A whopping 100,000 entangled photons have been detected for the first time, beating the previous record of just 12. The technique for spotting this delicate quantum link among so many photons could prove useful for safely sharing keys used in encrypted communications.

Entangled photons have linked quantum states, such that measuring the state of one photon determines the state of the others, no matter how far apart they are.

Detecting entanglement usually involves coincidence measurement – the simultaneous detection of multiple photons in the same quantum state at different locations. This method has so far worked for up to 12 photons. Although it is possible more photons have been entangled in previous experiments, the detectors are not sensitive enough to sort out entangled from non-entangled particles when a large number of photons are involved.

“When you go to high numbers you need a completely different technique,” says Maria Chekhova of the Max Planck Institute for the Science of Light in Erlangen, Germany.


Novel measurement

Another way to look for entanglement is to detect whether photons share polarisation under certain conditions. Chekhova’s team exploited this by setting up an experiment that allowed them to make a novel type of measurement.

The researchers shot a laser through a device called a polarising beam splitter, which creates two beams of photons with different polarisations. The beams were sent through a pair of barium crystals to generate photons with the same polarisation but different wavelengths. These beams were then recombined to create pulses of light, each containing up to 100,000 photons.

The team split each pulse one more time into beams with two different polarisations, then used detectors to count the number of photons in each beam and calculate the difference. The values the team measured were consistent with entanglement.

Christoph Simon at the University of Calgary in Canada points out that the jump from 12 entangled photons to 100,000 is not as dramatic as it sounds. The 12 photons shared a superposition of two states, while Chekhova’s photons shared about a million states, making the nature of the entanglement quite different.

“One has to be a little careful when comparing this experiment to the previous one,” he says. However, working with a large number of states at the same time could prove useful for quantum key distribution, which uses entanglement to transmit the cryptographic key for quantum communication.

Journal reference: Physical Review Letters, doi.org/jm5