Spying on photons with photons: quantum interference and information

TL;DR

This paper proposes an experiment where one photon can gain which-path information about another photon, demonstrating that the quantum state manipulation, not the setup type, limits interference visibility, allowing full interference with proper state engineering.

Contribution

It introduces a novel method to spy on photon paths via quantum state modification, challenging traditional views on wave-particle duality and interference limitations.

Findings

Which-path information limits interference visibility

Full interference can be achieved with engineered quantum states

Quantum state manipulation replaces physical setup modifications

Abstract

The quest to have both which-path knowledge and interference fringes in a double-slit experiment dates back to the inception of quantum mechanics (QM) and to the famous Einstein-Bohr debates. In this paper we propose and discuss an experiment able to spy on one photon's path with another photon. We modify the quantum state inside the interferometer as opposed to the traditional physical modification of the "wave-like" or "particle-like" experimental setup. We are able to show that it is the ability to harvest or not which-path information that finally limits the visibility of the interference pattern and not the "wave-like" or "particle-like" experimental setups. Remarkably, a full "particle-like" experimental setup is able to show interference fringes with 100 % visibility if the quantum state is carefully engineered.