Scientists split a single photon and observed an important law of physics in action

A single particle of light can behave in surprising ways, but it still follows the strict rules of physics. A new experiment shows that even when one photon is split into two, the total angular momentum remains exactly the same.

The work tested conservation at the smallest possible scale and did not cut corners.

Lead author Lea Kopf of Tampere University, and colleagues built a setup sensitive enough to catch only a few successful events out of billions.

Split photons and angular momentum

Angular momentum is the quantity that measures how much an object or field is rotating. In light, one form is orbital angular momentum (OAM), which describes a structured wavefront and comes in whole-number steps.

Conservation laws tie directly to symmetries in nature, an idea formalized by Noether’s theorem. When a system has rotational symmetry, total angular momentum stays constant.

In the lab, researchers use spontaneous parametric down conversion (SPDC) to convert one higher energy photon into a pair of lower energy photons.

The team sent single photons into a second crystal to see whether each quantum of OAM is conserved, not just the average over many photons.

The study reported its main result in May 2025, confirming that OAM adds up correctly for each photon pair.

The peer reviewed paper describes how a pump photon with zero OAM always produced two photons whose OAM values cancel to zero.

How to split photons

The researchers built two down conversion stages in sequence, a design called Cascaded SPDC.

The first stage generated single photons that acted as the pump for the second stage, which produced pairs of photons whose OAM could be measured.

This approach let them move from averages to one photon at a time. It was hard, because the nonlinear process is very inefficient and backgrounds can wash out real signals.

Only about one in a billion pump photons converted into a pair, so patience and stability were everything.

The team reported that they overcame this by running extremely stable optics, using low noise detectors, and collecting data long enough to build reliable statistics.

To check conservation, the group varied the OAM carried by the input photon. They verified the rule for inputs of zero, minus one, and plus two, always finding outputs whose OAM numbers add up to the input.

For these checks, the team shaped and analyzed spatial modes with programmable optics and single mode fibers.

They then built correlation matrices that show which input and output OAM values appear together and compared them with runs driven by a weak classical pump, finding the same pattern.

What the data showed

Most detections occurred for the simple case that maximizes efficiency, where the input carries no OAM and the outputs cancel. Still, the team recorded the tiny event rates needed to see conservation at the single photon level.

The authors also looked for early signs of entanglement, the uniquely quantum connection between particles that share a joint state.

Their measurements pointed in that direction, and the article notes that a more definitive test will require steadier conditions and higher rates.

They quantified similarities by computing correlation coefficients between single photon and classical pump runs. The close match indicated that the pump statistics do not alter the selection rule tested here.

“Our experiments show that the OAM is indeed conserved even when the process is driven by a single photon. This confirms a key conservation law at the most fundamental level, which is ultimately based on the symmetry of the process,” said Dr. Kopf.

Split photons fit with old physics

Experiments with strong lasers had already shown that OAM is conserved on average in SPDC. Two decades ago, a landmark experiment demonstrated OAM entanglement using a laser pump, a key step for high dimensional quantum information.

The new work goes further by using single photons as the pump. That closed the remaining gap by showing that each quantum of OAM is accounted for, not just the mean of a large ensemble.

The selection rule itself is simple to state. The OAM numbers of the output photons must add up to the OAM of the input photon, which the team verified for zero, minus one, and plus two.

Real world implications

Photons that carry OAM can encode information in many levels, not only zeros and ones. That expands the alphabet available to quantum communication and may raise error tolerances and data rates.

These ideas are already well studied across quantum optics and communications. A comprehensive review explains how high dimensional OAM states can boost capacity and strengthen security in realistic channels.

“This work is not only of fundamental importance, but it also takes us a significant step closer to generating novel quantum states, where the photons are entangled in all possible ways, i.e., in space, time, and polarization,” added Prof. Robert Fickler, who leads the Experimental Quantum Optics group at Tampere University.

The immediate goal is efficiency. Better crystals, faster and more efficient detectors, and improved OAM measurement tools could lift the event rate so that richer quantum states can be verified and used.

If that happens, cascaded SPDC with OAM control could become a compact source for complex photonic states. That would support more ambitious tests of quantum foundations and more capable quantum networks.

The study is published in Physical Review Letters.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–