Title : A quantum circuit framework for extended Wigner’s friend scenarios: consistent reasoning without assuming objective events

Location: Sorbonne Université – LKB – Room  210 –  Tour 13/23 – 2e – Campus Jussieu – Paris 5e

Meet us at 1.45pm if you’d like to share a cup of coffee or tea with us!
The workshop starts at 2pm.

Based on https://arxiv.org/abs/2209.09281 (joint work with Mischa Woods)

Abstract : When using quantum theory, observers or agents are usually treated classically. Extended Wigner’s Friend Scenarios (EWFS) capture multi-agent protocols where the agents are also modelled as unitarily evolving quantum systems, and several no-go results assert that EWFS would have radical implications for physics. Frauchiger and Renner (FR) have claimed that quantum agents reasoning using quantum theory will arrive at logical paradoxes, while other works show the fundamental absence of an objective, agent-independent notion of measurement events in EWFS. This raises a crucial question: is it possible to reliably make and test scientific predictions, and consistently reason about the world when quantum theory is applied universally (also to agents), and without assuming that observed measurement outcomes are absolute and objective? We give a positive answer by developing a general quantum circuit framework for EWFS that achieves this. In particular, we provide concrete rules by which quantum agents can reason and make predictions in a logically and causally consistent manner. We formalise the concept of Heisenberg cuts by mapping distinct cuts to distinct channels in the circuit, and prove that FR-type paradoxes can be fully resolved by making explicit the conditioning on the quantum channels (which model the cuts) that are used in the reasoning process. Despite allowing measurement events to be fundamentally subjective, our framework describes all perspectives and predictions of an EWFS within a single, well-defined causal structure. Moreover, we show that an objective notion of measurement events which we perceive in real-world experiments naturally emerges as a special case. Our work demonstrates the possibility of a fundamentally relational and yet operational framework overcoming challenges to logical and causal reasoning in EWFS, without modifying the Born rule, quantum unitarity or the axioms of classical logic and probability theory applied to measurement outcomes. This also provides a unifying formalism for analysing several EWFS arguments, including epistemic/agent-centric aspects as well as more ontological/meta-physical aspects.