Researchers at the National Institute of Standards and Technology (NIST) have bested the previous quantum teleportation fiber-optic distance record four times over, achieving a span of 100 kilometers. While physicists have teleported over farther distances in free space—via open-air laser beams, that is—the ability to information across vast spans using fiber-optic cabling offers a new degree of practicality to quantum-based networking. But also a new challenge.
The accomplishment is described in the current issue of Optica.
Quantum teleportation is (in a sense) a way of transmitting information about the state of a given quantum particle to other particles within its same quantum system. It's often misunderstood or misstated as a mechanism of instantaneous faster-than-light-speed communication, when there's quite a bit more to it (the devil's in the details, and it's all details). In any case, the important thing to understand is that when information is transmitted via entanglement, it's meaningless without additional information about the "sender" particle, and that information is sent the old-fashioned way. So, entanglement is only as fast as information can be sent the old fashioned way.
But, still it's a potentially very useful property, particularly for distributing keys in future quantum encryption schemes. Until now, the fiber-optic entanglement record was 25 kilometers. The difficulty is in detection. Conventional single-photon detectors register incoming particles with relatively low efficiency, which makes the likelihood that a particle will first make the 100 km trip and then actually be detected poor—poor enough to pose a major barrier.
Only 1 percent or so of photons sent along a 100 km fiber-optical channel will actually make it to the detector. The NIST group's accomplishment came only thanks high-detection-efficiency superconducting nanowire single-photon detectors (SNSPDs). The basic idea of a SNSPD is that an incoming photon causes a local disturbance in the superconductivity of a material by breaking apart a Cooper pair, which can then be registered as a change in electrical resistance.
As for what's actually being sent, we're not exactly talking about 80-bit keys. The particle property being entangled—what's actually tasked with representing information—is its position in time. As a "time-bin qubit," it occupies a particular location or slot among several possibilities in a sequence. The slots are each only a nanosecond long, but the difference is enough to distinguish between them. These differences can be used to then encode information.
So, cool. But if we can already achieve quantum entanglement for larger distances in free-space, why bother with fiber at all? It seems like ditching Wi-Fi for an ethernet cable.
"Free-space is fine if you have a clear line of sight between sender and receiver, but not very practical if you want to implement this in a typical landscape with trees, buildings, and other tall objects in the way," Martin Stevens, a study co-author and NIST physicist, told me. "A free-space link also requires a telescope at each end and will typically only work at night."
The technology might be featured in future quantum repeaters, for example, e.g. the relay points required by a hypothetical "quantum internet." As quantum information is sent in such infinitesimal packages, it needs to be amplified constantly to remain coherent. This is all quite a ways off yet.
"As for how close this is to being practically implemented, that's hard to say," Stevens said. "We merely demonstrated that teleportation is possibly over such a long distance in fiber. This could be used in some future quantum communications implementation to transmit information more securely."