Talks

In recent years the quantum foundations community has seen increasing interest in the possibility of using retrocausality to restore locality to quantum physics. In this talk I will argue that this motivation for retrocausality is ill-founded, and that the most coherent conceptual approach to retrocausality makes locality redundant in any case. I will then present an alternative motivation for retrocausal models based on accepting genuine nonlocality and demanding the nonexistence of a preferred reference frame.

Causal influence in quantum theory

Jonathan Barrett
_{University of Oxford}

Any account of causality in the physical world should answer questions such as the following. If A is a cause of B, then what sort of thing are A and B? What does it mean to say that A is a direct cause of B? Do causal concepts involve, in an essential way, interventions by agents? Are causal relations directed in time? What, in the overall picture, is ontic (i.e., factual and independent of the agent), and what is epistemic (i.e., relative to an agent’s knowledge or beliefs)? I will give an account of causality in quantum theory that answers these questions, leading to a formalism for quantum causal modelling. The classical formalism for causal modelling can be recovered from this account, as a special case when quantum channels are all diagonal. Towards the end I will make some more speculative remarks about a possible new direction in the interpretation of quantum theory that is suggested by the work. The talk is based on (bits of):

arXiv:1609.09487
arXiv:1906.10726
arXiv:2001.07774
arXiv:2002.12157
arXiv:2011.08120

Present and future precision tests of spontaneous wave function collapse models

Angelo Bassi
_{University of Trieste}

Quantum mechanics is grounded on the superposition principle, which is the source both of its tremendous success and technological power, as well as of the problems in understanding it. The reason why superpositions do not propagate from the microscopic to the macroscopic world are subject to debate. Spontaneous wave function collapse models have been formulated to take into account a progressive breakdown of quantum superpositions when systems are large enough; they do so by modifying the Schrödinger dynamics, and therefore they are empirically testable. Deviations are tiny, and require precision measurements. I will review the most recent tests of such models, with a focus on gravity-related ones.

Quantum Darwinism and classical objectivity: A collision model viewpoint

Steve Campbell
_{University College Dublin}

Understanding how a quantum system exchanges information or energy with its surroundings and how this can lead to a classically objective state is a ubiquitous problem. Quantum Darwinism, and the more stringent strong quantum Darwinism and spectrum broadcast structures, provide a framework to understand how specific information is redundantly encoded across the environmental degrees of freedom resulting in classical objectivity. Various tools have been employed to study the phenomenon of quantum Darwinism and in this talk I will focus on one such technique, namely collision models, which are particularly well suited to the task. Their simple structure endows them with great flexibility to probe the limitations of the Darwinistic framework which we will explore in simple spin-systems.

Underground tests of Quantum Mechanics – Collapse models and Pauli Exclusion Principle

Catalina Oana Curceanu
_{LNF-INFN, Frascati (Italy)}

We are experimentally investigating possible departures from the standard quantum mechanics’ predictions at the Gran Sasso underground laboratory in Italy.

In particular, with radiation detectors we are searching signals predicted by the collapse models (spontaneous emission of radiation) which were proposed to solve the “measurement problem” in quantum physics and signals coming from a possible violation of the Pauli Exclusion Principle.

I shall discuss our recent results published in Nature Physics under the title “Underground test of gravity-related wave function collapse”, where we ruled out the natural parameter-free version of the gravity-related collapse model. I shall then present more generic results on testing CSL (Continuous Spontaneous Localization) collapse models and discuss future perspectives.

Finally, I shall briefly present the VIP experiment with which we look for possible violations of the Pauli Exclusion Principle by searching for “impossible” atomic transitions and comment the impact of this research in relation to Quantum Gravity models.

