Zenith Grant Awardee

Chiara Marletto

Oxford University

Co-Investigators

Marco Genovese, INRIM Torino

Project Title

Agent-based irreversibility in quantum theory: theory and experiment

Project Summary

The microscopic physical laws seem to have no place for agents. Yet agents are key in macroscopic phenomena such as life, the physics of engines and computers. Here we propose a radically new route to accommodating agents in fundamental physics. We use agents to crack an old-age problem, that of reconciling macroscopic irreversibility with the reversibility of the microscopic world. For example, the macroscopic process dissipating the energy of a piston into heat inside an engine is irreversible: it is impossible to revert the heat back into useful work. This irreversibility is mandated by the second law of thermodynamics. Yet, at the microscopic scale, atoms constituting the engine and its surroundings display perfectly reversible trajectories: they can move both forward and backwards in time. Here we show that agents offer a surprising way to reconcile these two views. First, we express irreversibility via agents – stating that an arbitrarily accurate agent performing a transformation in one direction is possible (e.g. converting heat into work), but it is impossible for the reverse transformation. Then we show theoretically and experimentally how agents ultimately reconcile the microscopic world, where dynamical trajectories are reversible; and the macroscopic world, where that agent-based irreversibility is allowed.

Technical Abstract

Agents are deeply problematic in fundamental physics. Not only does the definition of agents seem arbitrary; also, once initial conditions and laws of motion are fixed, there is little room left for agents in fundamental physics – if not as approximate, accessory entities. In sharp contrast, here we use agents to address the long-standing problem of reconciling time-reversal symmetric laws with irreversible behaviours – such as that of a process that increases the temperature of some liquid by mechanical stirring. To this end, we propose an objective, operational definition of agents, within the recently proposed constructor theory. Then, we define a particular type of irreversibility – ‘agent-based’ irreversibility; and we show its compatibility with time-reversal symmetric laws. We then model agent-based irreversibility in unitary quantum theory, resorting to two tools: homogenisation machines (generalising thermalisation to quantum information scenarios) and a powerful generalisation of quantum states, called pseudo-density operators, which encapsulate both spatial and temporal correlations. Finally, we propose to perform an experimental verification of agent -based irreversibility with multiple photons, thus demonstrating experimentally for the first time how agents can reconcile irreversibility with time-reversal symmetric laws.