Seminars

Because of the approaching deadline of 15/12/2020 for many NCN grant applications, the Director, himself a member of NCN and a co-author in many of its projects, will share his expertise in writing them.

The aim of this journal club talk will be the discussion of a topic that is getting more attention in the quantum thermodynamics community. In fact, numerous efforts, both theoretically and experimentally, are devoted to design technologies able to control and to route the heat flow in qubit systems suitable for the realization of quantum circuits.
In particular, the purpose is to discuss the experimental implementation with superconducting qubits in J. Senior at al., arXiv:1908.05574.

What makes quantum theory stand out against general stochastic classical theories? Such a question necessitates a precise evaluation of the assumptions that go into the definition of classicality. To be of foundational, as well as technological significance, these assumptions must be operationally falsifiable. In this seminar, we shall derive operational properties, specifically statistical equalities, for any number of generalized (with unbounded cardinality) classical ensembles. These properties rely exclusively on operationally falsifiable assumptions. We shall then demonstrate the operational quantum violation of the equality for three preparations. This stems from (hidden-variable) incompleteness of quantum formalism, which in-turn powers quantum advantage.
This forms a follow-up to arXiv:1909.07293v2.

We describe the dynamics of a quantum field coupled to a moving heat bath, in the formalism of the Markovian master equation for the field considered as an open system. We apply this to the superradiance of a rotating black hole, which provides a useful paradigm for understanding other irreversible active processes. Fermions can’t superradiate, but work may be extracted from their motion-induced population inversion in the presence of two baths. We argue that this describes the triboelectric effect (the charging of rough surfaces by rubbing). We also apply this formalism to shock waves, fleshing out Zel’dovich’s intuition that in this case “quantum mechanics helps understand classical mechanics”, and Ginzburg’s insight that “radiation during the uniform motion of various sources is a universal phenomenon rather than an eccentricity”. Finally, we argue that our interpretation of the triboelectric effect offers a qualitatively new mechanism for CP violation in fundamental physics.

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