Teleportation: Behind the Science of Quantum Computing

[Jennie McGrath]

Teleportation: Behind the Science of Quantum Computing
Researchers were able to reliably teleport information between quantum bits.



A quantum teleportation circuit, with an ant for scale.   

A quantum teleportation circuit, with an ant for scale.

Photograph courtesy Jonas Mlynek, ETH Zurich

Melody Kramer

National Geographic

Published August 14, 2013

It might seem like something straight from the Star Trek universe, but two new research experiments—one involving a photon and the other involving a super-conducting circuit—have successfully demonstrated the teleportation of quantum bits.

If that sounds like gobbledygook, don't worry. We got in touch with one of the researchers, physicist Andreas Wallraff, of the Quantum Device Lab at the Swiss Federal Institute of Technology Zurich, to explain how his team and a team based at the University of Tokyo were able to reliably teleport quantum states from one place to another.

People have done this before but it hasn't necessarily been reliable. The new complementary research, which comes out in Nature today, is reliable—and therefore may have widespread applications in computing and cryptography.

Before we talk about the nitty-gritty part of teleportation, we need to define a few key words. Let's start with a regular, classical bit of information, which has two possible states: 1 or 0. This binary system is used by basically all computing and computing-based devices. Information can be stored as a 1 or a 0, but not as both simultaneously. (Related: "The Physics Behind Schrodinger's Cat.")

But a quantum bit of information—called a qubit—can have two values at the same time.

"With the qubit, you can store more information because you have information in all of its possible states," Wallraff says. "Whereas in the classical memory system, only one can be stored." (More physics: "The Physics Behind Waterslides.")

Quantum teleportation relies on something called an entangled state. An entangled state, in the words of Wallraff, is a "state of two quantum bits that share correlations." In other words, it's a state that can't be separated.

If you have a classical 1 and a 0, for example, you can separate them into a 1 and a 0. But if you have qubits, the bits can be assigned both a 1 and a 0 at the same time—meaning they can't be separated into their individual components and must be described relative to each other. (If you'd like to know more about this, I recommend delving into "Quantum Entanglement" on the Caltech website.)



 
A magnified view of a quantum bit, or qubit, used in the experiment.
A magnified view of a quantum bit, or qubit, used in the experiment.

Photograph courtesy Arkady Fedorov and Lars Steffen, ETH Zurich

[Jennie McGrath]

Diving Into Teleportation

Now that we have a small working vocabulary, we can delve into what Wallraff and team actually did.

Let's go back to Star Trek.

"People automatically think about Star Trek when they hear teleportation," says Wallraff. "In Star Trek, it's the idea of moving people from point A to B without having the person travel that distance. They disappear and then reappear."

What happens in quantum teleportation is a little bit different. The bits themselves don't disappear, but the information about them does.

"That's where the relation to Star Trek comes in," says Wallraff. "You can make the information disappear and then reappear at another point in space."

So how does this work? Remember, we're talking about quantum bits—which can hold two possible states at the same time.

"You can ask yourself, 'How can I transport the information about this bit from one place to another?'" says Wallace. "If you want to send the information about the qubit from point A to B, the information at point A [contains] 0 and 1 simultaneously."

[Jennie McGrath]

Sending and receiving quantum bits could help secure sensitive information.

Illustration courtesy Akira Furusawa

 

It's impossible using classical bits to transmit this information because, as we learned earlier, the information can be stored as 1s or 0s but not both. Quantum teleportation gets around this problem. (Related: "Physicists Increasingly Confident They've Found the Higgs Boson.")

This is where those entangled states I mentioned earlier come into play. In quantum teleportation, a pair of quanta in an entangled state is sent to both a sender—which I'll call A—and a receiver—which I'll call B. A and B then share the entangled pair.

"The sender takes one of the bits of the entangled pair, and the receiver takes the other," says Wallraff. "The sender can run a quantum computing program measuring his part of the entangled pair as well as what he wants to transport, which is a qubit in an unknown state."

Let's untangle what he said: The sender—A—makes a measurement between his part of the entangled pair and what he wants to transport.

Back to you, Wallraff.

"So we have this measurement, and that's what is sent to the receiver via a classical bit," he says.

The receiver—B—receives the measurement between A's part of the entangled pair and the unknown qubit that A wants to send. After B receives this measurement, he runs a quantum computing algorithm to manipulate his part of the entangled pair in the same way. In the process, B re-creates the unknown qubit that A sent over—without receiving the qubit itself.

I realize this is confusing.

[Jennie McGrath]

The Tokyo research team used more than 500 mirrors and beam splitters.

Photograph courtesy Akira Furusawa

 

But Why Is It Useful?

The advances these two research groups have made may improve the way quantum bits are sent, leading to faster processors and larger-scale encryption technologies.

Encryption technology—which is used by everyone from credit card companies to the NSA—is based on the fact that it's really, really hard to find factors of very large prime numbers. And quantum computing is extremely useful for factoring very large prime numbers.

Dividing or multiplying numbers is fairly easy for any computer, but determining the factors of a really large 500- or 600-digit number is next to impossible for classical computers. But quantum computers can process these numbers easily and simultaneously.

Credit card companies, for instance, assign users a public key to encode credit card information. The key is the product of two large prime numbers, which only the website seller knows. Without a quantum computer, it would be impossible to figure out the two prime numbers that are multiplied together to make the key-which protects your information from being shared. (For more info, read this really useful guide about the basics of quantum computing from the University of Waterloo.)

"If you wanted to use classical bits to do this, it wouldn't be efficient," says Wallraff. In other words, classical computers—the ones we use now for most stuff—can't do any of the things quantum computers can do on a large scale.

So while we might not be beaming Scotty up just yet, our computers, it appears, are one step closer to doing so.

http://news.nationalgeographic.com/news/2013/08/130814-physics-quantum-computing-teleportation-star-trek-qubit-science/