Seminars

Status of quantum wavefunction is one of the most debated issues in quantum foundations — whether it corresponds directly to the reality or just represents knowledge or information about some aspect of reality. In this work we propose a ψ-ontology theorem addressing this question. Our theorem invokes an assumption, called `no ontic retro-causality’, about the underlying ontological model. We provide physical rationale for this assumption and discuss novelty of the present theorem over the existing ones. At the core of our derivation we utilize another seminal no-go result by John S. Bell that rules out any it local realistic world view for quantum theory. We show that Bell nonlocality excludes the ontological explanations where quantum wavefunction is treated as mere information, viz. the ψ-epistemic explanations. In fact, models even with some degree of epistemicity can not fully incorporate the phenomena of Bell nonlocality observed in quantum theory.

Link to the paper: https://arxiv.org/abs/2005.08577

Wave-particle duality is considered to be one of the true misteries of quantum theory. However its core phenomenology can be reproduced by a local and noncontextual ontological model for particularly designed interferometric setup. Open is the question whether there is a connection between wave-particlle duality and contextuality in general scenarios. In this talk we focus on Englert’s inequality that provides a quantitatively description of wave-particle duality in terms of visibility and distinguishability. The goal of this project is to explore whether contextuality is necessary for saturating the inequality for any values of visibility and distinguishability.

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

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

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