A no-go theorem on the nature of the gravitational field beyond quantum theory

Thomas D. Galley and Flaminia Giacomini
_{Perimeter Institute} _{for Theoretical Physics}

Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the gravitational field, provided that two quantum systems become entangled due to solely the gravitational interaction. Typically, this question has been addressed by assuming an underlying physical theory to describe the gravitational interaction, but no systematic approach to characterise the set of possible gravitational theories which are compatible with the observation of entanglement has been proposed. Here, we introduce the framework of Generalised Probabilistic Theories (GPTs) to the study of the nature of the gravitational field. This framework has the advantage that it only relies on the set of operationally accessible states, transformations, and measurements, without presupposing an underlying theory. Hence, it provides a framework to systematically study all theories compatible with the detection of entanglement generated via the gravitational interaction between two non-classical systems. Assuming that two quantum systems interact via the gravitational field we show that the following conditions are incompatible: 1) entanglement is generated between the two quantum systems ; ii) the interaction between the two quantum systems is mediated by the gravitational field ; iii) the gravitational field is classical. We further show what the violation of each condition implies, and in particular, when iii) is violated, we provide examples of non-classical and non-quantum theories which are logically consistent with the other conditions.

Objectivity in and from quantum mechanics

Jarosław K. Korbicz
_{Center for Theoretical Physics PAS Warsaw}

I will discuss how to construct classical-like, objective features in quantum mechanics, focusing on so-called Spectrum Broadcast Structures (SBS). This adds some new understanding of the celebrated quantum-to-classical transition by showing that what is commonly perceived as ‘objectivity’ might be a specific property of quantum states, produced during decoherence.

The measurement postulates are redundant

Lluis Masanes
_{University College London}

The measurement postulates specify: the mathematical structure of measurements, the Born Rule and the state-update rule. The rest of postulates are referred to as “unitary quantum mechanics”. We will characterise the (possibly empty) family of theories consisting of unitary quantum mechanics supplemented with non-quantum measurement postulates. And we will prove that any such theory has the following problematic feature: the set of mixed state of any finite-dimensional Hilbert space has infinite dimension, which makes state estimation impossible. Therefore, if we disregard theories with this problematic feature (if any such theory exists) then the only measurement postulates compatible with unitary quantum mechanics are the good old quantum measurement postulates. This result reveals the true identity of the “measurement postulates” as “measurement theorems”.

Quantum theory from simple principles

Markus P. Müller
_{IQOQI Vienna}

I describe some recent reconstructions of quantum theory from information-theoretic principles, and give some suggestions for what we can possibly learn from them.

A quantum crash on Darwin

Mauro Paternostro
_{Queen’s University Belfast}

Quantum non Markovianity and quantum Darwinism are two phenomena linked by a common theme: the flux of quantum information between a quantum system and the quantum environment it interacts with. In this work, making use of a quantum collision model, a formalism initiated by Sudarshan and his school, I will analyse the efficiency with which the information about a single qubit gained by a quantum harmonic oscillator, acting as a meter, is transferred to a bosonic environment. I will show how, in some regimes, such quantum information flux is inefficient, leading to the simultaneous emergence of non Markovian and non Darwinistic behaviours. I will then combine the collisional picture for open system dynamics and the control of the rate of decoherence provided by the quantum (anti-)Zeno effect to illustrate the temporal unfolding of the redundant encoding of information into a multipartite environment that is at the basis of Quantum Darwinism, and to control it. The rate at which such encoding occurs can be enhanced or suppressed by tuning the dynamical conditions of system-environment interaction in a suitable and remarkably simple manner. This would help the design of a new generation of quantum experiments addressing the phenomenology of Quantum Darwinism and thus its characterization.

Entanglement and objectivity in pure dephasing models

Katarzyna Roszak
_{Wroclaw University of Science and Technology}

We study the relation between the emergence of objectivity and qubit-environment entanglement generation. We find that although entanglement with the unobserved environments is irrelevant (since sufficiently strong decoherence can occur regardless), entanglement with the observed environments is crucial. In fact, the appearance of an objective qubit-observed-environment state is strictly impossible if their joint evolution does not lead to entanglement. Furthermore, if a single observer has access to a single environment (no macrofractions), then the required orthogonality of the observed environmental states comes only as a consequence of the system-environment state becoming strongly entangled.

