Comment on "Particle Path Through a Nested Mach-Zehnder Interferometer"

TL;DR

This paper defends the counterfactual nature of a quantum communication protocol by analyzing consistent-histories, showing that the photon does not travel through the communication channel despite different interpretations.

Contribution

It clarifies the consistent-histories analysis of the protocol, demonstrating the protocol's counterfactuality and addressing previous criticisms.

Findings

No consistent histories with photon in the channel for the Mach-Zehnder version.

Existence of consistent histories with photon in the channel for the Michelson-type protocol.

Probability of photon being in the communication channel is zero.

Abstract

In a recent paper, arXiv:1604.04596, Griffiths questioned - based on an informative consistent-histories (CH) argument - the counterfactuality, for one of the bit choices, of Salih et al.'s protocol for communicating without sending physical particles, Phys. Rev. Lett. 110, 170502 (2013). Here, we first show that for the Mach-Zehnder version used to explain our protocol, no family of consistent histories exists where any history has the photon travelling through the communication channel, thus rendering the question of whether the photon was in the communication channel meaningless from a CH viewpoint. We then show that for the actual Michelson-type protocol, there are consistent-histories families that include histories where the photon travels through the communication channel. We show that the probability of finding the photon in the communication channel is zero - thus proving complete counterfactuality.