# Experimental observation of anomalous trajectories of single photons

## TL;DR

This paper introduces a novel method to define and measure single-photon trajectories using spectral composition, revealing anomalous, non-classical paths that challenge traditional notions of particle trajectories.

## Contribution

It presents an operational technique to track single photons via spectral degrees of freedom, demonstrating anomalous trajectories with experimental validation.

## Key findings

- Spectral composition can serve as a quantum measurement device.
- Photons exhibit non-continuous, anomalous trajectories.
- Two-state vector formalism explains the observed trajectories.

## Abstract

A century after its conception, quantum mechanics still hold surprises that contradict many "common sense" notions. The contradiction is especially sharp in case one consider trajectories of truly quantum objects such as single photons. From a classical point of view, trajectories are well defined for particles, but not for waves. The wave-particle duality forces a breakdown of this dichotomy and quantum mechanics resolves this in a remarkable way: Trajectories can be well defined, but they are utterly different from classical trajectories. Here, we give an operational definition to the trajectory of a single photon by introducing a novel technique to mark its path using its spectral composition. The method demonstrates that the frequency degree of freedom can be used as a bona fide quantum measurement device (meter). The analysis of a number of setups, using our operational definition, leads to anomalous trajectories which are non-continuous and in some cases do not even connect the source of the photon to where it is detected. We carried out an experimental demonstration of these anomalous trajectories using a nested interferometer. We show that the Two-state vector formalism provides a simple explanation for the results.