Scientists Confirm Quantum Conservation Law at Single-Photon Level

In a painstaking experiment that required 168 hours of data collection to capture just 57 photon events, researchers at Tampere University have confirmed that angular momentum conservation—a fundamental law of physics—holds true even when individual particles of light split into pairs.

The achievement validates quantum theory’s most basic principles while opening pathways to advanced quantum technologies.

The team used an extraordinarily delicate process where single photons carrying orbital angular momentum were converted into photon pairs, then meticulously tracked whether the conservation law held. Like finding needles in haystacks, only one in a billion input photons successfully converted, making each detection precious.

Testing Physics at the Quantum Limit

Conservation laws govern which processes nature allows or forbids. When billiard balls collide, their motion transfers according to momentum conservation. Light can also carry angular momentum through its spatial structure—think of a corkscrew-shaped beam twisting through space.

Previous experiments had verified angular momentum conservation using powerful laser beams containing trillions of photons. But quantum mechanics demands that conservation laws work for individual particles, not just large collections. No one had tested this fundamental principle at the single-photon level until now.

The researchers created a cascaded system where photons from one conversion process pumped a second conversion. They imprinted specific amounts of orbital angular momentum onto single photons, then watched them split into pairs through spontaneous parametric down-conversion in nonlinear crystals.

Needle-in-Haystack Detection

The experimental challenges were immense. Key technical hurdles included:

• Only one billionth of input photons successfully converted to pairs
• Detection rates as low as 1.3 heralded photon pairs per hour
• Measurement sessions lasting up to 168 hours for statistical significance
• Suppression of background noise to near-zero levels
• Extremely stable optical alignment over days of operation

The team used superconducting nanowire detectors with 80% efficiency and spatial light modulators to shape and analyze the photons’ twisted wavefronts. Each detection required precise timing correlations to distinguish real events from random background noise.

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

Quantum Math in Action

The conservation rule follows simple arithmetic: when a photon with zero angular momentum splits, the two resulting photons must have equal and opposite values. So if one photon gains +1 unit, its partner must have -1 unit, ensuring the total remains zero.

The researchers tested this with photons carrying up to two units of orbital angular momentum. In every case, the pairs obeyed conservation laws with 76% of detections showing perfect compliance—the remainder attributed to experimental imperfections rather than physics violations.

Remarkably, when they compared single-photon results with traditional laser-pumped experiments, the correlation reached 99.5%. This near-perfect match confirms that quantum and classical physics operate by the same conservation principles, just at different scales.

Entanglement Signatures

Beyond conservation verification, the team observed preliminary evidence of quantum entanglement in the photon pairs. When particles become entangled, measuring one instantly affects its partner regardless of distance—Einstein’s “spooky action at a distance.”

The orbital angular momentum entanglement could enable quantum computers to process information in higher dimensions than current polarization-based systems. Instead of simple 0-1 bits, these “qudits” could represent multiple values simultaneously, dramatically expanding computational power.

Professor Robert Fickler, who leads the research group, emphasized broader implications: “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.”

The researchers now plan to improve conversion efficiencies and develop better detection schemes to make these quantum needles easier to find. Their ultimate goal involves creating three-photon entangled states that could enable more sophisticated quantum communication networks and computing architectures.