Big step for quantum teleportation won’t bring us any closer to Star Trek. Here’s why

By Adrian Cho, Sep. 19, 2016 , Science (AAAS)

Two teams have set new distance records for quantum teleportation: using the weirdness of quantum mechanics to instantly transfer the condition or “state” of one quantum particle to another one in a different location. One group used the trick to send the state of a quantum particle of light, or photon, 6.2 kilometers across Calgary, Canada, using an optical fiber, while the other teleported the states of photons over 14.7 kilometers across Shanghai, China.

Both advances, reported today in Nature Photonics, could eventually lead to an unhackable quantum internet. But what else is quantum teleportation good for? And will we ever be able to use it to zip painlessly to work on a frigid January morning?

When will this stuff enable us to travel by teleportation?

Sorry to disappoint, but the answer is never. In spite of its name, quantum teleportation has nothing to do with the type of teleportation depicted in the television show Star Trek and other science fiction stories. Such teleportation generally involves disintegrating a material object, somehow beaming the contents through space, and instantly and perfectly reassembling the object in some distant location. In quantum teleportation, nothing is disintegrated and reassembled and no matter travels anywhere. What’s more, the process works only at the level of individual quantum particles: photons, electrons, atoms, etc. Long and short, quantum teleportation and “real” teleportation have nothing in common but the name.

But if quantum teleportation doesn’t move things, then what does it do?

Compared with sending an away team to a planet’s surface, quantum teleportation aims to do something both much less ambitious and much more subtle. Quantum teleportation instantly transfers the condition or “state” of one quantum particle to another distant one without sending the particle itself. It’s a bit like transferring the reading on one clock to a distant one.

What’s so impressive about reading one clock and setting a second the same way?

The quantum state of a particle like a photon is more complex and far more delicate than the reading of a clock. Whereas you can simply read the clock and then set the other clock to the same time, you generally cannot measure the state of a quantum particle without changing it. And you cannot simply “clone” the state of one quantum particle onto another. The rules of quantum mechanics don’t allow it. Instead, what you need to do is find a way to transfer the state of one quantum particle to another without ever actually measuring that state. To continue with the clock analogy, it’s as if you’re transferring the setting of one clock to another without ever looking at the first clock.

How could that possibly work?

It’s a bit complicated. To get a feel for it you need to know something about quantum states. Consider a single photon. A photon is a fundamental bit of an electromagnetic wave, so it can be “polarized” so that its electric field points vertically or horizontally. Thanks to the weirdness of quantum mechanics, the photon can also be in both states at once—so the photon can literally be polarized both vertically and horizontally at the same time. The amounts of vertical and horizontal help define the state of the photon.

But it gets even more complicated than that. In addition to the mixture of vertical and horizontal, the photon’s state is defined by a second parameter, which is a kind of angle called the “phase.” So the actual state of the photon consists of both the mixture of vertical and horizontal and the phase. It can be visualized with the help of an abstract sphere or globe, on which the north pole stands for the pure vertical state and the south pole stand for the horizontal late state.

The precise state of the photon is then a point on the globe, with the latitude giving the balance of vertical and horizontal in the state and the longitude giving the phase. Thus, for example, every point on the equator stands for a state in which the photon is in an equal mixture of vertical and horizontal, but in which the phase, which can be probed in certain more complicated measurements, is different.

So why can’t you just read the point off the globe?

You can’t because measurements of quantum particles provide only limited information. Given a photon in some unknown state, you cannot ask what the “coordinates” of the state on the globe are. Instead, you must perform an either/or measurement. The most simple would be: Is the photon polarized vertically or horizontally? That measurement will give one result or the other with probabilities that depend on the exact mixture of vertical and horizontal in the state. But it won’t tell you the phase. And it will “collapse” the original state, so that the photon is left pointing at one pole or the other, in a state that is either purely vertical or horizontal. That disturbance of the original state is unavoidable in quantum theory.

A photon’s state is described by a point on a “Bloch sphere.” The point’s latitude (angle θ) determines the mixture of horizontal and vertical polarization. The longitude (angle φ) has no classical analog but leads to many weird quantum effects.

A photon’s state is described by a point on a “Bloch sphere.” The point’s latitude (angle θ) determines the mixture of horizontal and vertical polarization. The longitude (angle φ) has no classical analog but leads to many weird quantum effects.

But if you can’t measure the exact state of the photon, how do you transfer it?

You need more photons and another weird bit of quantum mechanics. Two photons can be linked through a subtle connection called “entanglement.” When two photons are entangled, the state of each photon is completely uncertain but the two states are correlated. So, on our abstract globe, the position of each photon remains completely undetermined—it is literally pointing in every direction at once. But, in spite of that uncertainty, the states of the two photons can be correlated so that they are guaranteed to be, say, identical. That is, if you did a fancy measurement that collapsed one photon in the direction on our globe of 40º north, 80º west, you would know the second one would instantly collapse into the same state, no matter how far away it is. Such pairs are crucial to quantum teleportation.

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 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.

But why does that happen?

The experiment depends crucially on the correlations inherent in entanglement. Beyond that, to see why the state of Alice’s photon ends up transferred to Bob’s, you pretty much have to go back and work through the math. Once you get used to the notation, anybody who has taken high school algebra can do the calculation. That is one of the things algebra is good for.

Is this what the physicists actually did?

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 is this possibly good for?

Even though it’s abstract, quantum teleportation could be used to make a quantum internet. This would be like today’s internet, but would enable users to transfer quantum states and the information they contain instead of classical information, which is essentially strings of 0s and 1s.

Currently, physicists and engineers have built partially quantum networks in which secure messages can be sent over optical fibers. Those technologies work by using single photons to distribute the numerical keys for locking and unlocking coded messages. They take advantage of the fact that an eavesdropper could not measure those photons without disturbing them and revealing his presence. But right now, those networks aren’t fully quantum mechanical in that the message needs to be decoded and encoded at every node in the network, making the nodes susceptible to hacking.

With quantum teleportation, physicists and engineers might be able to establish an entanglement connection between distant nodes on a network. In principle, this would enable users at those nodes to pass encoded messages that could not be decoded at intermediary nodes and would be essentially unhackable. And if physicists ever succeed in building a general-purpose quantum computer—which would use “qubits” that can be set to 0, 1, or both 0 and 1 to do certain calculations that overwhelm a conventional computer—then such a quantum network might enable users to load in the computer’s initial settings from remote terminals.

When is that going to happen?

Who knows? But a quantum internet seems likely to show up a lot earlier than a general-purpose quantum computer.

Huh. Cool! But no beaming to work during the winter?

Sorry, you’ll still have to bundle up and face the cold.

http://www.sciencemag.org/news/2016/09/big-step-quantum-teleportation-won-t-bring-us-any-closer-star-trek-here-s-why

___________