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Quantum communication relies on the principle of quantum entanglement, or the idea that two particles can interact and become linked across space in such a way that the state of one invariably changes the state of the other, even over a very long distance. One basic idea about quantum physics is that particles exist in many states at once until they're observed, or measured (you might recall how Schrödinger's cat is both alive and dead until you open the box). Entanglement makes it so once you measure one entangled particle, it takes on a particular state and instantaneously forces the other in the pair to take on its linked state. That's how quantum communication works: you can make a bunch of particles interact, send one from each pair to a separate location, then measure the particles when you want to read the message.

The fact that measuring one particle immediately changes the state the other is also a failsafe way to encrypt a message. If an eavesdropper tries to mess with even a single particle, the other particle will react, the recipient will know, and the message will probably be destroyed.

The problem with all this quantum communication talk is that we live in the real world, and sending particles—often photons, or particles of light—over a long distance runs into mundane annoyances like air molecules and optical fiber fidelity. That's why so many scientists are looking to space: particles travel undisturbed in a vacuum and it's a shorter distance. China's Micius satellite, for example, sits 500 kilometers (310 miles) above the Earth's surface, but has successfully sent a quantum message between ground stations 1200 kilometers (745 miles) apart.

They did that by sending a laser beam through a light-altering crystal on the Micius satellite, which emitted pairs of photons that would be entangled in opposing positions once they were measured—one spinning clockwise, the other counterclockwise. Then they sent one photon from each pair to a station in Delingha, China, and the other to a station in Lijiang. (Both stations were high in the mountains of Tibet, which cuts down on the number of air molecules the photons might bump into). Then, they simultaneously measured more than 1,000 photon pairs and found that they took on opposite polarizations more often than you'd expect them to by chance. That made the experiment break a record for the longest-distance quantum communication performed so far.

This is just one step in a long journey for quantum satellites, but experts are hopeful that the internet of the future—not to mention digital security—will be quantum.

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