Last year, a Long March 2D rocket took off from the Jiuquan Satellite Launch Centre in the Gobi Desert carrying a satellite called Micius, named after an ancient Chinese philosopher who died in 391 B.C. The rocket placed Micius in a Sun-synchronous orbit so that it passes over the same point on Earth at the same time each day.
Micius is a highly sensitive photon receiver that can detect the quantum states of single photons fired from the ground. That’s important because it should allow scientists to test the technological building blocks for various quantum feats such as entanglement, cryptography, and teleportation.
Today, the Micius team announced the results of its first experiments. The team created the first satellite-to-ground quantum network, in the process smashing the record for the longest distance over which entanglement has been measured. And they’ve used this quantum network to teleport the first object from the ground to orbit.
Teleportation has become a standard operation in quantum optics labs around the world. The technique relies on the strange phenomenon of entanglement. This occurs when two quantum objects, such as photons, form at the same instant and point in space and so share the same existence. In technical terms, they are described by the same wave function.
The curious thing about entanglement is that this shared existence continues even when the photons are separated by vast distances. So a measurement on one immediately influences the state of the other, regardless of the distance between them.
Back in the 1990s, scientists realized they could use this link to transmit quantum information from one point in the universe to another. The idea is to “download” all the information associated with one photon in one place and transmit it over an entangled link to another photon in another place.
This second photon then takes on the identity of the first. To all intents and purposes, it becomes the first photon. That’s the nature of teleportation and it has been performed many times in labs on Earth.
Teleportation is a building block for a wide range of technologies. “Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks and distributed quantum computation,” says the Chinese team.
In theory, there should be no maximum distance over which this can be done. But entanglement is a fragile thing because photons interact with matter in the atmosphere or inside optical fibers, causing the entanglement to be lost.
As a result, the distance over which scientists have measured entanglement or performed teleportation is severely limited. “Previous teleportation experiments between distant locations were limited to a distance on the order of 100 kilometers, due to photon loss in optical fibers or terrestrial free-space channels,” says the team.
But Micius changes all that because it orbits at an altitude of 500 kilometers, and for most of this distance, any photons making the journey travel through a vacuum. To minimize the amount of atmosphere in the way, the Chinese team set up its ground station in Ngari in Tibet at an altitude of over 4,000 meters. So the distance from the ground to the satellite varies from 1,400 kilometers when it is near the horizon to 500 kilometers when it is overhead.
To perform the experiment, the Chinese team created entangled pairs of photons on the ground at a rate of about 4,000 per second. They then beamed one of these photons to the satellite, which passed overhead every day at midnight. They kept the other photon on the ground.
Finally, they measured the photons on the ground and in orbit to confirm that entanglement was taking place, and that they were able to teleport photons in this way. Over 32 days, they sent millions of photons and found positive results in 911 cases.