Achieving higher-orbit quantum communications remains an objective for all institutional and private players with enough expertise and funding to consider it. And while quantum computing and the capability to communicate in unbreakable, unsnoopable channels is of interest to most entities, only China has manifested a low-orbit satellite — Micius — that enables two-way research and quantum information traffic between space and the surface. This was back in 2016 — the US doesn’t have a publicly-known, operational Quantum Key Distribution satellite system, and Europe’s is only expected to launch next year.
Not one to rest on its laurels, China is nonetheless aiming to take QKD (Quantum Key Distribution) communication to new heights, and is plotting out the ways to break its current, 310-mile (~500 km) geostationary orbit limit towards an impressive 6,200 mile (10,000 km) radius.
"Low-orbit quantum key satellite networking and medium- and high-orbit quantum science experiment platforms are the main development directions in the future," said Wang Jianyu, dean of the Hangzhou Advanced Research Institute of the Chinese Academy of Sciences (CAS). While timelines weren’t given for medium or high-orbit QKD, work is underway in understanding what problems need to be solved to get there.
Of course, satellites sitting at higher orbits could cover larger portions of the surface and additional ground stations, enabling a wider and more resilient quantum network coverage. But distance isn’t exactly helpful in increasing the survival of information-carrying qubits, and high-orbit satellites will require improved on-board micro-vibration suppression technology so spacecraft can send precise optical or laser signals. Fortunately, photons within the 1550nm band (used in our day-to-day fiber optics communications) can be leveraged for this, facilitating a number of implementation and adaptation steps.
Current satellite-based quantum communications leverages the entanglement susceptibility of photons — individual light particles that can be quantized — towards using them as information carriers. Much like the binary system of information, a single photon can be polarized in one way or another — in being able to discern more than one state, they can be encoded into information.
Due to this ability to encode useful information within photons, QKD leverages the property of entanglement to make it so that two separate photons become a qubit pair — a single system, where to describe one of them requires describing the other. Because they’re light-based, photonic qubits showcase a higher resilience to outside interference, placing them as the prime candidates towards ferrying sensitive information across long distances — and specifically between the Earth, its atmosphere, and space.
At this stage, the information (the entangled photon) reaching its destination or not becomes dependent on the absence of interference that could lead to a collapse of its entangled state. This collapse would also lead to the loss of all in-transit information.
What light-speed quantum key distribution and quantum-key-encrypted communications will lead to is to a future where certain communications streams will become unhackable but, up to a point, blockadeable (up to a point) by savvy-enough opponents. This has implications in the design of quantum communications systems for higher reliability and redundancy, as interrupted communications can have just as dire consequences as it being unencrypted.
Micius was recently used to successfully distribute quantum keys between the cities of Delingha and Nashan (756 miles apart) and, in 2018, between the Austrian city of Braz and the Chinese city of Xinglong — an intercontinental quantum key distribution separated by some 4,700 miles (7,600 kilometres). Meanwhile, Europe’s own QKD system as orchestrated by the European Space Agency (ESA) expects to see the first European QKD satellite — Eagle-1 — in space from 2024.
It’s clear that China is looking to capitalize on the years of experience it has low-orbit QKD system, and plans to increase its resiliency and redundancy. Considering the limited throughput of current QKD systems, however, it’ll likely be decades before these applications become pervasive — and even more before they’re used for communications in non-critical systems.
