In the quest for quantum gravity, we expect causal structure and quantum theory to have a deep relationship. In the last decade (2007), Lucien Hardy has suggested that the radical aspects of quantum physics (probabilistic nature) and relativity (dynamic causality) would both manifest in such a theory giving rise to indefinite causal orders between events. Since then there has been an active interest in studying causally neutral formulations of quantum theory (states and channels at the same footing); and in particular the study of indefinite causal orders, that exhibit new phenomenon, such as the quantum switch. These frameworks often employ minimum space-time considerations rendering the ontology of this indefiniteness obscure.

In this work, I construct a toy theory set in 1+1 D that has indefinite causal structures emerging from local definite causal structures through graph coloring rules and simple operational rules. In the setting of this toy model, I study indefinite causal structures that emerge from these operational rules and that naturally forbid close-time-like-curves. I reproduce a version of the quantum switch and more such curious possibilities in this toy model and attempt to illuminate the ontology of indefinite causal orders.

We study the implications of introducing vacuum state of the battery for arbitrary thermodynamic processes. Using the framework of thermal operations we derive a form of the second law which holds for batteries with bounded energy spectrum. In this form the second law gains corrections which vanish when battery is initialized far from the bottom of its spectrum. Furthermore, by studying a paradigmatic example of Landauer erasure we show that the existence of battery ground state leads to a thermodynamic behaviour which cannot be realized using an ideal weight. Surprisingly, this remains true even when battery operates far from its bottom. Our results are formulated in the language of quantum mechanics, but they can be similarly applied to classical (stochastic) systems as well.

We introduce data-driven inference as a protocol that, given a set of observed data, infers the mathematical description of the device that is both consistent with the observed data, in that it can reproduce the observed data, and is least committal, in that its range is of minimal volume in the space of output distributions. We introduce the notion of observational completeness, where an observationally complete set of states gives the same statistical information as the entire state space when data-driven inference is applied. Observational completeness thus plays in data-driven inference a role analogous to that of informational completeness in conventional quantum tomography. We characterize observational completeness and show that for qubit systems, a set of states are observationally complete for all informationally complete measurements if and only if the states are a 2-design.

The paradox of Wigner’s friend—a long-standing illustration of the measurement problem in quantum theory—has recently received resurging interest regarding its implications for the nature of quantum reality. In [Brukner, Entropy 20, 350 (2018)], Brukner proposed a no-go theorem for observer-independent facts based on an extended version of Wigner’s friend. In that work, he argued that a certain set of assumptions (which we here term “Local Friendliness”) leads to a Bell-local (i.e. a local hidden variable) model, and thus to Bell inequalities. Here, we show that, in fact, these assumptions imply a set of correlations that is, in general, a superset of the Bell-local correlations, and that they allow for theory-independent testing (unlike another recent no-go theorem by Frauchiger and Renner). We characterize and explicitly derive the full set of inequalities that define Local Friendliness correlations in scenarios involving three measurement settings per party. We show theoretically and via a toy experiment that some Bell-nonlocal quantum correlations satisfy Local Friendliness, thereby demonstrating that Local Observer-Independence is a weaker assumption than Local Causality. We further show that quantum correlations can violate the new inequalities that arise in the three-setting scenario. This work provides the first rigorous proof that the statement of Brukner’s theorem is correct, even though the set of correlations it generates is richer than previously realized, leading to new conceptual and information-theoretic implications.

Wave-particle duality is one of the most counterintuitive and debated aspects of quantum theory: a quantum object can behave like a classical particle or wave depending on the measurement apparatus. Wheeler with his Delayed Choice Experiment (DCE), enlightened even more the contrast between classical and quantum theories: after a photon enters in a Mach-Zehnder interferometer, the experimenter chooses to insert or not the second beam splitter revealing the wave or particle behaviour respectively. The goal is to rule out a classical objective description of the duality in which a quantum system is intrinsically either a wave or a particle.

On the other hand, exploiting Causal Inference, one can map the DCE into a prepare and measure scenario and study it in a causal framework, as shown by Chaves et al. in [Physics Review Letters 120, 190401 (2018)]. Chaves et al. showed that a local classical description can explain the quantum predictions of a DCE. However they also propose a modification of the DCE, by which it can be rejected any classical model with the only assumption of its dimensionality.

Here we implement this modified version of the DCE using a photonic platform and exploiting the polarization degree of freedom of single photons. By violating two dimensional witnesses we experimentally rule out, in a device-independent way, any classical model accounting of the modified DCE. The crucial hypothesis needed to rule out any non retro-causal classical model is the natural assumption that dimensionality of the studied system is 2.

