Seminars

The predictions of quantum theory resist generalised noncontextual explanations. In addition to the foundational relevance of this fact, the particular extent to which quantum theory violates noncontextuality limits available quantum advantage in communication and information processing. In the first part of this work, we formally define contextuality scenarios via prepare-and-measure experiments, along with the polytope of general contextual behaviours containing the set of quantum contextual behaviours. This framework allows us to recover several properties of set of quantum behaviours in these scenarios, including contextuality scenarios and associated noncontextuality inequalities that require for their violation the individual quantum preparation and measurement procedures to be mixed states and unsharp measurements. With the framework in place, we formulate novel semidefinite programming relaxations for bounding these sets of quantum contextual behaviours. Most significantly, to circumvent the inadequacy of pure states and projective measurements in contextuality scenarios, we present a novel unitary operator based semidefinite relaxation technique. We demonstrate the efficacy of these relaxations by obtaining tight upper bounds on the quantum violation of several noncontextuality inequalities and identifying novel maximally contextual quantum strategies. To further illustrate the versatility of these relaxations, we demonstrate monogamy of preparation contextuality in a tripartite setting, and present a secure semi-device independent quantum key distribution scheme powered by quantum advantage in parity oblivious random access codes.

This talk is based on Quantum 5, 484 (2021).

In this talk I will discuss some recent work with Thomas D. Galley and Flaminia Giacomini, in which we apply the formalism of generalised probabilistic theories to the study of the nature of the gravitational field. 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. The formalism allows us to study this problem without having to make any precommitments to any particular model of gravity or ontological notions. By analysing these experiments within the framework of GPTs we prove that the following are inconsistent i) the gravitational field is the mediator of an interaction between two systems; ii) entanglement is generated between the two systems; iii) the field is classical. I will discuss the particularly interesting case which is a violation of condition (iii), a violation of which has commonly been viewed as evidence for the quantum nature of the gravitational field. From the perspective of GPTs, however, we see that there are other possibilities. That is, I will discuss other examples of non-classical but non-quantum theories which are nonetheless consistent with conditions (i) and (ii). This leaves an important open question, what evidence do we actually need in order to conclude that the gravitational field is quantum?

The indistinguishability of quantum identical particles has been widely used as a resource for the generation of entanglement. In this talk, I discuss the multipartite entanglement generation of identical particles with spatial overlap and internal state rotation, which can be realized as a linear quantum network (LQN) of identical particles. For the tripartite case, I explain schemes to generate two fundamental classes of genuine entanglement, i.e., GHZ and W classes, which are experimentally demonstrated with three photons. The tripartite entanglement class decays from the genuine entanglement to the full separability as the particles become more distinguishable from each other. To extend the tripartite results to an arbitrary N-partite case, I introduce a graph picture of LQNs, which provides a powerful tool for analyzing and designing LQNs to generate multipartite entanglement. Perfect matching diagrams (PM diagrams) in our graph picture furnish rigorous criteria for the entanglement of a given LQN and solid guidelines for designing suitable LQNs for the genuine entanglement.

This talk is based on arXiv:2101.00392 and arXiv:2104.05937.

We present the state of the art of TEDOPA (Time Evolving Density operator with Orthogonal PolynomiAls), a numerically exact simulation method for open quantum systems interacting with structured baths. We show how, by exploiting the chain mapping of the bath modes, TEDOPA is able to unleash the power of Tensor Networks methods to exponentially reduce the effective dimension of the extended system+bath state. We moreover show how the application of a thermofield-like transformation allows to efficiently extend the application of TEDOPA to open quantum systems interacting with finite temperature baths.

In this talk I will review the concept of “quantum batteries”: quantum systems where energy is stored and from which work can be extracted. I will focus on many-body batteries, systems composed by N identical quantum cells. In this context, I will discuss the relation between extractable work, charging power and entanglement.

We analyze the relationship between qubit-environment entanglement that can be created during the pure dephasing of the qubit and the effectiveness of the spin echo protocol. We focus here on mixed states of the environment. We show that while the echo protocol can obviously counteract classical environmental noise, it can also undo dephasing associated with qubit-environment entanglement, and there is no obvious difference in its efficiency in these two cases. Additionally, we show that qubit-environment entanglement can be generated at the end of the echo protocol even when it is absent at the time of application of the local operation on the qubit (the π pulse). We prove that this can occur only at isolated points in time, after fine-tuning of the echo protocol duration. Finally, we discuss the conditions under which the observation of specific features of the echo signal can serve as a witness of the entangling nature of the joint qubit-environment evolution.

In this presentation, we will make a wide introduction to topological quantum computing. Quantum computers use qubits in order to function and topological quantum computers use anyons, 2D particles, to encode qubits. Anyons can be determined based on the magnetic flux and the charge they are consisted of. In the first part, we will discuss the basics of group theory in order to construct an anyon model. For our anyon model, we used the group D4 and calculated the different combinations of fluxes and charges for every anyon of this model. In the second part, we will talk about the differences between anyons and 3D particles and that we (in principle) could construct a quantum computer based on the properties of anyons. Quantum gates are very sensitive to decoherence, but gates based on the rotation of anyons can guarantee a much better protection of the information. Using the Aharonov-Bohm effect, we will examine how the quantum states acquire a phase which can be used for the coding of the information. On our study, we further examined the operations which can occur between anyons and how they can be simulated with F and R matrices. In the end, we will study the simplest of all anyon models, the Fibonacci anyon model, which allows (in principle) universal quantum computing. To conduct this study, we constructed the F and R matrices for the Fibonacci anyons using some conditions known as pentagon and hexagon equations.

We discuss compatibility between various quantum aspects of bosonic fields, relevant for quantum optics and quantum thermodynamics, and the mesoscopic formalism of reduced state of the field (RSF). In particular, we derive exact conditions under which Gaussian and Bogoliubov-type evolutions can be cast into the RSF framework. To strengthen the link between the RSF formalism and the notion of classicality for bosonic quantum fields, we prove that RSF contains no information about entanglement in two-mode Gaussian states. For the same purpose, we show that the entropic characterisation of RSF by means of the von Neumann entropy is qualitatively the same as its description based on the Wehrl entropy. Our findings help bridge the conceptual gap between quantum and classical mechanics.

From the core of the matter to the stellar objects, the underlying physics relies significantly on how we understand atomic nuclei. On the other hand, the ever-puzzling nuclear interaction can be studied at various energy scales ranging from thermal neutrons to relativistic heavy-ion collisions and various sizes up to those of stars. I will introduce the correlations between finite nuclei and infinite matter, based on the liquid drop model. With the help of quantum field theory (QHD), I will elucidate modelling the nuclear interaction to describe infinite matter and neutron stars. We will discuss how the corroboration of terrestrial nuclear physics experiments, observation of neutron stars with our model could reveal properties of high-dense matter in general. I will highlight the importance of studying some exotic nuclei like the hypernuclei and proton emitters, in this regard.

In my presentation about my PhD work I will talk about Single Ion Magnets and their possible applications as the new contrast agents used in MRI. In order to obtain the magnetic properties of the selected Single Ion Magnets I have used ab-initio methods which are going to be described briefly during my talk. I will also show the accuracy of the theoretical results in comparison with the experimental data (absorption spectra and magnetic molar susceptibility).