Negativity Bounds for Weyl-Heisenberg Quasiprobability Representations

The appearance of negative terms in quasiprobability representations of quantum theory is known to be inevitable, and, due to its equivalence with the onset of contextuality, of central interest in quantum computation and information. Until recently, however, nothing has been known about how much negativity is necessary in a quasiprobability representation. Zhu proved that the upper and lower bounds with respect to one type of negativity measure are saturated by quasiprobability representations which are in one-to-one correspondence with the elusive symmetric informationally complete quantum measurements (SICs). We define a family of negativity measures which includes Zhu's as a special case and consider another member of the family which we call "sum negativity." We prove a sufficient condition for local maxima in sum negativity and find exact global maxima in dimensions $3$ and $4$. Notably, we find that Zhu's result on the SICs does not generally extend to sum negativity, although the analogous result does hold in dimension $4$. Finally, the Hoggar lines in dimension $8$ make an appearance in a conjecture on sum negativity.Comment: 21 pages. v2: journal version, added reference

Informationally Complete Measurements and Optimal Representations of Quantum Theory

Minimal informationally complete quantum measurements (MICs) furnish probabilistic representations of quantum theory. These representations cleanly present the Born rule as an additional constraint in probabilistic decision theory, a perspective advanced by QBism. Because of this, their structure illuminates important ways in which quantum theory differs from classical physics. MICs have, however, so far received relatively little attention. In this dissertation, we investigate some of their general properties and relations to other topics in quantum information. A special type of MIC called a symmetric informationally complete measurement makes repeated appearances as the optimal or extremal solution in distinct settings, signifying they play a significant foundational role. Once the general structure of MICs is more fully explicated, we speculate that the representation will have unique advantages analogous to the phase space and path integral formulations. On the conceptual side, the reasons for QBism continue to grow. Most recently, extensions to the Wigner\u27s friend paradox have threatened the consistency of many interpretations. QBism\u27s resolution is uniquely simple and powerful, further strengthening the evidence for this interpretation

Quantum and Classical Bayesian Agents

We describe a general approach to modeling rational decision-making agents who adopt either quantum or classical mechanics based on the Quantum Bayesian (QBist) approach to quantum theory. With the additional ingredient of a scheme by which the properties of one agent may influence another, we arrive at a flexible framework for treating multiple interacting quantum and classical Bayesian agents. We present simulations in several settings to illustrate our construction: quantum and classical agents receiving signals from an exogenous source, two interacting classical agents, two interacting quantum agents, and interactions between classical and quantum agents. A consistent treatment of multiple interacting users of quantum theory may allow us to properly interpret existing multi-agent protocols and could suggest new approaches in other areas such as quantum algorithm design.Comment: 45 pages, 17 figure