In cosmology there are different models concerning the last phase of the universe. In my work I consider the assumption, that certain macroscopically different potential histories lead to the same endstate. I.e., that certain states of the universe in its last phase are potentially being reached via different histories. I will then investigate the question, what different interpretations of quantum mechanics would predict for the given assumption. And in particular, whether we can expect interference in these endstates.

The ontological framework provides a basis for describing the underlying nonclassical nature of an operational theory. The notion of classicality should be motivated by some principle or proposition that reflects fundamental understanding of nature. All the notions of classicality so far are related to the Leibniz’s `identity of indiscernibles’ which states that the operational equivalence should be imposed on the ontological level. In this work, we take a novel approach to the fundamental principle of classicality in terms of indistinguishability of preparation, measurement, transformation procedures and show their incompatibility with operational quantum theory. The principle states that the ontological description of these procedures is not more distinguishable than their operational distinguishability. This is the first robust ontological notion of classicality and captures a broader class of non-classicality than the previously proposed ones.

We propose a geometric multiparty extension of Clauser-Horne (CH) inequality. The standard CH inequality can be shown to be an implication of the fact that statistical separation between two events, A and B, defined as $P(A\oplus B)$, where $A\oplus B=(A-B)\cup (B-A)$, satisfies the axioms of a distance. Our extension for tripartite case is based on triangle inequalities for the statistical separations of three probabilistic events $P(A\oplus B\oplus C)$. We show that Mermin inequality can be retrieved from our extended CH inequality for three subsystems in a particular scenario. With our tripartite CH inequality, we investigate quantum violations by GHZ-type and W-type states. Our inequalities are compared to another type, so-called N-site CH inequality. In addition we argue how to generalize our method for more subsystems and measurement settings. Our method can be used to write down several Bell-type inequalities in a systematic manner.

A central problem in the study of resource theories is to find functions that are nonincreasing under resource conversions—termed monotones—in order to quantify resourcefulness. Various constructions of monotones appear in many different concrete resource theories. How general are these constructions? What are the necessary conditions we need in order to be able to apply them? To answer these questions, we introduce several general ways to construct monotones and show how standard constructions arise as special cases of these.

Generally, accounts of science as the different forms of realism, of empiricism and antirealism, are accompanied by a definition of observation and (un)observability. Each account endorses different definitions, but these latter have in common the fact that the limits of the observable heavily depend on limits of our means of empirical investigation of the world – e.g., such limits can be physiological if observing is equated with perceiving, or technological otherwise. In other words, the criteria of observability are external to the particular scientific theory one is considering. (See Shapere (1982), and Van Fraassen (1980)).
In this presentation I advance the idea that observation is subject to different limits in QM. Namely, the theory itself seems to posit limitations to what can be empirically observed in the study of quantum objects (“systems”), which are independent of the empirical means employed to carry out the observations. In order to outline this idea clearly, I am going to present two examples of this type of theory-dependent – and thus principled – unobservability in QM, which are uncertainty relations and the measurement problem.

The thesis I intend to put forth is the idea that QM provides a consistent, empirically adequate and predictively successful picture of the world, which does however contain a representation of reality at the fundamental level of which it is not possible to obtain directly corresponding observational evidence. Since the reasons for this impossibility are contained within the axioms of the theory itself, I submit that the unobservability of simultaneous precise values for non-commuting observables (1st example) and of states such as superposition and entanglement (2nd example) is a type of principled unobservability. I conclude with an open section about what consequences this type of unobservability could bear in relation with our epistemic access to the objects to which QM applies.

In Daniela Frauchiger and Renato Renner’s “Quantum theory cannot consistently describe the use of itself” (FR), a no-go theorem is recommended that given their thought experiment, at least one of three primary assumptions that underlie an alternative quantum reasoning (designated (Q), (S) and (C)) must be rejected. Here, it is proposed that if one analyzes particular premises of the reasoning, it might be shown that affirmations of contradictory premises regarding whether certain qubits have been measured or not might be primary source of the contradiction formed with the more conventional quantum calculative reasoning. This may be shown in a number of ways, including an analysis of FR’s employment of isometry-type operators. These ways are explored in brief in this poster.

We introduce a simple single-system game inspired by the Clauser-Horne-Shimony-Holt (CHSH) game. For qubit systems subjected to unitary gates and projective measurements, we prove that any strategy in our game can be mapped to a strategy in the CHSH game, which implies that Tsirelson’s bound also holds in our setting. More generally, we show that the optimal success probability depends on the reversible or irreversible character of the gates, the quantum or classical nature of the system, and the system dimension.

