Conservation of Angular Momentum on Splitting a Single Photon

This article THAT has been published in Physical Review Letters IS contributed by L. Kepf R Barre, S. Prabhakar, E. Giese and R. Fickler (Doi: 10.1103 /Physical Revlett. 134. 203601). The work is by the researchers at Tampere university, collaborating with colleagues in India and Germany. They have demonstrated for the first time that the angular momentum remains conserved when a photon is split into two. It confirms a basic physics principle at quantum level and could apply to computing, communication and sensing technologies.

At quantum level, the amount of orbital angular momentum (OAM) must be preserved while interacting with matter. In billiards, a momentum of one ball transforms to another when they collide. Spinning objects also prove this principle. Light too can possess angular momentum. It leads to spatial shape of a light beam. The work examines how far limits of conservation can be pushed —to the minutest small scale.

A photon with no OAM splits and the resulting two values OAM cancel each other out — a photon carries one unit of OAM and the second carries negative one. The equation 1+(-1) = 0 always holds. Even when a single photon is involved, the principle holds.

Measurments here are delicate. The processes are inefficient. Every billionth photon converts into a photon pair. It is a search for the needle in a haystack.

A stable optical setup, low background noise, highly sensitive detection efficiency and experimental endurance is conducive for enough successful conversions proving the basic conservation law.

OAM conservation gets substantiated. At the same time, there were first indications of quantum entanglement in the generated photon pairs. It means this technique can be extended to create more complex photonic quantum states — novel quantum states where photons are entangled in all possible ways.

They are trying to make the research more advanced in the future, especially detection of the needle in lab haystack. Even multi-photon states could be leveraged for new quantum states and applications.