. Campisi, P. Hänggi, & P. Talkner, Colloquium: quantum fluctuation relations: foundations and applications, Review Modern Physics 83, 771–791 (2011).
[17] A. J. Roncaglia, F. Cerisola, & J. P. Paz, Work measurement as a generalized quantum measurement, Physical Review Letters 113, 250601 (2014).
[18] M. Perarnau-Llobet, E. Bäumer, K.V. Hovhannisyan, M. Huber, & A. Acin, No-go theorem for the characterization of work fluctuations in coherent quantum systems, Physical Review Letters 118 (7), 070601 (2017).