For the second time this summer, China’s quantum computer scientists have blown the rest of the world out of the water. In early July, a team of 32 researchers reported achieving quantum teleportation from Ngari in Chinese-occupied Tibet, to the Micius satellite up in space — across layers of interfering atmosphere and 870 miles. That’s about the distance from Los Angeles to Montana.
The previous record was about 62 miles — roughly the distance from New York City to Philadelphia.
No physical matter or particles were teleported here in the way some people might remember from Star Trek, but what the Chinese scientists achieved is, arguably, even more mind blowing. Quantum teleportation refers to the instant transfer of a “quantum state,” or a bit of quantum information, from one particle to another across a distance.
For quantum teleportation to happen, two or more particles have to be “entangled.” In the strange world of quantum physics, this means that they’re really the same particle, even though they exist in two different places. If one is measured as having a certain polarization, the other will have the same polarization, even if they’re extremely far apart from each other.
(This was the discovery in physics that Albert Einstein famously thought was so bizarre it could not be true, dismissing it as “spooky action at a distance.”)
The ability to teleport quantum states is a big deal for quantum computing because two entangled particles can serve as a kind of “password” for sharing encrypted information. It’s not quite as simple as encoding information onto one such that it pops out of the other, resulting in faster-than-light communication (that, so far, is impossible), but it is still incredibly useful.
Here’s how, as Inverse reported in June when the same Chinese team first reported sending entangled particles from Micius down to Earth:
Without quantum entanglement, the only way for people in two different spots to share a secret password is to send a carrier — a trusted ally, a letter, a pigeon with a tattoo on its feet, whatever — across the intervening distance. Quantum entanglement, in theory, allows interlocutors to go through the steps of secret communication in a bizarre, out-of-order sequence. First, send entangled particles to two different spots and compose a message in one spot. Encode that message using the pattern of entangled particle states on that end. Send it, thus encoded, using normal wires or radio waves. Finally, decode it using the pattern of particles on the other end.
No legible information passes between the two spots for others to read, and the passcode transfer is instant.
The Chinese experiment is impressive, because, while in theory, two entangled particles can be separated by the whole expanse of the observable universe, in practice it’s pretty hard to make that happen. Standard media for moving photons from one place to another, like fiber optic cables, tend to gobble up individual photons over long trips, ruining the entanglement.
Even sending them straight up to space is tricky. The transmitter has to be precise enough to overcome atmospheric wobbling and distortion, and launch a particle directly at the satellite. And the satellite’s receiver has to be sensitive enough to detect the particle arriving.
But the Chinese pulled it off — six times. In each instance, two particles were entangled on the ground and one was sent into space. A quantum bit of information was teleported from one to the other. And, once again, China moved a big step forward toward true quantum communication.
Abstract: An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated. However, quantum teleportation allows faithful transfer of unknown quantum states from one object to another over long distance, without physical travelling of the object itself. Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks and distributed quantum computation. However, the previous teleportation experiments between distant locations were limited to a distance on the order of 100 kilometers, due to photon loss in optical fibres or terrestrial free-space channels. An outstanding open challenge for a global-scale “quantum internet” is to significantly extend the range for teleportation. A promising solution to this problem is exploiting satellite platform and space-based link, which can conveniently connect two remote points on the Earth with greatly reduced channel loss because most of the photons’ propagation path is in empty space. Here, we report the first quantum teleportation of independent single-photon qubits from a ground observatory to a low Earth orbit satellite - through an up-link channel - with a distance up to 1400 km. To optimize the link efficiency and overcome the atmospheric turbulence in the up-link, a series of techniques are developed, including a compact ultra-bright source of multi-photon entanglement, narrow beam divergence, high-bandwidth and high-accuracy acquiring, pointing, and tracking (APT). We demonstrate successful quantum teleportation for six input states in mutually unbiased bases with an average fidelity of 0.80+/-0.01, well above the classical limit. This work establishes the first ground-to-satellite up-link for faithful and ultra-long-distance quantum teleportation, an essential step toward global-scale quantum internet.
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