Scientists Just Split a Single Photon. Here’s What They Found

Physicists have, for the first time, shown that even a single photon obeys one of nature’s strictest rules: conservation of angular momentum.

Achieved only once in a billion attempts, this needle-in-a-haystack success not only proves a cornerstone law of physics at the smallest scale but also opens a pathway to advanced quantum technologies, from entangled states to secure communication.

Quantum-Level Confirmation of Angular Momentum Conservation

Researchers at Tampere University, working with colleagues in Germany and India, have demonstrated for the first time that angular momentum remains conserved when a single photon splits into two. This result confirms a core principle of physics at the quantum scale and marks a milestone that could help in creating advanced quantum states for use in computing, communication, and sensing technologies.

Conservation laws are central to science because they determine which processes are possible and which are not. A familiar example is seen in billiards, where the momentum of one ball transfers to another during a collision. A similar principle applies to objects that spin, which carry angular momentum. Light, too, can possess angular momentum, specifically orbital angular momentum (OAM), which relates to the spatial shape of a light beam.

At the quantum level, this means that individual photons carry specific amounts of OAM that must be preserved when they interact with matter. In a study recently published in Physical Review Letters, the Tampere-led team investigated whether this conservation rule still applies when a lone photon is divided into a pair. Their work pushed the limits of conservation testing to the smallest possible scale.

One Minus One Equals Zero

According to the rule, if a photon without OAM splits into two, the angular momentum values of the resulting photons must cancel each other out. For instance, if one photon emerges carrying one unit of OAM, the second must carry negative one. Put simply, the equation 1 + (-1) = 0 must always hold. While similar rules have been tested many times in laser-based optics experiments, this had never before been confirmed for a single photon.

“Our experiments show that the OAM is indeed conserved even when a single photon drives the process. This confirms a key conservation law at the most fundamental level, which is ultimately based on the symmetry of the process,” explains Dr. Lea Kopf, who is the lead author of the study.

Finding the Photonic Needle in the Laboratory Haystack

The team’s experiments rely on delicate measurements as the required nonlinear optical processes are very inefficient. Only every billionth photon is converted to a photon pair, such that measuring the conservation of OAM for single photons resembles the proverbial search for the needle in the haystack.

An extremely stable optical setup, low background noise, a detections scheme with the highest possible efficiency, and a lot of experimental endurance enabled the researchers to record enough successful conversions such that they could confirm the fundamental conservation law.

First Signs of Quantum Entanglement

In addition to confirming OAM conservation, the team observed first indications of quantum entanglement in the generated photon pairs, which suggests that the technique can be extended to create more complex photonic quantum states.

“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,” adds Prof. Robert Fickler, who leads the Experimental Quantum Optics group where the experiment was performed.

Future Directions in Quantum Photonics

Looking forward, the researchers plan to improve the overall efficiency of their scheme and develop better strategies for measuring the generated quantum state such that in the future these photonic needles can be found easier in the laboratory haystack. Moreover, the researchers aim at leveraging the generated multi-photon quantum states for novel fundamental quantum tests and quantum photonics applications such as quantum communication and network schemes.

Reference: “Conservation of Angular Momentum on a Single-Photon Level” by L. Kopf, R. Barros, S. Prabhakar, E. Giese and R. Fickler, 20 May 2025, Physical Review Letters.
DOI: 10.1103/PhysRevLett.134.203601

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