# Single-photon test proves angular momentum conservation — a quantum first
*A needle-in-a-haystack experiment shows a lone photon still obeys nature’s twist-keeping rule.*
Researchers at Tampere University, working with partners in Germany and India, have shown that angular momentum is conserved when a single photon splits into a pair. Reported on 17 August 2025, the result pushes a core law of physics to the quantum limit and hints at new technologies in computing, communication and sensing. The work appears in Physical Review Letters.
## What they actually proved
The team confirmed that orbital angular momentum (OAM) is conserved even when the process starts with just one photon. If the original photon had zero OAM, the two new photons’ OAM must add to zero — 1 + (–1) = 0.
## How they caught an ultra-rare event
Converting one photon into a pair is extremely inefficient — roughly one in a billion attempts works. An ultra-stable optical setup, very low background noise and high-efficiency detection let the team record enough events to test the rule.
## Why it matters for quantum tech
Conservation laws decide which processes nature allows. Proving OAM conservation at the smallest scale supports designs for more complex photonic states that could power future quantum computing, secure communication and precision sensing.
## Who did it — and what they say
The study was led by Tampere University’s Experimental Quantum Optics group with collaborators in Germany and India. “Our experiments show that the OAM is indeed conserved even when the process is driven by a single photon,” said lead author Dr Lea Kopf. Group lead Prof Robert Fickler called the result “of fundamental importance”.
## What’s next
The team aims to improve the conversion efficiency and develop better strategies to measure the generated quantum states. Early signs of entanglement suggest the method could scale to multi-photon states for future quantum networks.
### Takeaway
A bedrock rule of physics holds firm at the smallest scale we can test it — and that reliability is exactly what quantum technologies need.
Read more details in the original report on ScienceDaily: [Scientists just proved a fundamental quantum rule for the first time](https://www.sciencedaily.com/releases/2025/08/250816113515.htm).