We focus on irreversibility and the use of quantum mechanics as the sources of computational advantages for the game. We analyze the former in light of Landauer’s principle, showing the entropic costs of the erasure associated with the game.

On the other hand, quantum advantages can be explained by appealing to the presence of contextuality. We compare the two analysis and explore a possible connection between erasure of information and contextuality.

Quantum entanglement is central to most philosophical puzzles related to quantum mechanics. Yet, it is not an exclusive property of quantum mechanics, as it is also found in theoretical generalizations thereof. Investigating the implications of entanglement in a broader context could shed a new light on the conceptual problems in quantum physics.

This work focuses on an operational theory called the process matrix formalism that generalizes quantum mechanics by relaxing the assumption of a well-defined causal structure. That broader theoretical context predicts the existence of quantum correlations that may violate the causal equivalent of Bell inequalities, extending the notion of entanglement to the geometry of spacetime.

Our objective is to identify the connection between these indefinite causal structures and quantum entanglement. In particular, we will discuss to what extent the formal analogy existing between the corresponding notions of spatial and causal non-separability carries any deeper conceptual significance.

We constructed Bell-inequalities that are maximally violated by graph-states in every prime dimension. We show that such inequalities are non-trivial since the exclusivity graph of the scenario has odd-cicles and with the sum of squares(SOS) for the Bell-operator we show that the graph-states maximally violate the Bell-inequalites. Looking for the SOS we got some self-testing statements for the graph-states in dimension 3.

Bell nonlocality as a tool for device independent certification schemes has been studied extensively in recent years. The strongest form of device independent certification is known as self-testing, which given a device certifies the promised quantum state as well as the quantum operations without any knowledge of the internal workings of the device. In recent years there has been a wave of results presenting self-testing protocols for various composite quantum systems and measurements. However, it remains a highly nontrivial problem to propose a certification scheme of d-dimensional quantum states based on violation of a single d-outcome Bell inequality that uses d-outcome measurements. Here we address this problem and propose a self-testing protocol for the maximally entangled state of any local dimension. Further, we self-test using minimum of measurements possible, i.e., 2 per site and provide the first self-testing protocol which certifies arbitrary d-outcome measurements. Our self-testing result can be used to establish unbounded randomness expansion from quantum correlations which is log_2d of perfect randomness, while it requires one random bit to encode the measurement choice.

Where does quantum advantage spring from? Such an investigation necessitates invoking an ontology on which non-classical features of quantum theory are unveiled. One such non-classical ontic-feature is preparation contextuality (PC) and advantage in oblivious communication tasks is one of its operational signatures. This article primarily pursues the ontic-feature underlying quantum advantage in communication complexity (CC). We construct oblivious communication tasks tailored to given CC problems. We upper-bound the classical success probability of these oblivious communication tasks, obtaining preparation non-contextual inequalities. We use the very states and measurements responsible for advantage in CC problems along with the orthogonal mixtures of these states to orchestrate an advantageous protocol for the associated oblivious communication tasks and the violation of the associated inequalities, thereby unveiling PC. We provide explicit quantum oblivious communication protocol i. prepare and measure, ii. two-way multi-round and, iii. entanglement assisted classical communication quantum CC protocols. To showcase the vitality of our results, we find a criterion for unbounded violation of these inequalities and demonstrate the same for two widely studied CC problems.

We derive topological nonlocal electromagnetic angular momenta that are the classical counterparts to the Aharonov-Bohm, Dual-Aharonov-Bohm, Aharonov-Casher and He-McKellar-Wilkens phases. We show that these quantum phases δ_{i} are connected with their classical electromagnetic angular momenta counterparts L_{i} via the generic relation δ_{i} = 2πL_{i}/h, which suggests an approach to find new quantum phases by first identifying their classical counterparts. Using this approach we derive the duality-invariant quantum phase δ = n(qΦ_{m}-gΦ_{e})/(hc) that accumulates a dyon upon encircling a solenoid enclosing electric and magnetic fluxes. The phase is topological because it depends on the number of windings the dyon carries out around the solenoid and is independent of the shape of the trajectory. The phase is nonlocal because there is no force acting on the dyon and then the electric and magnetic fluxes have no local consequences at any point of the trajectory. We argue that the dynamical nonlocality of quantum phases is attributable to the topology of the involved non-simply connected region rather than to the quantum equations of motion.

In 1970’s Eugene Wigner proposed the new test of Local Realism in inequalities for probabilities (so called Wigner inequalities). After some years professor Bertlmann proposed to consider for this goal the experiments with the neutral kaons systems. All theoretical calculations was prepared for the pure states of entangled kaons pair. But in the real experiments we have the mixing states of pure state of entangled kaons and background. How to change the conditions of Wigner’s inequalities violations if we take into account the existiong of background component? My report will be able to answer this question.

