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China’s Micius satellite sets distance record for quantum entanglement in space
A schematic shows how China’s Micius satellite could theoretically enable secure quantum communications. (SMOC Graphic)Chinese researchers report that they’ve set a new distance record for quantum teleportation through space, the phenomenon that Albert Einstein once scoffed at as “spooky action at a distance.”
The technology isn’t yet ready for prime time, but eventually it could open the way for a new type of unbreakable encryption scheme based on the weirdness of quantum physics.
The experiment, reported today in the journal Science, involved transmitting pairs of entangled photons from China’s orbiting Micius satellite to ground stations in the mountains of the Tibetan plateau, separated by more than 745 miles (1,200 kilometers).
Quantum entanglement and teleportation sound like science-fictiony terms, but it’s a real phenomenon. Quantum physics allows for two particles to be linked in such a way that you can immediately determine the state of one particle – for example, its polarization – by measuring the state of the other particle.
That would hold true even if the two entangled particles end up being separated by light-years. That’s what led Einstein to complain about the “spooky” nature of entanglement.
Over the past couple of decades, a series of experiments have verified that photons and electrons can indeed become entangled, but it’s not easy. Scientists have found that the entangled pairs can degrade and become disentangled as they pass through air or optical fiber.
That shouldn’t be as much of a problem in the near-vacuum of space, and China’s space agency launched the Micius satellite last August to find out. The Micius mission, which is named after an ancient Chinese philosopher, is part of the country’s $100 million Quantum Experiments at Space Scale program, or QUESS.
Aboard the satellite, a light beam was passed through a crystal of potassium titanyl phosphate, which emitted pairs of entangled photons. One of the photons was sent to a ground station in Delingha, and the other to a facility in Lijiang. Photons were also sent to another ground station in Nanshan.
The mountain locations were chosen to minimize the distance traveled through the atmosphere.
When more than 1,000 pairs of photons were measured and compared via a ground-based link, the researchers found that the paired photons’ polarizations were opposite far more often than not. That verified that the quantum entanglement was adequate, if not perfect.
The technology is tricky, because quantum signals can’t be amplified by classical means. The researchers could recover only about one photon out of the 6 million per second that were transmitted. But it’s a start.
If researchers can find ways to boost the quantum signal’s strength and reliability, that could open the way for quantum-based transmission of encryption keys. Such a mode of communication is considered super-secure, because any attempt to eavesdrop on transmissions en route would basically result in the collapse of the signal.
Thomas Jennewein, a physicist at the University of Waterloo in Canada, told Science that the Chinese team’s feat is a “huge, major achievement” for the field.
The Chinese team has plans for more satellite experiments, to be conducted in cooperation with Australian researchers. Canada is also working on a quantum satellite. Meanwhile, European and U.S. scientists have talked about sending quantum communication devices to the International Space Station for testing.
Principal author for the study published in Science, “Satellite-Based Entanglement Distribution Over 1,200 Kilometers,” is Juan Yin of the University of Science and Technology of China. Corresponding authors are Cheng-Zhi Peng, Jian-Yu Wang and Jian-Wei Pan. There are 30 additional co-authors.
