Satellite Calls Earth on the Quantum Line
The Micius satellite served as a quantum "switchboard" for communicating entangled photos from space to Earth. | Jian-Wei Pan
In a record-breaking study, Chinese scientists report the successful transmission of entangled photons between suborbital space and Earth. The results , which could improve the speed and security of global telecommunications, are published in the June 16 issue of Science.
Entanglement is the curious phenomenon where particles like photons or electrons are "linked" in such a way that the quantum state of each particle cannot be described separately. When particles are entangled, we can determine the state of one entangled partner if we know the state of the other particle. In effect, this means that the particles can instantly communicate with each other, no matter how far apart they are separated. This phenomenon could be harnessed for instantaneous communication anywhere around the world or possibly beyond. Even Einstein struggled to understand entanglement, calling it "spooky action at a distance."
In the Science study, Jian-Wei Pan and colleagues at the University of Science and Technology of China report the successful transmission of entangled photons by satellite across a distance of roughly 1,200 kilometers, smashing the previous quantum entanglement record of 100 kilometers.
Along with other potential applications in computing and coding, quantum entanglement also could offer a more secure means of communication. For example, if anyone tries to "eavesdrop" on a quantum communication line, this will disrupt the state of the particles, alerting users that the channel has been compromised.
To date, however, efforts to transmit entangled particles have been limited to distances of roughly 100 kilometers, as the quantum state of a particle is highly susceptible to being altered by environmental influences.
One way to transmit entangled particles is through a protective optical fiber; however, the "message" must be passed from one particle to another down the fiber in segments, like children whispering a message down a line to each other in a game of telephone. As the message is transferred from one person to another — or in this case from one particle to another — the chances that the message becomes misconstrued increases.
A different solution is to transmit entangled photons using satellites and laser beams, as laser beams can create a vacuum, minimizing environmental interference on a quantum level. A satellite-based system is particularly appealing, as satellites can interact with any part of Earth, paving the way for a global quantum network.
On August 16, 2016, Chinese scientists working with the Quantum Experiments at Space Scale (QUESS) program launched a satellite, called Micius, with quantum technology onboard. Pan and his colleagues had worked for more than five years to ensure the launch was a success.
"The necessary technical requirements for doing our reported experiment is similar as clearly seeing and tracking a moving single human hair at a distance of 300 meters away, and detecting a single photon on the Earth from the fire of a single match lit at the Moon."
Pan said he had mixed feelings of excitement before the launch, but that occasionally it also occurred to him that the project would collapse and never work. "As scientists, we always have doubts about our experiments that are very close to our heart," he said. "And it was truly a relief that the Micius was successfully launched."
Only 10 days after the launch, the team managed to establish an initial connection between Micius and ground stations located in China.
A laser beam onboard the satellite was passed through a beam splitter, which gave the beam two distinct polarized states. One polarized beam was used to receive entangled photons while the other was used to transmit entangled photons. In this way, the satellite was able to communicate with three different receiving satellites on Earth scattered across China by distances as much as 1,200 kilometers.
Directing entangled photons through the atmosphere to the ground stations thousands of kilometers apart is no easy feat, especially considering that the satellite is speeding through suborbital space at eight kilometers per second, according to the research team. Many different factors could affect communication, such as beam diffraction and atmospheric turbulence.
Pan explained, "The necessary technical requirements for doing our reported experiment is similar as clearly seeing and tracking a moving single human hair at a distance of 300 meters away, and detecting a single photon on the Earth from the fire of a single match lit at the Moon."
After more than ten years of collaborative work, the team at the University of Science and Technology of China now has a new platform for quantum networks, as well as for probing the interaction of quantum mechanics with gravity. "For quantum networking, in this work, we have already achieved a two-photon entanglement distribution efficiency a trillion times more efficient than using the best telecommunication fibers," said Pan.
Next, the team plans to conduct further quantum experiments on a large scale using Micius. They are also developing new technology for a fleet of quantum-communicating satellites that can function at higher orbit.