Physicists transmit data via Earth-to-space quantum entanglement

July 11, 2017 by Bob Yirka report
Overview of the set-up for ground-to-satellite quantum teleportation of a single photon with a distance up to 1400 km. Credit: arXiv:1707.00934 [quant-ph]

(Phys.org)—Two teams of researchers in China have advanced the distance that entangled particles can be used to send information, including encryption keys. In their papers, both uploaded to the arXiv preprint sever, the two groups outline their work and suggest their achievement represents an essential step toward the development of a global-scale quantum internet.

Quantum entanglement is the shared state of two separate particles—what happens to one happens to the other. Scientists have not yet figured out how this occurs, but they have learned how to create on demand, typically by firing a laser through a crystal. As physicists learn more about entangled particles, they've designed more experiments to take advantage of their . One such area of research involves using them to build quantum networks. Such networks would be much faster than anything we have now, and they would also be much more secure because of the nature of entangled particles—disruptions to encryption keys, for example, could be instantly noted, allowing for prevention of hacking. In this new effort, the researchers have extended the entanglement distance of two —one on the surface of the Earth and the other in space, courtesy of a . They have also shown that it is possible to send entangled from a satellite to an Earth-based receiving station.

In the first experiment, the research team transferred the properties of an entangled particle housed in a facility in Tibet to its partner, which was beamed to a satellite passing overhead, far surpassing the distance record by other researchers. In this case, the information transfer occurred with photons that were approximately 500 to 1,400 kilometers apart, depending on the location of the satellite.

In the second experiment, equipment aboard a satellite created a random string of numbers to represent an encryption key. The key was then beamed to an Earth station as part of an entangled photon stream that used polarization as a means of transmission security.

Illustration of the experimental set-up from "Satellite-to-ground quantum key distribution" arXiv:1707.00542 [quant-ph]

Explore further: Big scientific breakthrough at sub-atomic level holds promise for secure comms

More information: * Ground-to-satellite quantum teleportation, arXiv:1707.00934 [quant-ph] arxiv.org/abs/1707.00934

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.

* Satellite-to-ground quantum key distribution, arXiv:1707.00542 [quant-ph] arxiv.org/abs/1707.00542

Abstract
Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty out space. Here, we develop and launch a low-Earth-orbit satellite to implement decoy-state QKD with over kHz key rate from the satellite to ground over a distance up to 1200 km, which is up to 20 orders of magnitudes more efficient than that expected using an optical fiber (with 0.2 dB/km loss) of the same length. The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission constitutes a key milestone for global-scale quantum networks.

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8 comments

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Porgie
1 / 5 (2) Jul 11, 2017
SO how long before we can transport Merchandise?
Osiris1
not rated yet Jul 13, 2017
There you have it....subspace radio a la Star Trek has been invented!
antialias_physorg
5 / 5 (2) Jul 13, 2017
There you have it....subspace radio a la Star Trek has been invented!

No.

Two teams of researchers in China have advanced the distance that entangled particles can be used to send information, including encryption keys.

I hate it when they get that wrong in these PR articles. They sent *quantum information*. Not (classical) information. The two are separate concepts.

(Classical) information: Can be used to send a message (or radio, or images or what-have-you-knowledge that you want to use to *inform* your opposite number of).
In information parlance:
Encode - Transmit - Decode

Quantum information: Can be used to make sure you and the receiver are talking about the same thing - but not *what* you are talking about (hence good for encryption but not messaging)
In information parlance:
Transmit - Decode
(NB: The 'encode' part is missing. I.e. you cannot purposefully assign a meaning to what you send)
SkyLy
5 / 5 (2) Jul 13, 2017
@antialias i reported your message for crushing readers' hopes.
dlethe
not rated yet Jul 13, 2017
But what if you then change the spin on one of the photons on purpose. The spin changes simultaneously on the other side .. ergo new information is on the other side. If you then use an extremely accurate clock as a timer, you could sequence out a message.

I don't care if the bit then flips back if they read it on the other side, since I am using the delays between changing the spin to create the knowledge that gets transferred.
antialias_physorg
not rated yet Jul 13, 2017
But what if you then change the spin on one of the photons on purpose.

You break entanglement.

The spin changes simultaneously on the other side ..

That doesn't happen.
Entanglement is visible when you measure (not set) the entangled property in one of the pair then the other part of the pair will show up as having a complementary property.

If you do something to either of the two (i.e. do a 'set' operation) then the other one will not know about it.

I am using the delays between changing the spin to create the knowledge that gets transferred.

You can measure an entangled pair only once due to the no-cloning theorem. You couldn't see it flip spins even if it did.
https://en.wikipe..._theorem

The no-cloning theorem is the reason (albeit a subtle one) why superluminal (classical) information transmission is not possible with quantum mechanics. On the plus side it's also the reason why quantum encryption is 100% safe.

dlethe
not rated yet Jul 13, 2017
Thank you. I wish the author would have taken the time to explain this. I appreciate the effort.
GoodElf
5 / 5 (1) Jul 16, 2017
Quantum key distribution was just one of many tests by Micius. This process mentioned uses a series of totally random quantum entangled states to distribute a completely random quantum key. No cloning is an impediment to "sending" information using only QE. To do that you need a non-realist teleport protocol. Most discussed was a one way "flying qubit", earth to orbit teleport protocol which actually worked. After receiving half of a qubit pair state, it is possible to code a retained unknown qubit at a ground station, and using a prepared state, destroying it and instantly setting other qubit on the orbiting platform. A further test was also performed sending two entangled photons from orbit as a resource to separate ground stations as a separate protocol that might be extended for use as a non-realist teleportation protocol with >4SD. Two orientation bits would be needed. See: Satellite-based entanglement distribution over 1200 kilometers - Science - J-W Pan etal - 16 June 2017.

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