Universal structure of objective states in all fundamental causal theories

Carlo Maria Scandolo
_{University of Calgary}

A crucial question is how objective and classical behaviour arises from a fundamental physical theory. In this talk, I will provide a natural definition of a decoherence process valid in all causal theories and show how its behaviour can be extremely different from the quantum one. Remarkably, despite this, I will show that the so-called spectrum broadcast structure characterizes all objective states in every fundamental causal theory, exactly as in quantum mechanics.

The sound of objective quantum jumps

Antoine Tilloy
_{Max Planck Institute of Quantum Optics}

I will explain how linearity at the master equation level severely limits what we can empirically know about collapse models. In a world in which collapse models are fundamental, and with parameters tuned such that random jumps of the wavefunctions make audible bangs, one still cannot distinguish between reasonable alternative continuous and even deterministic explanations. My objective will be to explain why that is the case and attempt to make clearer the separation between the ontological commitments and empirical predictions of collapse models. Based on arXiv:2007.15420

Eavesdropping on the Decohering Environment: Quantum Darwinism, Amplification, and the Origin of Objective Classical Reality

Akram Touil
_{University of Maryland, Baltimore County}

“How much information about a system S can one extract from a fragment F of the environment E that decohered it?” is the central question of Quantum Darwinism. To date, most answers relied on the quantum mutual information of SF, or on the data extracted by measuring S directly. These are reasonable upper bounds on what is really needed but much harder to calculate — the channel capacity of the fragment F for the information about S. We consider a model based on imperfect c-not gates where all the above can be computed, and discuss its implications for the emergence of objective classical reality. We find that all relevant quantities, such as the quantum mutual information as well as the channel capacity exhibit similar behavior. In the regime relevant for the emergence of objective classical reality this includes scaling independent from the quality of the imperfect c-not gates or the size of E, and even nearly independent of the initial state of S.

Revealing the emergence of classicality in nitrogen‐vacancy centers

Michael Zwolak
_{Biophysical and Biomedical Measurement Group, NIST}

We examine a nitrogen vacancy center evolving naturally in the presence of its environment to study the proliferation of information about preferred quantum states via the environment. This redundantly imprinted information accounts for objective behavior, as it is independently accessible by many without perturbing the system of interest. To observe this process, we implement a novel dynamical decoupling scheme that enables the measurement/ control of several nuclear spins (the environment E) interacting with a nitrogen vacancy (the system S). In addition to showing how to create entangled SE states relevant to quantum metrology, we demonstrate that under the decoherence of S, redundant information is imprinted onto E, giving rise to classical objectivity – a consensus of the nuclear spins about the state of S. This provides laboratory verification of the objective classical behavior emerging from the underlying quantum substrate.

Contributed Talks – Abstracts

Titles List:

Sahil Gopalkrishna Naik and Manik Banik – On composition of multipartite quantum systems: perspective from time-like paradigm

Martin Plavala – Operational Theories in Phase Space: Toy Model for the Harmonic Oscillator

Shubhayan Sarkar – Universal notion of classicality based on ontological framework

David Schmid – Unscrambling causation and inference: towards a compelling realist interpretation of quantum theory

Tian Zhang – Quantum correlations in time

On composition of multipartite quantum systems: perspective from time-like paradigm

Sahil Gopalkrishna Naik and Manik Banik
_{IISER Thiruvananthapuram}

Figuring out the physical rationale behind natural selection of quantum theory is one of the most acclaimed quests in quantum foundational research. This pursuit has inspired several axiomatic initiatives to derive mathematical formulation of the theory by identifying general structure of state and effect space of individual systems as well as specifying their composition rules. This generic framework can allow several consistent composition rules for a multipartite system even when state and effect cones of individual subsystems are assumed to be quantum. Nevertheless, for any bipartite system, none of these compositions allows beyond quantum space-like correlations. In this talk I will show that such bipartite compositions can admit stronger than quantum correlations in the time-like domain and, hence, indicate pragmatically distinct roles carried out by state and effect cones. I will also discuss the consequences of such correlations in a communication task, which accordingly opens up a possibility of testing the actual composition between elementary quanta.

