In 2016 Jian-Wei Pan of the University of Science and Technology of China and colleagues launched Micius into low-Earth orbit to investigate the potential of satellite-initiated quantum communication. Now, just months after setting a distance record for the distribution of entangled photons (see Physics Today, August 2017, page 14), Pan’s team has demonstrated a more important and practical capability: the establishment of a secure quantum communication channel between distant parties. By employing the satellite as a photon emitter and relay, team members in Graz, Austria, and Xinglong, China, developed and shared a 100-kilobyte key that they used to securely exchange photos and hold a video conference.
Micius initiated the link between Graz and Xinglong through a combination of quantum and classical signals, following a version of the BB84 protocol devised by Charles Bennett and Gilles Brassard (see the article by Daniel Gottesman and Hoi-Kwong Lo, Physics Today, November 2000, page 22). As the satellite passed over a station, it emitted photons that were each prepared in a random polarization state; the station (the Graz one is shown in the composite photo above) performed one of two polarization measurements on each received photon. Using the measurement types and results for the exchanged photons, the satellite and station established a unique key. Once Micius developed a key with both stations, it performed a logic operation (specifically, an exclusive OR) on the two strings of bits and sent the results via a classical radio channel to one of the stations. The station with that extra information compared those bits with its own key to determine its counterpart’s key.
To test the secure connection, the Austrian and Chinese researchers exchanged 5-kilobyte JPEG images that were indecipherable without the shared quantum key. They then used most of the remaining bits of the key to ensure the security of a 75-minute intercontinental video conference.
Micius is just one step toward achieving a secure global quantum network, which would require multiple satellites and ground stations working in parallel. Ideally, those satellites would orbit at higher altitudes to cover more ground area and would transmit at longer, telecom wavelengths to enable daytime operation. (S.-K. Liao et al., Phys. Rev. Lett. 120, 030501, 2018.)