Seminars

General probabilistic theories are shown to admit a Gleason-type theorem if and only if they satisfy the no-restriction hypothesis, or a “noisy” version of the hypothesis. Therefore, in precisely these theories we recover the state space by assuming that (i) states consistently assign probabilities to measurement outcomes and (ii) there is a unique state for every such assignment.

Link to the paper: https://arxiv.org/abs/2005.14166

The minimal-coupling quantum heat engine is a thermal machine consisting of an explicit energy storage system, heat baths, and a working body, which couples alternatively to subsystems through discrete steps – energy conserving two-body quantum operations. Within this paradigm, it is presented a general framework of quantum thermodynamics, where a process of the work extraction is fundamentally limited by a flow of non-passive energy (ergotropy), while energy dissipation is expressed through a flow of passive energy. The main result is finding the optimal efficiency and work extracted per cycle of the three-stroke engine with the two-level working body. One of key new tools is the introduced “control-marginal state” – one which acts only on a working body Hilbert space but encapsulates all the features of total working body-battery system regarding work extraction.
Link to the paper: https://arxiv.org/abs/2003.05788

Going beyond the simple two-party scenario of quantum key distribution, we consider N parties who wish to certify security against a potential eavesdropper in a cryptographic task. Moreover, we consider the very adversarial scenario in which the parties make no assumption about the underlying quantum system or the internal working of their measurement devices. This is the device-independent scenario. In the device-independent scenario, security is certified by the violation of a Bell inequality. In this talk I will present our recent results on bounds on Eve’s uncertainty as a function of the violation of the multipartite MABK Bell inequality. I will discuss the implication of these results to cryptographic tasks, such as randomness expansion and conference key agreement. Finally, I discuss the challenges and possibilities to extend our results to other Bell inequalities, which can lead to better cryptographic protocols.
Link to the paper: https://arxiv.org/abs/2004.14263

I am going to give a controversial seminar concerning entropy production in quantum thermodynamics. I will show that there exists a fundamental difference between microscopic quantum thermodynamics and macroscopic classical thermodynamics. It will be proved that the entropy production in quantum thermodynamics always vanishes for both closed and open quantum systems! This novel and the very surprising result is derived based on the genuine reasoning Clausius used to establish the science of thermodynamics. This result will interestingly lead to defining the generalized temperature for any non-equilibrium quantum system.

10:00 Opening of the session including Golden, Silver and Bronze KCIK Award results for 2019
A) keynote speakers
10:15 – 10:55 David de Vincenzo (Juelich): Blind Oracle Quantum Computation
11:00 – 11:40 Nicolas Gisin (Geneva): Non-locality in Networks
11:45-12:00 coffee break
B) talks of Laureates
12:00 – 12:25 distinguished Ph.D. Thesis
12:30 – 12:55 Bronze prize – awarded Master Thesis
13:00 – 13:25 Silver prize – awarded Ph.D. Thesis

Going beyond the simple two-party scenario of quantum key distribution, we consider N parties who wish to certify security against a potential eavesdropper in a cryptographic task. Moreover, we consider the very adversarial scenario in which the parties make no assumption about the underlying quantum system or the internal working of their measurement devices. This is the device-independent scenario. In the device-independent scenario, security is certified by the violation of a Bell inequality. In this talk I will present our recent results on bounds on Eve’s uncertainty as a function of the violation of the multipartite MABK Bell inequality. I will discuss the implication of these results to cryptographic tasks, such as randomness expansion and conference key agreement. Finally, I discuss the challenges and possibilities to extend our results to other Bell inequalities, which can lead to better cryptographic protocols.
Link to the paper: https://arxiv.org/abs/2004.14263

As quantum technologies advance, the certification of high-dimensional quantum devices becomes more and more relevant. This is essential both for verifying the outcome of quantum computations, and for guaranteeing the security of cryptographic devices. In this talk, I will focus on the certification of quantum measurement devices performing measurements in mutually unbiased bases (MUBs) in arbitrary dimension. MUBs have a myriad of applications in quantum information processing, and therefore these measurement devices constitute basic building blocks of computational and cryptographic devices. I will discuss how to give a characterisation of MUBs that is suitable for semi-device-independent (SDI) certification, that is, certification under the assumption of fixed Hilbert space dimension. Then I will present a protocol that allows for experimental SDI certification of MUBs in arbitrary dimensions (that has since been demonstrated experimentally in dimension 4). Furthermore, I will discuss a relaxed device-independent (DI) characterisation of MUBs, referred to as mutually unbiased measurements (MUMs). While MUMs are strictly more general than MUBs, they share many operational characteristics, and I will present a protocol that allows for their DI certification

Links to the papers: https://arxiv.org/abs/1803.00363,
https://arxiv.org/abs/1912.03225

A standard approach to quantifying resources is to determine which operations on the resources are freely available and to deduce the ordering relation among the resources that these operations induce. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, that is, entanglement, then it is typically presumed that the appropriate choice of free operations is local operations and classical communication (LOCC). We here argue that, in spite of the near-universal endorsement of the LOCC paradigm by the quantum information community, this is the wrong choice for one of the most prominent applications of entanglement theory, namely, the study of Bell scenarios. The nonclassicality of correlations in such scenarios, we argue, should be quantified instead by local operations and shared randomness (LOSR). We support this thesis by showing that various perverse features of the interplay between entanglement and nonlocality are merely an artifact of the use of LOCC-entanglement and that the interplay between LOSR-entanglement and nonlocality is natural and intuitive. Specifically, we show that the LOSR paradigm (i) provides a resolution of the “anomaly of nonlocality”, wherein partially entangled states exhibit more nonlocality than maximally entangled states, (ii) entails a notion of genuine multipartite entanglement that is distinct from the conventional one and which is free of several of its pathological features, and (iii) makes possible a resource-theoretic account of the self-testing of entangled states which simplifies and generalizes prior results. Along the way, we derive some fundamental results concerning the necessary and sufficient conditions for convertibility between pure entangled states under LOSR and highlight some of their consequences, such as the impossibility of catalysis for bipartite pure states.
Link to the paper: https://arxiv.org/abs/2004.09194

Modern communication strives towards provably secure systems which can be widely deployed. Quantum key distribution provides a methodology to verify the integrity and security of a key exchange based on physical laws. However, physical systems often fall short of theoretical models, meaning they can be compromised through uncharacterized side-channels. The complexity of detection means that the measurement system is a vulnerable target for an adversary. Here, we present secure key exchange up to 200 km while removing all side-channels from the measurement system. We use mass-manufacturable, monolithically integrated transmitters that represent an accessible, quantum-ready communication platform. This work demonstrates a network topology that allows secure equipment sharing which is accessible with a cost-effective transmitter, significantly reducing the barrier for widespread uptake of quantum-secured communication.

Link to the paper: https://arxiv.org/abs/1908.08745