A time-reversible quantum causal model

TL;DR

This paper explores whether the causal arrow in quantum models is an observer-dependent feature rather than an intrinsic property, proposing a time-symmetric quantum causal model that challenges traditional notions.

Contribution

It introduces a quantum observational scheme within a non-mechanistic causal framework, demonstrating time-symmetry in certain quantum processes and suggesting the causal arrow may be observer-dependent.

Findings

Quantum processes preserving the maximally mixed state are time-symmetric.

The causal arrow may be an observer-dependent feature in quantum models.

The model offers a way to reconcile causality with time-symmetric physical laws.

Abstract

Modern approaches to causal modeling give a central role to interventions, which require the active input of an observer and introduces an explicit `causal arrow of time'. Causal models typically adopt a mechanistic interpretation, according to which the direction of the causal arrow is intrinsic to the process being studied. Here we investigate whether the direction of the causal arrow might be a contribution from the observer, rather than an intrinsic property of the process. Working within a counterfactual and non-mechanistic interpretation of causal modeling developed in arXiv:1806.00895, we propose a definition of a `quantum observational scheme' that we argue characterizes the observer-invariant properties of a causal model. By restricting to quantum processes that preserve the maximally mixed state (unbiasedness) we find that the statistics is symmetric under reversal of the time-ordering. The resulting model can therefore accommodate the idea that the causal arrow is observer-dependent, indicating a route towards reconciling the causal arrow with time-symmetric laws of physics.