INQNET Pizza Seminar

Time: 12:30pm *Pizza arrives at 12pm!
Place: in the Downs-Lauritsen 257
Zoom: https://caltech.zoom.us/j/81370143470

Speaker: Paul Erker

Title: Autonomous Quantum Processing Unit: What does it take to construct a self-contained model for quantum computation?

Abstract: Computation is an input-output process, where a program encoding a problem to be solved is inserted into a machine that outputs a solution. Whilst a formalism for quantum Turing machines which lifts this input-output feature into the quantum domain has been developed, this is not how quantum computation is physically conceived. Usually, such a quantum computation is enacted by the manipulation of macroscopic control interactions according to a program executed by a classical system. To understand the fundamental limits of computation, especially in relation to the resources required, it is pivotal to work with a fully self-contained description of a quantum computation where computational and thermodynamic resources are not be obscured by the classical control. To this end, we answer the question; "Can we build a physical model for quantum computation that is fully autonomous?'', i.e., where the program to be executed as well as the control are both quantum. We do so by developing a framework that we dub the autonomous Quantum Processing Unit (aQPU). This machine, consisting of a timekeeping mechanism, instruction register and computational system allows an agent to input their problem and receive the solution as an output, autonomously. Using the theory of open quantum systems and results from the field of quantum clocks we are able to use the aQPU as a formalism to investigate relationships between the thermodynamics, complexity, speed and fidelity of a desired quantum computation.

Speaker: Maximilian Lock

Title: The Emergence of Irreversibility in Quantum Theory: Entropy and Measurement

Abstract: The second law of thermodynamics states that the entropy of an isolated system can only increase over time, thereby distinguishing the past from the future. This seems to conflict with the reversible evolution of isolated quantum systems, which preserves the von Neumann entropy. However, counterintuitively, many observables in large isolated systems do reach equilibrium, despite the unitary evolution of the system's state. We characterise the extent to which any observable exhibits this emergent irreversibility, as determined by the relationship between the microstates associated with the reversible evolution and the macrostates associated with the observable. We demonstrate how a version of the second law of thermodynamics can be recovered in isolated quantum systems, and analyse the fluctuations from equilibrium that reveal the underlying reversible dynamics, finding that these fluctuations diminish as the system size increases. We then explore the hypothesis that the apparent irreversibility of the quantum measurement process is a manifestation of the second law of thermodynamics, resulting in possible criteria for when a physical system constitutes an observer.