Quantumness via Discrete Structures

TL;DR

This thesis explores how discrete mathematical structures like graphs and hypergraphs can be used to understand the fundamental aspects of quantumness, including contextuality, causality, and measurement incompatibility.

Contribution

It introduces a unified framework using discrete structures to analyze various quantum phenomena, extending previous work with new invariants and operational interpretations.

Findings

Graph invariants characterize contextuality and generalized contextuality.

Hypergraph models elucidate measurement incompatibility and Bell nonlocality.

Directed graphs reveal connections between indefinite causal order and nonclassicality.

Abstract

Quantum theory departs from classical probabilistic theories in foundational ways. These departures--termed quantumness here--power quantum information and computation. This thesis charts the role of discrete structures in assessing quantumness, synthesizing elements of my postdoctoral research through this lens. After an introduction to the necessary background concepts, I present my work under three broad categories. First, I present work on contextuality that extensively relies on (undirected) graphs and hypergraphs as the discrete structures of interest; more specifically, it relies on invariants associated with them. This work includes Kochen-Specker (KS) contextuality and its operationalization to generalized contextuality, expressed via (hyper)graph-theoretic frameworks. I also present work on KS-contextuality in multiqubit systems and an application of generalized contextuality to a one-shot communication task, both of which rely on hypergraphs. Second, I present work on causality, where the discrete structures of interest are directed graphs. This includes work on indefinite causal order, specifically its connections to the gap between local operations and classical communication (LOCC) and separable operations (SEP), and a device-independent notion of nonclassicality--termed antinomicity--that generalizes Bell nonlocality without global causal assumptions. Finally, I present work on the incompatibility of quantum measurements, its connection to Bell nonlocality, and its role in discriminating between quantum and almost quantum correlations in the single-system setting. The discrete structures of interest here are hypergraphs that model joint measurability relations between quantum measurements. I conclude with a summary and an overview of work that is not covered in this thesis.