On angular momentum of photons: the role of transversality condition in quantum mechanics

TL;DR

This paper clarifies how the transversality condition affects the angular momentum of photons, revealing that proper representation allows separation of spin and orbital parts and identifying the role of Berry gauge in quantization.

Contribution

It introduces a two-component wavefunction representation that properly incorporates the transversality constraint, enabling clear separation of photon spin and orbital angular momentum.

Findings

Separation of photon spin and orbital angular momentum is possible with correct representation.

The Berry gauge plays a crucial role in the canonical quantization of photon states.

Observable quantum effects depend on the choice of Berry gauge.

Abstract

Whether the total angular momentum of the photon can be separated into spin and orbital parts has been a long-standing problem due to the constraint of transversality condition on its vector wavefunction. A careful analysis shows that the situation arises from a misuse of the constraint in quantum mechanics. To use the constraint properly, we convert the vector representation into a two-component representation in which the wavefunction is not constrained by any conditions. Upon doing so, we not only separate the spin conceptually from the orbital angular momentum but also identify the Berry gauge in which the two-component wavefunction can be canonically quantized. A corollary is that only in one particular Berry gauge can a radiation field be canonically quantized in terms of the plane waves. The degree of freedom to fix the Berry gauge, which reflects a symmetry of the constraint, turns out to have observable quantum effects.