Earlier today, China's pioneering quantum satellite Micius facilitated the first-ever intercontinental video conference using a quantum communications network. The video call connected Chunli Bai, the president of the Chinese Academy of Sciences in Beijing, with Anton Zeilinger, president of the Austrian Academy of Sciences in Vienna, a distance of over 4,600 miles. It was the first real-world demonstration that showed that a global quantum internet is not only possible, but within reach.
A little over a year ago, Bai and his colleagues launched Micius, the first quantum satellite that was meant to serve as a testbed for technologies that would pave the way for a global, space-based quantum communications network. Unlike the normal internet, this quantum internet would be perfectly secure, an issue of paramount importance as we stand on the threshold of the age of quantum computing.
Today, satellites are the backbone of many internet technologies that we use on a day-to-day basis, such as the GPS systems that power Google Maps, satellite televisions, ATMs, and many of the consumer products that make up the Internet of Things. These satellites help route data between internetworked objects all around the globe and when the data is sensitive, such as with banking applications, it is generally encrypted as it is passed between ground stations and satellites. The encryption algorithms used to protect this data are generally based on difficult math problems, such as factoring astronomically large prime numbers.
"This is a very important step towards a world-wide and secure quantum internet."
While contemporary encryption standards are robust enough that an attacker would be unable to crack them even if they had access to all the computing power on Earth, these same encryption standards will be rendered obsolete with the advent of large-scale quantum computers. Unlike a traditional computer, which traffics in binary bits, where data is stored as either a 1 or 0, quantum computers make use of qubits, which is data that is either a 1, 0, or a combination of these states at the same time. In practice, this means that a quantum computer will be able to crack today's encryption without much difficulty.
This impending crypto-apocalypse has resulted in a race to create quantum-resistant cryptography, and one of the leading candidates in this area is known as quantum key distribution (QKD). This is a method of manipulating the quantum states of individual photons to encode an encryption key. This key is then used to secure another key, which would work with a non-quantum encryption algorithm that is used to actually encode the data being sent. In other words, QKD is using particles of light to create a quantum encryption key that secures a traditional encryption key.
One way of implementing QKD is by entangling photons. Entanglement is a way of linking two different particles, in this case photons, at a distance, so that each share the same quantum state. Entanglement is kind of like having one particle exist in multiple locations at once. As far as the quantum internet is concerned, entanglement could be used to transfer quantum keys between two distant locations.
For example, in June of this year, Chinese researchers demonstrated that they were able to transmit entangled photons from the Micius satellite to two ground stations in China that are 750 miles apart while maintaining entanglement between the particles. The entangled particles were generated on board the satellite and then delivered to two different ground systems using a split laser beam. Each beam sent one of the entangled photons to a ground station, where the quantum state of the photon could be measured, thus distributing the same quantum key to two remote locations.
The quantum states of entangled photons are also a more secure cryptographic basis than difficult math problems because as soon as an attacker tries to measure the state of the photon, it alters the photon's state and renders decryption impossible. Today, these researchers took these same principles a step further by using the Micius satellite to facilitate the first intercontinental communication secured using QKD. According to a press release from the Austrian Academy of Sciences, using QKD to secure the video call made it "at least a million times safer" than securing it using conventional methods of encryption.
In the case of today's video conference, QKD was used to encrypt the video signal being routed between ground stations near Vienna and Beijing. Prior to the video call, the quantum state of photons generated on board Micius were generated and sent to a ground station near Vienna where these states were measured. These same quantum states were then translated into binary code (1s and 0s) on the satellite and sent to the ground station near Vienna. There, researchers compared their measurement of the quantum state of the photon with the binary translation of this quantum state. If these values didn't match exactly, it would alert the researchers that an attacker was trying to eavesdrop on the transmission.
This same process was then repeated between the Micius satellite and the ground station near Beijing. At this point, the researchers in Austria and China both had unique quantum keys that were stored on board the satellite. These keys were then combined to generate a new quantum key that was transmitted to both China and Austria. Each station was then able to use its unique quantum key in combination with the shared quantum key to securely encrypt the video call that was routed between them, effectively establishing the first intercontinental communication secured using quantum encryption.
"The exchange of quantum encrypted information over inter-continental distances confirms the potential of quantum communication technologies as opened up by fundamental research," Zeilinger said in a statement after the video call. "This is a very important step towards a world-wide and secure quantum internet."
Prior to the launch of Micius, the world-record for QKD using entanglement was 64 miles. Although both open-air and fiber-optic cable can be used to transmit photons, both of these mediums degrade the entanglement and over a long enough distance the effect is lost entirely. Space, however, provides a nearly lossless medium for the transmission of entangled particles, making it an optimal way to route quantum information between two distance points and lasers help the quantum state of photons survive their turbulent journey through Earth's atmosphere.
These ground stations were connected to the research institutions via terrestrial quantum communication networks built with optical fiber. Such networks have been used by government institutions for a few years, but these networks are limited in the distance that they can sent quantum information (around 60 miles).
In the future, the combination of local quantum communication networks on the ground and quantum satellites in orbit will enable the creation of a truly global quantum internet. For now, however, Micius remains an orbital laboratory testing the fundamental technologies that will make the quantum internet possible, but the video conference makes the quantum internet feel closer than ever.