Seminars

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

Recent theoretical and experimental studies have shown significance of quantum information scrambling for problems encountered in high-energy physics, quantum information, and condensed matter. Due to complexity of quantum many-body systems it is plausible that new developments in this field will be achieved by experimental explorations. Therefore, a better theoretical understanding of quantum information scrambling in systems affected by noise is needed. To address this problem I will discuss indicators of quantum scrambling – out-of-time-ordered correlation functions (OTOCs) in open quantum systems. As most experimental protocols for measuring OTOCs are based on backward time evolution, two possible scenarios of joint system-environment dynamics reversal will be considered. Derivation of general formulas for OTOCs in those cases as well as a study of the spin chain model coupled to the environment of harmonic oscillators will be presented.

Link to the paper: https://arxiv.org/abs/1903.05025

The standard benchmark for teleportation is the average fidelity of teleportation and according to this benchmark not all states are useful for teleportation. It was recently shown, however, that all entangled states lead to nonclassical teleportation, with there being no classical scheme able to reproduce the states teleported to Bob. Here we study the operational significance of this result. On the one hand, we demonstrate that every state is useful for teleportation if a generalization of the average fidelity of teleportation is considered which concerns teleporting quantum correlations. On the other hand, we show the strength of a particular entangled state and entangled measurement for teleportation—as quantified by the robustness of teleportation—precisely characterizes their ability to offer an advantage in the task of subchannel discrimination with side information. This connection allows us to prove that every entangled state outperforms all separable states when acting as a quantum memory in this discrimination task. Finally, within the context of a resource theory of teleportation, we show that the two operational tasks considered provide complete sets of monotones for two partial orders based on the notion of teleportation simulation, one classical and one quantum.
Link to the paper: https://arxiv.org/abs/1908.05107

Contextuality is a key feature of quantum mechanics, as was first brought to light by Bohr and later realised more technically by Kochen and Specker. Isham and Butterfield put contextuality at the heart of their topos-based formalism and gave a reformulation of the Kochen-Specker theorem in the language of presheaves. Here, we broaden this perspective considerably (partly drawing on existing, but scattered results) and show that apart from the Kochen-Specker theorem, also Wigner’s theorem, Gleason’s theorem and Bell’s theorem relate fundamentally to contextuality. We provide reformulations of the theorems using the language of presheaves over contexts and give general versions valid for von Neumann algebras. This shows that a very substantial part of the structure of quantum theory is encoded by contextuality.
Link to the paper: https://arxiv.org/abs/1910.09591

Communication networks have multiple users, each sending and receiving messages. A multiple access channel (MAC) models multiple senders transmitting to a single receiver, such as the uplink from many mobile phones to a single base station. The optimal performance of a MAC is quantified by a capacity region of simultaneously achievable communication rates. We study the two-sender classical MAC, the simplest and best-understood network, and find a surprising richness in both a classical and quantum context. First, we find that quantum entanglement shared between senders can substantially boost the capacity of a classical MAC. Second, we find that optimal performance of a MAC with bounded-size inputs may require unbounded amounts of entanglement. Third, determining whether a perfect communication rate is achievable using finite-dimensional entanglement is undecidable. Finally, we show that evaluating the capacity region of a two-sender classical MAC is in fact NP-hard.
Link to the paper: https://arxiv.org/abs/1909.02479