Another Step Closer to Quantum Teleportation

Came across this interesting nugget of science news this week and had to share….

Who would have thought that commercial optical fiber networks would be instrumental in developing quantum teleportation technology? But, yes, it’s true. Now, we’re not talking about Star Trek-type teleportation, here. (Sniff!) Quantum teleportation is limited to transmitting information, not physical things. Even so, it’s pretty cool. See, there’s a weird phenomenon called “quantum entanglement”, which Einstein once called “spooky action at a distance”. But, I’m getting ahead of myself….

quantum-entanglementAny subatomic particle is said to have a “quantum state”. For a photon, this is a combination of its “phase” and a mixture of vertical and horizontal “polarization”. If we wanted to send that “state” (i.e., information about the particle) to another location, how would we do it? Well, we can’t just make a copy or take a snapshot. Any measurement of the particle’s state only captures limited information, plus it results in the “collapse” of the original state. (It’s a pain, but that’s how quantum theory works.) What can we do? That’s where “entanglement” comes in, as well as the use of a third particle as a middle-man… sort of.

When two particles (e.g., photons) are entangled, they have a strange-yet-subtle connection, such that what affects the quantum state of one affects the state of the other, and the distance between them is irrelevant. “[T]he state of each photon is completely uncertain but the two states are correlated.” So, despite taking certain necessary measurements, we can use this to transfer that state to another particle. As Hamish Johnston points out in Physics World,

“Crucially, the original particle loses all of the properties that are teleported. This satisfies the ‘no-cloning’ theorem of quantum mechanics, which dictates that it is impossible to make a perfect copy of a quantum state.”

Science‘s Adrian Cho breaks it down for us (using a “point on a globe” as analogy for the precise quantum state):

“Here’s how it works. Suppose you have two people, Alice and Bob, with a third, Charlie, in the middle. Alice prepares a photon that she wants to teleport — that is, she sets its position on the [aforementioned] abstract globe. She sends it down an optical fiber to Charlie. At the same time, Charlie prepares a pair of entangled photons. He keeps one and sends the second one on to Bob.

Now, here’s the tricky part. When Charlie receives Alice’s photon he can take it and the one he’s kept and do a particular type of “joint” measurement on them both. Because quantum measurements collapse the states of photons, Charlie’s measurement actually forces those two photons into an entangled state. (Charlie’s measurement actually asks the either/or question: Are the photons in one particular entangled state or a complementary one?)

But as soon as Charlie does the entangling measurement on the two photons he has — the one he got from Alice and the one he kept from the original entangled pair — a striking thing happens. The photon he sent to Bob instantly collapses into the state of Alice’s original photon. That is, the globe setting of Alice’s photon has been teleported to Bob’s even if Bob is kilometers away from Charlie — as he was in these two experiments.”

Why was this in the news this week?

quantum-cryptology-7Two independent teams — one at the University of Calgary in Canada, and the other at the University of Science and Technology of China in Hefei, China — recently performed quantum experiments that took advantage of the local communications infrastructure. The Canadian team successfully used entanglements to send quantum information 6.2km across optical fiber; the Chinese team used a little different method and sent quantum information more than twice as far: 12.5km. These aren’t the furthest distances achieved, but they are the furthest outside of a lab. This was crucial in demonstrating the “viability and practicality of the process.” The teams both published their findings in the latest edition of the journal Nature Photonics.

The Canadian and Chinese teams didn’t do things exactly as described above, but close.

“The only difference is that they used two slightly different arrival times for the basic states of the photons, not different polarizations. The hard part in the experiments was guaranteeing that the two photons sent to Bob arrived at the same general time and were identical in color and polarization. If they were distinguishable, then the experiment wouldn’t work. Those were the technical challenges to teleportation over such long distances.”

So, what’s this quantum teleportation good for, if it can’t “beam me up?” As Avaneesh Pandey put it in the International Business Times,

“From the development of super fast quantum computers to quantum crypotography and the creation of extremely secure ‘quantum internet,’ feasible and reliable teleportation has the capability to revolutionize communications.”

That sounds pretty good, and if it requires a little “spookiness”, that’s find by me.

[For more details on the experiment(s) and the quantum physics behind them, as well as the implications for computing, follow the above links to Physics World and Science.]

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