Quantum-Repeater Trial Ignites Hopes for Long-Distance Quantum Cryptography and Computation

New technology teases simple, photon-only alternative to existing high-maintenance quantum repeater technologies

Hoi-Kwong Lo in the lab
Photo: Jessica MacInnis
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A repeater for quantum communications has been prototyped that requires no expensive cryogenics or elaborate ion traps—only photons. As a result, international quantum cryptography and perhaps the beginnings of a long-distance quantum “internet” could be one step closer to reality.

The Internet and indeed all modern digital communications piggyback off the hard-working repeater, a laser or LED light source that amplifies weakening digital light signals as they propagate down a fiber optic line. Without a repeater, the Internet would extend no farther than the width of a single metropolitan area or small country—in other words, a footprint as wide as a single light pulse could reliably travel inside a fiber optic cable.

And an internet that with an effective reach of a few hundred kilometers is no internet at all; it would be not much more than a country-wide digital bulletin board like France’s nationwide Minitel network in the 1980s.

Of course there was never any danger that conventional, fiber optic signal repeaters wouldn’t have been invented. These devices, crucial as they are for modern, global digital communications, were not even bleeding edge technology in ‘90s during the early explosive growth phase of the Internet.

Not so the quantum signal repeater.

Quantum communications rely on single photons carrying delicate information, often involving their polarization state, that can be nullified in a nanosecond courtesy of an impurity in the fiber optic cable. So sending and receiving individual quantum bits (qubits) via single photons is challenging enough.

Add to that the fact that quantum physics actually prevents any single quantum state from being copied. So you technically can’t even create a quantum repeater that lives up to its own name. No single photon’s quantum state can ever be replicated without first destroying the original photon’s state.

The lack of reliable quantum repeaters is the reason quantum cryptography technologies today are often geographically constrained—and why people even bother keeping track of the distances over which the most far-reaching quantum cryptography-secured communications occur.

Examples of quantum repeater technology, which are still solidly in the experimental stage, typically translate a photon’s quantum state into a quantum memory equivalent in matter, like an atomic state in a laser ion trap. That ion trap state is then quantum teleported to another laser ion trap atom somewhere closer to the receiver of the quantum communication. And then the atomic state is translated back again into the quantum state of a photon on the other end of the signal relay race.

It’s like the quantum equivalent of trying to transmit a poem written in English by first translating it into Japanese or Russian and then handing off the translation to someone else down the line who then translates the translated text back into English.

The process would be simpler, and possibly less error-prone, if a quantum communication sent via photon could remain in that form throughout the length of the communication.

Hoi-Kwong Lo, professor of physics and electrical and computer engineering at the University of Toronto, agrees. He and a team of collaborators from the Japanese Universities of Osaka and Toyama and NTT Corporation in Japan have conducted the first proof of principle experiment that establishes the viability of the all-photon quantum repeater technology that they first proposed in 2015.

Their new research, published in this week’s issue of the journal Nature Communications, admittedly does not result in a working, all-photon quantum signal repeater.

Rather, it establishes the workability of perhaps the most important link in the chain: the translation of the initial photon’s quantum state into an intermediary swarm of photons that then convey the original photon’s quantum state into a receiving photon’s quantum state.

“The ultimate objective is to send quantum communication over an arbitrarily long distance,” Lo says. “However, if we want to do a real quantum repeater, we need more than one component.”

So the group is now working on increasing the robustness of that intermediate swarm of quantum state-transmitting photons — performing a quantum triple axel jump maneuver called the “time-reversed adaptive Bell measurement.”

In the process, they’re inching closer to real-world implementation of the all-photon quantum repeater. Lo admits that the all-photonic repeater might ultimately still prove less attractive than the more conventional, matter-quantum-memory-based repeater technologies already in development.

“I’m not saying focus on our [repeater] is the only way,” says Lo, who adds that he’s not against any of the matter-based quantum repeaters other research groups are testing out today. He allows for the possibility that maybe the others will still come out with the most practicable quantum repeater technology in the end. But “I’m just saying, using photons alone, you can achieve the same goals.”

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