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."Jian-Wei Pan
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.