Scientists Split a Photon and Discovered Quantum Perfection Still Reigns

In a quiet Finnish lab, a single photon blinked into existence and met its quantum fate—not with a bang, but with a beautiful symmetry. What happened next not only confirmed one of the most sacred principles in physics, but also cracked open the door to a dazzling new future in quantum technology.

For the first time ever, scientists have experimentally confirmed that angular momentum is conserved when a single particle of light—a photon—is split into two. This subtle, almost imperceptible moment proves that one of nature’s deepest rules holds even in the most delicate, quantum-scaled interactions.

At the heart of this breakthrough, reported in Physical Review Letters, is a global team led by Tampere University in Finland, working alongside collaborators from Germany and India. Their discovery is more than just a textbook victory—it’s a glimpse into a new era of photonic quantum states that could redefine how we compute, communicate, and measure the universe.

The Sacred Laws That Shape Reality

To appreciate what the team has achieved, you need to understand the rule they put to the test: the conservation of angular momentum. This isn’t just physics jargon—it’s one of the bedrock laws of the universe, governing how objects spin and transfer motion.

In our everyday world, you can see angular momentum in action when a spinning figure skater pulls in their arms to spin faster, or when a rotating planet keeps whirling through space. Angular momentum isn’t just conserved—it’s stubborn. It doesn’t vanish. It doesn’t appear from nowhere. It simply transforms.

Even light—massless, weightless, pure energy—can carry angular momentum. This is called orbital angular momentum (OAM), and it’s tied to the structure of a light beam, almost like a tiny corkscrew twisting through space. A single photon can carry a specific quantum of this twist, which we describe as +1, -1, or zero.

But could that twisting nature of light persist even when we’re working at the smallest imaginable scale—when a single photon is split into two? Would the symmetry survive, or break apart?

That’s the question Tampere University’s team dared to ask. And they weren’t just asking for curiosity—they were pushing at the edge of what we know about quantum behavior, information, and entanglement.

One Minus One Equals… Quantum Harmony

Here’s the paradox at the core of their experiment. Imagine you have a photon with zero orbital angular momentum—meaning it carries no twist. When that photon undergoes spontaneous parametric down-conversion—a fancy way of saying it’s split into two partner photons by passing through a special crystal—what should happen?

If the laws of physics hold true, then the two new photons must cancel each other out. If one gains a twist of +1, the other must take on -1. The total must remain zero. That’s the quantum math: 1 + (-1) = 0.

This sounds obvious. But until now, no one had been able to test this with a single photon at the input. In the past, experiments involved bright laser beams containing countless photons. While these large-scale tests confirmed OAM conservation in the classical realm, they left a big open question:

Does this law still apply when the universe deals its cards one photon at a time?

“We wanted to see what happens at the most fundamental quantum level,” said Dr. Lea Kopf, lead author of the study. “Our experiments show that OAM is indeed conserved even when a single photon drives the process. It confirms that the symmetry we expect from nature really does hold, even in its quietest whisper.”

Finding a Photon in a Quantum Haystack

But turning theory into reality was no easy feat. Detecting this subtle process—where only one out of every billion photons successfully splits into a pair—was like trying to find a single golden thread in a storm of noise.

It required everything: a rock-steady optical setup, ultra-sensitive detectors, and a refusal to give up. The researchers built an apparatus capable of extremely precise control of the light’s shape, and employed nonlinear crystals that enable the rare transformation of one photon into two.

“The measurement is incredibly delicate,” explained Professor Robert Fickler, head of the Experimental Quantum Optics group at Tampere University. “It’s like searching for a needle, but in a haystack made of needles—only one of which is special.”

To catch these rare photonic events, the team had to filter out all background noise, ensure total experimental stability, and collect data over long periods of time. But their persistence paid off. Not only did they confirm that OAM is conserved, but they also observed tantalizing hints of something even deeper.

Entangled by Design: Twisting into the Quantum Future

As they monitored the photon pairs, the researchers began to see early signs of quantum entanglement—that famous, mind-bending phenomenon where two particles become linked across space, such that measuring one instantly affects the other.

This suggests that the photons born from a single parent might not just balance each other—they might share a deeper, more mysterious connection, entangled not only in angular momentum, but potentially in space, time, and polarization.

“This work is not only of fundamental importance,” said Prof. Fickler, “but it also takes us a significant step closer to generating novel quantum states where photons are entangled in all possible ways.”

That’s more than just a scientific curiosity. If these complex states can be harnessed, they could serve as the building blocks for next-generation quantum computers, ultra-secure quantum communication networks, and highly sensitive quantum sensors capable of detecting gravitational waves, minute biological changes, or even dark matter.

Beyond the Limits: What Comes Next

Now that the door is open, the researchers plan to make their process more efficient, hoping to generate these precious quantum states with greater reliability. They’ll refine their detection methods, sharpen their control of the input photons, and explore new ways to verify entanglement across multiple properties.

The ultimate goal? To not only find more of these “quantum needles,” but to learn how to weave them together—creating entangled webs of photons that could power the quantum internet of the future.

At the same time, this breakthrough reminds us of something older, something deeper. It shows that even at the most granular level, the universe plays by its own rules—elegant, unwavering, and strangely poetic.

The conservation of angular momentum isn’t just a mathematical constraint. It’s a whisper from the universe, telling us that symmetry still reigns, even in the chaos of the quantum world.

And all it took to prove it… was one photon.

Reference: L. Kopf et al, Conservation of Angular Momentum on a Single-Photon Level, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.203601