Optical interference by amplitude measurement

TL;DR

This paper demonstrates that direct measurement and addition of optical field amplitudes via homodyne detection can reveal interference even with distinguishable paths, challenging traditional complementarity principles.

Contribution

It introduces a novel amplitude measurement technique for observing interference, applicable to both classical and quantum fields with distinguishable paths, expanding understanding of optical coherence.

Findings

Interference can be observed through amplitude measurement despite path distinguishability.

The technique works for both classical and quantum fields.

It enables interference recovery in unbalanced interferometers beyond coherence length.

Abstract

Interference effects are usually observed by intensity measurement. Path indistinguishability by quantum complementarity principle requires projection of the interfering fields into a common indistinguishable mode before detection. On the other hand, the essence of wave interference is the addition of amplitudes of the interfering fields. Therefore, if amplitudes can be directly measured and added, interference can occur even though the interfering fields are in well-distinguishable modes. Here, we make a comprehensive study in both theory and experiment of a technique by homodyne measurement of field amplitudes to reveal interference. This works for both classical and quantum fields even though there exists distinguishability in the interfering paths of light. This directly challenges complementarity principle. We present a resolution of this issue from the viewpoint of measurement that emphasizes either particle or wave. This technique is particularly useful for recovering interference in unbalanced interferometers with path-imbalance beyond coherence length of the input field and can be applied to remote sensing to extend applicable range. Since the amplitude-based interference phenomena studied here are fundamentally different from the traditional intenisty-based interference phenomena, our approach leads to a new paradigm to study coherence between optical fields.