Quantum Mechanics

Physicists May Have Finally Cracked Counterfactual Quantum Communication

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Quantum mechanics is utterly strange, but little by little quantum physicists have been able to use its mind-bending quirks to their advantage. In 2017, a team of Chinese scientists announced that they had used quantum principles to transfer an image from one location to another—without any transfer of particles. The achievement they claimed is known as counterfactual quantum communication.

Send A Message Without Sending A Message

Counterfactual quantum communication is different than your everyday, garden-variety quantum communication in one amazing way. You see, quantum communication, sometimes called quantum teleportation, relies on the principle of entanglement: the idea that two particles can interact and become linked, or "entangled," so that they affect each other's behavior over any distance. If you do something to one, the other reacts in a predictable way, and scientists have used this principle to send data from one point to another without actually "sending" anything at all.

In contrast, counterfactual quantum communication doesn't require particles to interact beforehand. Instead, it uses what's known as the quantum Zeno effect. That works on the fact that in the quantum world, objects take on many states until they're observed, or measured. If you've got an unstable quantum system, you can effectively "freeze" that system by repeatedly measuring it.

Counterfactual quantum communication uses this effect to transfer a quantum state from one location to another, without any transfer of particles between them. To achieve that, you need a quantum channel that runs between the two sites, which opens up the possibility for a particle to stray into the channel. If that happens, the system is discarded, and a new one is put in its place.

Do The Wave

To achieve their breakthrough, the team set up an array of beam splitters and placed two single-photon detectors at the end. To get the quantum Zeno effect rolling, they also set up a series of nested interferometers, which continually measured the state of the system to make sure it wouldn't change. That made it possible to predict which of the detectors would be triggered by a passing photon—if one detector clicks, you're almost certain a photon has a specific quality, not the loosey-goosey probability of qualities that quantum objects usually have.

Passing photons, you say? Wouldn't that count as a transfer of particles? No, and here's why: in the quantum realm, light particles can be described as waves. This technique uses the phase of the light wave to carry information instead of the intensity of an individual photon. These waves were each flagged by one of detectors, which recorded either "logic 0" for a black pixel or "logic 1" for a white pixel. That let researchers reassemble the data into a simple black-and-white picture of a Chinese knot. How useful this is remains to be seen, but it's certainly a cool demonstration of something that up until now has been no more than a thought experiment.

Watch And Learn: Our Favorite Content About Quantum Communication

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