A team of Chinese scientists have realized the satellite-based distribution of entangled photon pairs over 1,200 kilometers. The photon pairs were demonstrated to be still entangled after travelling long distances. Credit: Handout.

Chalk up another amazing achievement for Chinese scientists.

A Chinese team has successfully established a satellite-based network using quantum entangled photons that can send passwords 1,120 kilometers away.

The distance is about 10 times further than the old, optic fiber-based quantum tech could do, according to a study published in the journal Nature on Monday.

The experiment shows quantum entanglement, described by Albert Einstein as a “spooky action,” still exists at such a distance.

Experts said this represents a major milestone in building a practical global-scale, ultra-secure quantum internet, but the actual technology might still be years away, China Daily reported.

Quantum key distribution, known as QKD, is a technique used to achieve secure communication that uses cryptographic protocols based on the laws of quantum mechanics. Today’s QKD is mostly conducted through optical fibers on the ground, with few exceptions using quantum satellites.

These protocols can generate secret security keys that enable more secured data transfer between devices by allowing authorities to spot eavesdroppers trying to intercept communications, China Daily reported.

“QKD is theoretically highly secured. But in reality, imperfections in equipment such as photon sources and detectors may lead to security risks,” said Pan Jianwei, noted quantum physicist and lead scientist at Quantum Experiments at Space Scale (QUESS).

Moreover, QKD via optical fibers are susceptible to interference and cannot be sent over ultralong distance without relying on intermediate repeaters to boost the signals, China Daily reported.

To tackle these issues, Pan and his team tested a new QKD protocol method that uses satellites instead of ground-based relays.

They achieved this by using China’s quantum science satellite Micius to send a secret key at the rate of 0.12 bits per second between the ground stations at Delingha and Urumqi, separated by around 1,120 kilometers, China Daily reported.

Peer-reviewed comments called the study a “groundbreaking experiment,” and a “significant step toward establishing a global network for QKD, and more generally, a quantum internet for quantum communication.”

Gilles Brassard, one of the founders of quantum cryptography, said if the technology for secured, long distance quantum communication finally arrives, “this would achieve the Holy Grail that all cryptographers have been dreaming of for thousands of years.”

Quantum entanglement is a phenomenon in quantum physics, which is so confounding that Einstein described it as a “spooky action at a distance” in 1948, China Plus reported.

Scientists found that when two entangled particles are separated, one particle can somehow affect the action of the far-off twin instantly.

Scientists liken it to two pieces of paper that are distant from each other: if you write on one, the other immediately shows your writing.

Quantum physicists have a fundamental interest in distributing entangled particles over increasingly long distances and studying the behavior of entanglement under extreme conditions.

In theory, this bizarre connection can exist over any distance, but scientists want to see if there’s some physical limit.

“Will gravity affect quantum entanglement? It needs long-distance experiment to test the different models. Although QUESS cannot test quantum gravity theories yet, we have developed the technologies needed for space-based experiments through this project,” said Pan Jianwei.

Due to the fact that the entangled photons cannot be amplified as classical signals, new methods must be developed to reduce the link attenuation in the satellite-to-ground entanglement distribution, China Plus reported.

To optimize the link efficiency, Chinese scientists combined a narrow beam divergence with a high-bandwidth and a high-precision acquiring, pointing, and tracking (APT) technique.

An accurate transmission of photons between the “server” and the “receiver” is never easy, as the optic axis of the satellite must point precisely toward those of the telescopes in the ground stations, said Zhu Zhencai, QUESS chief designer.

What makes it much harder is that the satellite flying over the Earth at a speed of 8 kilometers per second can be continuously tracked by the ground station for just a few minutes, China Plus reported.

“It is like tossing a coin from a plane at 100,000 meters above sea level exactly into the slot of a rotating piggy bank,” said Wang Jianyu, QUESS project’s chief commander.