Recently a novel quantum advantage over classical information processing has been developed in the context of randomness processing, under the title of “Bernoulli factories”. Such resampling schemes find modern application in classical Bayesian statistics, for example in genetics when one encounters intractable likelihood functions. However, since quantum Bernoulli factories are more powerful than their classical counterparts, an open question is if this advantage can be exploited within this setting. We show that uploading the classical input to the quantum Bernoulli factory eliminates the quantum advantage, suggesting it can only be exploited within a fully coherent protocol. We compare and contrast this with the resampling scheme of Kitaev & Webb, discussing the fidelity of the protocol and its implementation on near-term quantum computers.

In quantum theory, the no-information-without-disturbance and no-free-information theorems express that those observables that do not disturb the measurement of another observable and those that can be measured jointly with any other observable must be trivial, i.e., coin tossing observables. We show that in the framework of general probabilistic theories these statements do not hold in general and continue to completely specify these two classes of observables. In this way, we obtain characterizations of the probabilistic theories where these statements hold. As a particular class of state spaces we consider the polygon state spaces, in which we demonstrate our results and show that while the no-information-without-disturbance principle always holds, the validity of the no-free-information principle depends on the parity of the number of vertices of the polygons.

We construct all of the qubit no-signaling channels that maximally violate the CHSH inequality, we show that that they have a block-diagonal structure and that all of them are measure-and-prepare channels. We also present a classical no-signaling protocol using post-selection that implements the PR-box correlations without the need for any quantum system, even though the protocol is based on quantum teleportation.

Weak values inferred from weak measurements have been proposed as a tool to investigate trajectories of pre- and post-selected quantum systems. Are the inferences drawn from the weak values about the past of a quantum particle fully true? Can the two-state vector formalism predict everything that the standard formalism of quantum mechanics can? To investigate these questions we present a “which-path” gedanken experiment in which the information revealed by a pre- and post-selected quantum system is surprisingly different from what one would expect from the weak values computed using the two-state vector formalism. In our gedanken experiment, a particle reveals its presence in locations where the weak value of the projection operator onto those locations was vanishingly small. Therefore our predictions turn out to be in contradistinction to those made based on the non-vanishing weak values as the presence indicators of the quantum particle. We propose a six port photon-based interferometer setup as a possible physical realization of our gedanken experiment.

In this work we study how the non-Markovian character of the dynamics can affect the thermodynamic performance of a quantum thermal engine, by analysing the maximum power output of Carnot and Otto cycles departing from the quasi-static and infinite-time-thermalization regime respectively. In our model, non-Markovianity is introduced by allowing some degrees of freedom of the reservoirs to be taken into account explicitly and share correlations with the engine by Hamiltonian coupling. It is found that the non-Markovian effects can fasten the control and improve the power output.

It is well-known that quantum particles are able to traverse classically un-traversable energy barriers; however, the value of the time taken by a quantum particle to complete this tunnelling process is a topic of extensive controversy in the physics literature. I set out to clarify the source of this controversy by highlighting the links between quantum tunnelling time and interpretations of quantum mechanics. I found that whether quantum tunneling time even has a coherent meaning depends on the interpretation of quantum mechanics you are working in. On the Copenhagen interpretation, it does not make sense to expect quantum theory to provide a unique, definite value for quantum tunneling time, because particles do not tunnel as localized particles; rather, they tunnel as delocalized probability distributions, and only collapse to localized states when detected. On the Bohm-deBroglie interpretation, however, quantum theory should provide a unique, definite value for quantum tunneling time, since every Bohm-deBroglie particle does tunnel through the barrier as a localized particle on a well-defined trajectory. Thus, based on the central tenets of the two interpretations, the Copenhagen theorists who try to find an expression for quantum tunneling time are bound to fail, whereas the Bohm-deBroglie theorists are bound to succeed. This link between quantum tunneling time and interpretations of quantum mechanics explains the current state of the physics literature: it explains why authors who subscribe to the Copenhagen interpretation cannot agree on an expression for quantum tunneling time, whereas Bohm-deBroglie theorists think they have a very clear answer to the tunneling time problem.

