# Research

**Quantum nonlocality**

The concepts of entanglement and nonlocality are now recognized as defining features of quantum theory. Distant observers sharing a quantum system prepared in an entangled state, can establish strong correlations, which could provably not been achieved in any theory satisfying a natural constraint of locality. The theoretical and experimental explorations of quantum entanglement and nonlocality achieved tremendous progress in recent years.

Moreover, the discovery of a general theory of nonlocality, featuring nonlocal correlations more powerful than those of quantum mechanics (but nevertheless consistent with relativity) has raised deep questions about the foundations of quantum theory. Why is nonlocality limited in quantum physics? Is there a physical principle from which quantum theory would emerge? There has been tremendous progress in this research area worldwide during the last years. This line of research offers potential to deepen our understanding of the foundations of quantum mechanics. ￼￼￼￼￼

(a) Sketch of a Bell experiment. (b) Geometric representation of correlations.

Taken from: B. Christensen et al. Physical Review X 5 (2015).

**Device-independent quantum information processing**

It was recently discovered that quantum information can be processed in a device-independent way. Specifically, it is possible to perform a protocol such as quantum cryptography, and to guarantee its security, based on experimental data only. That is, no assumptions about the internal working of the devices used in the protocol are required in this approach, therefore termed ‘device-independent’. This represents the ultimate form of security for quantum information processing, and appears as a promising solution for overcoming recent hacking attacks on commercial quantum cryptography devices. Conceptually, this approach also opens novel perspectives on the foundations of quantum theory, a clear understanding of which is still missing today. Our main objective is to develop an intensive and systematic research program on the device-independent approach to quantum theory and quantum information processing. Using tools of quantum information theory, we address questions related to (i) theory/foundations, (ii) applications, and (iii) experiments. The spirit of this research is that all aspects go hand in hand, and that progress in one direction may lead to progress in another. Particular attention is devoted to applied aspects, with a clear focus on their practical implementation. Given the great potential of device-independent protocols, we believe that their experimental implementation, which is still very challenging with today's technology, represents a major avenue for future research.

Self-testing quantum random number generator. Lunghi et al. Physical Review Letters 114 (2015).

**Quantum thermodynamics**

Quantum thermodynamics aims at understanding the thermodynamic properties of quantum systems, and hence the interplay between two fundamental theories of physics. Given the strong connection between thermodynamics and the concept of information (e.g. Maxwell’s demon), it is not surprising to see that concepts and methods of quantum information are relevant in the context of quantum thermodynamics. For instance, based on entanglement theory, a resource theory of quantum thermodynamics was recently developed. From a more practical perspective, these studies are relevant as experiments with quantum systems are attaining regimes in which thermodynamical considerations cannot be ignored, and could potentially be used advantageously.

A fundamental question in this area is to undertand the role played by quantum properties (such as entanglement and coherence) in the context of quantum thermodynamics. Loosely speaking, what is quantum in quantum thermodynamics? Our aim is to explore this question in the context of small autonomous quantum thermal machines. These machines represent an ideal testbed for discussing these questions, as they function without any source of work or external control, but using only thermal interactions with heat baths at different temperatures. In particular, we investigate how entanglement and coherence can enhance performance of these machines, for instance by achieving better cooling or extracting more work from given resources. Moreover, we also explore the possibility of creating stable entanglement from dissipative coupling to thermal reservoirs. Finally, we discuss the practical relevance of these ideas, and investigate the feasibility of experimental realizations of autonomous quantum thermal machines in mesoscopic systems, such as superconducting qubits and semiconductor quantum dots.

The smallest quantum thermal machines. Brunner et al. Physical Review E 85 (2012).

Image by Paul Skrzypczyk.