China’s Quantum Satellite Transmits Entangled Photons to Earth

An ambitious quantum entanglement experiment launched aboard China’s Quantum Science Experiment Satellite last year has proven to be a success, having successfully transmitted pairs of entangled photons to two facilities on the ground 750 miles apart. The success of this experiment means that the quantum-based encryption of long-distance transmitted information will be possible using satellites of this nature.

The Quantum Science Experiment Satellite, nicknamed "Micius" after the 5th-century Chinese scholar, was launched last August, in an effort to develop quantum encryption and quantum teleportation techniques. This current experiment involved long-range quantum key distribution (QKD), where a pair of entangled photons were sent to two different sites on the Earth’s surface, 1,200 kilometers (750 miles) apart, one in Delingha, and the other in Lijiang.

While the entangled photon pairs themselves only move at the speed of light, the fact that they’re entangled can be used as an information encryption key for the data being conveyed by the light beam: if the transmission is uninterrupted, sensing devices at both sites will read the incoming entangled photons as being identical. But if someone were to intercept the transmission, the photons would fall out of their entangled state, alerting the devices at the receiving stations that someone was eavesdropping on the transmission.

While entangled photon pairs — produced by splitting photons using a specially-prepared prism — can be transmitted reliably via optical cable, the material in the cable eventually absorbs the light, limiting the distance that the particles can be sent. Open-air transmission has proven to be able to beam particles further, but they require a clear line-of-sight between the transmitter and receiver, once again meaning that the range is limited. However, photons transmitted from (and possibly in the future, to) a high-flying satellite can be sent much, much further, and for the most part the transmission isn’t fettered by atmospheric interference. This new satellite-based transmission breaks the previous record distance for transmitting entangled particles of 140 kilometers (86 miles) by over eight and a half times.

"It’s a really stunning achievement, and I think it’s going to be the first of possibly many such interesting and exciting studies that this particular satellite will open up," explains Shohini Ghose, a physicist with Canada’s Wilfrid Laurier University. "Who knows, maybe there’ll be a space entanglement race?"