# An Improved Experiment to Determine the `Past of a Particle' in the Nested Mach-Zehnder Interferometer

## TL;DR

This paper critiques a recent experiment modification claiming to determine a photon's past in a nested interferometer, clarifying that it measures a different operator and proposing a variation that aligns with previous findings.

## Contribution

The paper clarifies the limitations of a recent experimental modification and proposes an alternative variation that better tests the photon's past in the interferometer.

## Key findings

- The modified experiment measures a degenerate operator, not the photon’s past.
- A simple variation can provide evidence consistent with earlier results.
- The critique emphasizes the importance of correctly interpreting experimental operators.

## Abstract

We argue that the modification proposed by Li et al. [Chin. Phys. Lett. 32, 050303 (2015)] to the experiment of Danan et al. [Phys. Rev. Lett. 111, 240402 (2013)] does not test the past of the photon as characterised by local weak traces. Instead of answering the questions: (i) Were the photons in A? (ii) Were the photons in B? (iii) Were the photons in C? the proposed experiment measures a degenerate operator answering the questions: (i) Were the photons in A? (ii) Were the photons in B and C together? A negative answer to the last question does not tell us if photons were present in B or C. A simple variation of the modified experiment does provide good evidence for the past of the photon in agreement with the results Danan et al. obtained.