While quantum correlations between two spacelike-separated systems are fully encoded by the bipartite density operator associated with the joint system, what operator encodes quantum correlations across space and time? I will describe the general theory of such "quantum states over time" as well as a canonical example that encodes the expectation values of certain observables measured sequentially in time. The latter extends the theory of pseudo-density matrices to arbitrary dimensions, not necessarily restricted to multi-qubit systems. In addition, quantum states over time admit a natural proposal for a general-purpose quantum Bayes' rule. Our results specialize to many well-studied examples, such as the state-update rule, the two-state vector formalism and weak values, and the Petz recovery map. This talk is based on joint work with James Fullwood and the two papers: arXiv: 2212.08088 [quant-ph] and 2405.17555 [quant-ph].
During the last 20 years, we have seen a tremendous development in the computational power of our devices. We have by now laptops and phones that are more powerful than the high-profile workstations of a few generations ago. This computational power has already severely impacted our lives and many scientific fields as well, with examples ranging from AI to biophysics. On the other hand, a large class of problems in the study of quantum many-body systems has benefited only marginally from these developments.In the talk, I will review this class of problems, emphasizing the conceptual reason behind the limited impact that traditional computational methods have brought. I will then move on to describe the most promising candidates (in my opinion) to overcome the current situation.
