Image: NASA

China’s Quantum Satellite Sent Entangled Photons to Earth for the First Time

We could be just five years away from a space-based quantum communication network.

Jun 15 2017, 6:00pm

Image: NASA

We are increasingly dependent on digital tools for almost every aspect of our daily lives, which means we also rely on two fundamental technologies that have made the digital revolution possible: satellites and encryption. Satellites are used for everything from GPS to television to processing credit card data, and encryption protocols are integral not only to communicating with these satellites, but also for matters as private as personal medical records, or as mundane as texting your friends.

The problem is that the advent of quantum computing threatens to render current methods of encryption obsolete, which will put your sexts—and every satellite communication network—at risk of exposure. Researchers and government intelligence agencies are scrambling to develop quantum-proof encryption techniques, but figuring out how to export these techniques to the uniquely hostile space environment has proven difficult.

Yet based on two papers independently released today by Chinese and European researchers, secure quantum satellite networks seem closer than ever. In China, a team used a new quantum satellite to smash the record for transmitting a pair of entangled particles. Meanwhile, a separate group in Germany demonstrated that existing communication satellites could be used to transmit quantum information.

Entanglement is kind of like having a single particle existing in multiple locations at once

Based on the new research, some of the scientists involved think that we could see the beginnings of a quantum satellite communication network in as soon as five years.

There's already one quantum-equipped satellite in orbit around Earth. Last August, China launched Micius, the world's first quantum satellite. It's intended to be proof of concept for China's global quantum satellite communications network, which it hopes to have up and running by 2030. But first, proponents had to prove that transmitting quantum states from orbit was practical.

Today, Chinese researchers report that not only is it possible, but that the quantum satellite managed to transmit entangled photons over a distance of nearly 750 miles, smashing the previous entanglement distribution record of about 64 miles on Earth set by a different team in 2012.

Entanglement is a quantum phenomenon allowing particles in different locations to share the same quantum state, which is kind of like having a single particle existing in multiple locations at once. Particle entanglement—usually light particles, or photons—is an ideal way to send secure information because any measurement of the transmission by an eavesdropper will alter the quantum state of the particles, letting the receiving party know that the transmission has been interfered with and rendering the message incoherent.

While the ability to instantly and securely relay information between two points by manipulating a photon's quantum state would have big implications for the future of communications, figuring out how to entangle particles over long distances is tricky.

Although both open air and optical fiber can be used to entangle particles, both of these mediums degrade entanglement and, over long enough distances, the effect will be lost entirely. In this sense, space is the ideal medium for transmitting the quantum states of particles, since this communication won't be subjected to degradation by the Earth's atmosphere, at least if the entanglement happens between two satellites based in space.

But having a bunch of satellites in orbit transmitting quantum states to one another in the vacuum of space doesn't really help us here on terra firma. The Chinese researchers had to figure out a way to get these quantum states from orbit to ground stations on Earth, which meant the signal would have to make a perilous journey through the atmosphere and risk destroying the entanglement. To circumvent this problem, the researchers turned to lasers—and as reported today in Science, it worked like a charm.

To make this happen, they used a beam splitter mounted on the satellite, which split the laser signal from the satellite into two distinct beams. These beams were then filtered through a crystal onboard the satellite, which produced a pair of entangled photons.

These photons then traveled through up to 1240 miles of space to two different ground stations in China that were nearly 750 miles distant from one another, while maintaining the entanglement between the particles.

Image: European Space Agency

This successful transmission of entangled photons from a satellite is a huge step toward a secure, space-based quantum communication network. Some hurdles still remain, such as actually creating a network of quantum satellites and using the entanglement to actually send information, rather than just single photons.

But as the researchers write in their paper, "the distributed entangled photons are readily useful for entanglement-based quantum key distribution," or QKD. This is essentially a method of manipulating the quantum states of individual photons to create a key. This key is then combined with any non-quantum encryption algorithm that is used to actually encode the data being sent, which can be transferred over traditional communication mediums, like optical fiber or lasers.

In short, quantum keys are basically encryption keys for encryption keys. But whereas the encryption protocols used today are vulnerable to being cracked by tomorrow's quantum computers, the inability to measure a quantum state without disturbing it would make a quantum key impossible to crack, even with all the computing power in the universe.

Designing, testing, and launching a whole new suite of quantum satellites will be an immensely costly affair. But according to new research from the Max Planck Institute in Germany, it should be possible to take advantage of satellite-based quantum key distribution using the existing satellite networks.

Read More: Scientists Set a New Distance Record for Quantum Teleportation

As detailed today in a paper in Optica, these researchers were able to successfully measure quantum states relayed to a ground station via laser from a satellite 23,600 miles above Earth. Unlike Micius, this satellite was just a run-of-the-mill communications satellite that was part of Europe's SpaceDataHighway and hadn't been optimized for sending quantum states via laser.

"We were quite surprised by how well the quantum states survived traveling through the atmospheric turbulence to a ground station," Christoph Marquardt, an optical physicist at the Max Planck Institute for the Science of Light, said in a statement. "From our measurements, we could deduce that the light traveling down to Earth is very well suited to be operated as a quantum key distribution network."

Over the course of 2015 and early 2016, Marquardt and his colleagues ran experiments from the Teide Observatory in Spain. Since the laser communications technology on the satellite was very similar to the laser technology developed by the Max Planck Institute to enable quantum key distribution, the researchers were able to use the satellite to create quantum states which were then transmitted to the ground and measured for accuracy.

Although transmitting a quantum state from space to Earth is not the same as transmitting entangled particle pairs, like the Chinese experiment, the researchers hope that this initial experiment can be built upon in order to turn existing communication satellites into the backbone of a future quantum communication network, which would allow for their rollout much more quickly than starting over from scratch.

"The paper demonstrates that technology on satellites can be used to achieve quantum-limited measurements, thus making a satellite quantum communication network possible," Marquardt said. "This greatly cuts down on the development time, meaning it could be possible to have such a system as soon as five years from now."

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