Asymmetry in cause and effect in a quantum world*

How can we observe an asymmetry in the temporal order of events when physics at the quantum level is time-symmetric? The source of time's barbed arrow is a longstanding puzzle. Causal asymmetry offers a provocative perspective. It asks how Occam's razor can privilege one temporal direction over another. That is, if we want to make statisticall correct future predictions then what is the minimum past information we must store? Are we forced to store more data if we model events in one particular temporal order over the other?

Take a glass shattering upon impacting the floor. In one direction, the future distribution of shards depends only on the glass's initial position, velocity and orientation. In the opposite, we may need to track relevant information regarding each shard to infer the prior trajectory. Does this require more or less information? For stochastic processes, this potential divergence is quantified in the theory of computational mechanics. It is not only generally non-zero, but can also be unbounded. This phenomenon implies a simulator operating in the 'less natural' temporal direction is penalized with potentially unbounded memory overhead, and is cited as a candidate source of time's barbed arrow. However these studies assumed models were implemented via classical physics. Could the observed causal asymmetry have been a consequence of this classicality constraint?

In this presentation, we answer this question in the affirmative, by directly constructing a process where there is a classical arrow of time, but at the quantum level this arrow vanishes [1], and reporting on the experimental observation of this effect in a photonic quantum processor [2]. Our work suggests that causal asymmetry could be an artefact of forcing classical causal explanations in a fundamentally quantum world.

[1] Thompson et al., Phys. Rev. X 8, 031013
[2] Ghafari et al., Phys. Rev. X 9, 041013