In Quantum Mechanics, there exist two fundamentally different types of channel attenuation, namely stochastic and deterministic attenuation. These two forms of loss are quantitatively different; in the case of the deterministic loss, although one may not know whether or not a particular particle will be lost or not, one may in principle obtain this information. However, with stochastic loss, even if one has complete knowledge of the system, one cannot predict whether or not a particular particle will be lost or not. The two types of loss therefore result in quantifiable different amplitudes in interference signals, as has been experimental demonstrated as early as 1984 using neutron interferometers. We conducted an experiment similar to the early neutron experiments in the optical regime instead and investigated the influence of both loss mechanisms on the heralding efficiency of correlated photons from type-II spontaneous parametric down-conversion (SPDC). Deterministic and stochastic losses were implemented via a chopping wheel and neutral density filter, respectively. By observing the time-averaged coincidence to single-photon count-rate-ratio of the collinear signal and idler photons, we were able to distinguish between additional implemented stochastic and deterministic attenuation without the need for an interferometer. How this notion of deterministic loss may be formalized for more generally, and the importance of these results in the context of quantum communications are discussed.

Nies and Scholz introduced the notion of a state to describe an infinite sequence of qubits. They defined quantum-Martin-Löf randomness for states analogously to the well known concept of Martin-Löf randomness for elements of Cantor space (the space of infinite sequences of (classical) bits). We formalize how `measurement’ of a state with respect a basis induces a probability measure on Cantor space. We define a state to be `measurement random’ (mR) if the measure induced by it with respect to any computable basis assigns probability one to the set of Martin-Löf random sequences. Equivalently, a state is mR if and only if measuring it with respect to any computable basis yields a Martin-Löf random sequence with probability one. Our main result is that while quantum-Martin-Löf random states are mR, the converse fails: there is a mR state, p, which is not quantum-Martin-Löf random. In fact, something stronger is true. While p is computable and can be easily constructed, measuring it with respect to any computable basis yields an ‘arithmetically random’ sequence with probability one. Roughly speaking, a sequence is said to be arithmetically random if it is Martin-Löf random relative to any iterate of the halting problem, considered as an oracle. By generalizing the construction of p, we exhibit a family of mR states which are not quantum-Martin-Löf random. We conclude by outlining work in progress towards characterizing those states for which mR and quantum-Martin-Löf random are equivalent notions.

The notion of a causal order between events is an essential ingredient in our understanding of the world. Our conventional view of causality is that events are ordered according to some global time parameter, with past events influencing future events, but not vice versa. It has been found that in quantum theory, the situation is not so clear and the causal order between events can be indefinite. For instance, if the laws of quantum mechanics are applied to causal relations, then one could imagine situations in which the causal order of events is not always fixed. There might exist an indefinite causal structure that somehow corresponds to a quantum superposition of “A causing B” and “B causing A”. Such indefinite causal structures could make new quantum information processing tasks possible and provide methodological tools in quantum theories of gravity.

An explicit example of such a process is the so-called “quantum switch”, where an auxiliary quantum system can control the order in which operations are applied and hence realize “superpositions of causal circuits”. This quantum switch was indeed proven to allow for the realization of new tasks, which are impossible with standard quantum circuits. The quantum switch was until now the only physical example of a superposition of causal orders. The associated process matrix is non-separable because of “interference” terms that disappear when one traces out the control qubit. So far, the only known implementable circuit involves two polarizing beam splitters (or analog routers, depending on the nature of system that is used to encode the control qubit), linked with each other with a “feedback wire”. I propose an implementable circuit different from the quantum switch, which can reproduce the same interference of causal orders.

We study the possibility of using dark-solitons in quasi one dimensional Bose-Einstein condensates to produce two-level systems (qubits) by exploiting the intrinsic nonlinear and the coherent nature of the matter waves. We calculate the soliton spectrum and the conditions for a qubit to exist. We also compute the coupling between the phonons and the solitons and investigate the emission rate of the qubit in that case. Remarkably, the qubit lifetime is estimated to be of the order of a few seconds, being only limited by the dark-soliton death due to quantum evaporation. Further, we explore the spontaneous creation of entanglement between two dark soliton qubits due to quantum fluctuations, by using the superposition of two maximally entangled states in the dissipative process of spontaneous emission. By driving the qubits with the help of Raman lasers, we observe the formation of long distance steady-state concurrence. Our results suggest that dark-soliton qubits are a good candidates for quantum information protocols based purely on matter-wave phononics.

Just as quantum nonlocality is a stronger version of a phenomenon present in classical physics, namely, correlation, we believe that the ‘inconsistency’ between the inferences of the different agents in Renner and Frauchiger’s paper (Frauchiger, 2018) is a special, quantum version of a classical phenomenon, also present in cases such as the Sleeping Beauty Problem. We call this phenomenon Probabilistic Consistency. What is Probabilistic Consistency, and what are the conditions for it to be present? We explore this question and several other related ideas.