Operational Theories in Phase Space: Toy Model for the Harmonic Oscillator

Martin Plavala
_{Universität Siegen}

We show how to construct an energy observable dependent on position and momentum in general probabilistic theories. The construction is in accordance with classical and quantum theory and allows for physical predictions, such as observing tunneling behavior. We demonstrate the construction by formulating a toy model for the harmonic oscillator that is neither classical nor quantum. The model features a discrete energy spectrum, a ground state with sharp position and momentum, an eigenstate with non-positive Wigner function as well as a state that has tunneling properties. The toy model demonstrates that operational theories can be a viable alternative approach for formulating physical theories.

Universal notion of classicality based on ontological framework

Shubhayan Sarkar
_{Center for Theoretical Physics of the Polish Academy of Sciences}

Existence of physical reality in the classical world is a well-established fact from day-to-day observations. However within quantum theory, it is not straightforward to reach such a conclusion. A framework to analyse how observations can be described using some physical states of reality in a theory independent way was recently developed, known as ontological framework. Different principles when imposed on the ontological level give rise to different observations in physical experiments. Using the ontological framework, we formulate a novel notion of classicality termed “universal classicality” which is based upon the physical principles that in classical theories pure states are physical states of reality and every projective measurement just observes the state of the system. We construct a communication task in which the success probability is bounded from above for ontological models satisfying the notion of universal classicality. Contrary to previous notions of classicality which either required systems of dimension strictly greater than two or at least three preparations, a violation of “universal classicality” can be observed using just a pair of qubits and a pair of incompatible measurements. We further show that violations of previously known notions of classicality such as preparation non-contextuality and Bell’s local causality is a violation of universal classicality.

Unscrambling causation and inference: towards a compelling realist interpretation of quantum theory

David Schmid
_{University of Gdańsk}

Quantum theory provides an algorithm for computing statistical correlations. A realist approach to science, however, demands more: a physical theory not only predict correlations, but also provide causal explanations of these correlations. It is well-known, however, that there are significant obstacles to understanding quantum theory in any straightforwardly realist sense. Most notable among these obstacles are locality and noncontextuality no-go theorems. That is, Bell’s theorem demonstrates that local hidden variable theories cannot reproduce the quantum predictions, which implies serious difficulties to providing a causal account of those predictions. Additionally, it has been shown that there can be no realist representation of quantum theory that satisfies the principle of generalized noncontextuality, which is our most broadly applicable foundational notion of classicality.

In this article, we describe a program for circumventing these no-go theorems—that is, for providing a realist account of quantum theory that salvages the spirit of locality and of noncontextuality. We present a mathematical framework for causation and inference that helps to identify which aspects of the conventional notions of causation and inference are indispensable for any notion of causation and inference, and which aspects can be modified. (This is analogous to how in nonEuclidean geometries, certain aspects of the conventional notions of points and lines are preserved while others are modified.) The possibility of such modifications within our framework opens the door to an intrinsically nonclassical notion of realism that can account for the statistical predictions of the quantum formalism.

Our framework is also useful for the important project of unscrambling causation from inference. For example, we provide a new framework for generalized physical theories, one which allows us to provide more refined descriptions than standard frameworks of generalized and operational probabilistic theories. Our approach also makes clear how certain geometric structures are common to all generalized physical theories. Furthermore, we our framework also allows us to provide more refined “ontological representations” of operational scenarios than is possible using the standard framework of ontological modeling. For example, we demonstrate (somewhat surprisingly) that standard “ontological models” inadvertently scramble together causation and inference, in a manner that we untangle.

Quantum correlations in time

Tian Zhang
_{University of Oxford}

Assuming that temporal correlations are treated on an equal footing as spatial correlations in quantum theory, we investigate quantum correlations in time in different space-time approaches, including process matrices in indefinite causal structures, consistent histories, and generalised quantum games, out-of-time-order correlations, path integrals and pseudo-density matrices. We claim that quantum correlations in time are operationally equivalent in these approaches, except the path integral representation. We further apply temporal correlations to understanding quantum time crystals and time translation symmetries. As a result, we find that quantum correlations in space and quantum correlations in time are quite different, which suggests that time is very different from space in terms of quantum correlations.