Australasian Science Magazine

The quantum satellite Micius distributes entangled photon pairs to two ground stations in China. (Courtesy Prof. Jian-Wei Pan, University of Science and Technology of China, Hefei).

By Paul J Edwards

Initial results from the world’s first quantum communications satellite Micius (Australasian Science, May/June 2017) launched last year have been published in Science, and show unequivocally that the strange phenomenon of “quantum entanglement” between pairs of light photons survives to distances of at least 1200 km. This greatly extends the previous record of 100 km and is another nail in the coffin of Einstein’s classical concept of physical reality. The results also strengthen the prospect of a secure global quantum internet that is immune to hacking.

A secure quantum link established between two parties, traditionally called “Alice” and “Bob” in the cryptographic literature, allows them to share a cryptographic key without fear of eavesdropping by a third party (“Eve”). This private key can then be used as a “one-time pad” to securely encrypt subsequent communications over an ordinary public network.

The new results show that a measurement of the state of one entangled photon by either Alice or Bob can instantaneously determine the outcome of a measurement by the other party, even though they may be separated by thousands of kilometres. Alice and Bob can create a shared secret key by making a series of measurements on pairs of entangled photons. Interception of the key by an Eve will destroy the correlation between the two sets of measurements and be easily detected.

Micius carries a bright source on board that generates six million entangled photon pairs per second. It delivers them to Alice and Bob on the ground at a rate of one pair per second via a highly sophisticated laser acquisition, tracking and pointing system, a technical and scientific tour de force.

Results have been reported from two Chinese high altitude ground stations separated by 1200 km using large (greater than 1 metre diameter) optical telescope receivers: one at Delingha on the Tibetan plateau, the other at Lijiang in Yunnan province (see accompanying picture showing the two satellite-to-ground links to these stations in simultaneous operation).

The enormous (64–82 dB) attenuation of the entangled photon-pair beams over distances of a thousand or more km from satellite to ground, together with atmospheric turbulence at low angles of elevation, impose major challenges. Because Micius is accessible to both stations for less than 5 minutes each night, potential quantum key data transfer is limited to a few hundred bits per day at most. Clearly Micius represents only the first step in the practical implementation of a global satellite-based quantum key courier system.