OK, WTF Is Quantum Teleportation?

Hypothetically, if a person could teleport from one place to another, their physical matter would somehow disappear from one location, and reappear instantaneously at another location, even if the destination is extremely far away. Not including the instantaneous travel abilities of characters in Star Trek or Dr Who, no one has ever witnessed teleportation of physical human.

In quantum physics, teleportation is not just possible, it’s been achieved, with scientists now making steps towards bringing teleportation to the masses. But unlike physical teleportation, the quantum version doesn’t involve any matter changing location in space. Instead, information is mysteriously and instantaneously teleported from one quantum particle or quantum system to another.

Teleporting information across huge distances could be the next revolution in communication and connectivity, with implications for telecommunication infrastructure, new techniques in quantum computing, a next-generation internet and cryptography. But when you’re dealing with unpredictable quantum systems, the story is never that simple.

Particles with “other halves”

Quantum teleportation is the result of a quantum phenomenon called “entanglement.” When two (or more) quantum particles are created or interact in a specific way, they can end up dependent on each other, or “entangled,” even when they are separated from each other by long distances.

In a way, entangled particles behave as if they are aware of how the other particle is behaving. Quantum particles, at any point, are in a quantum state of probabilities, where properties like position, momentum, and spin of the particle are not precisely determined until there is some measurement. For entangled particles, the quantum state of each depends on the quantum state of the other; if one particle is measured and changes state, for example, the other particle’s state will change accordingly.

This seems normal enough, until the particles are separated. Still, a change in the state of one particle induces a change in the other, no matter how far away the particles are from each other. For Albert Einstein, this implied that information could be sent from one particle to the other faster than the speed of light, which would violate his own rule from special relativity which says nothing with mass can travel faster than the speed of light because it would require infinite energy to do so. He referred to entanglement as “spooky action at a distance,” and proposed that, rather than entangled particles depending on each other, there were some properties of each particle that were “hidden” and contained the information supposed to be transmitted instantaneously.

Experiments on entangled quantum particles have shown results that make Einstein’s explanation is a lot less likely than the simpler explanation: entangled particles really are each other’s “other halves,” forming an interdependent quantum system no matter where they are.

How to teleport

Quantum particles can become entangled naturally, for example if a particle decays into two less massive or less energetic particles, or entanglement can be created purposefully, for example by forcing photons to mix together under specific conditions.

In contrast, quantum teleportation using entangled particles has to be set up in a particular way. The approach involves techniques from quantum information theory and quantum computing, with quantum versions of traditional computing features like “bits” and “logic operations”.

In the simplest case, there are three quantum states involved in the teleportation, generally described as “qubits”—the quantum equivalent of computer bits encoding information—or referred to as quantum particles. Two of the three qubits are in an entangled state, and the third is in an independent quantum state which contains the information to be teleported. The entangled qubits are separated, with one at the “receiving end” and the other, together with the independent qubit, at the “sending end.”

The sender performs a joint measurement on both the entangled qubit and the independent qubit, in a way that doesn’t directly measure the specific state that the entangled qubit is in. In fact, another quirk of quantum physics is that it’s actually not possible to observe the state of a single qubit, which is why the sender needs another qubit to measure alongside the entangled qubit.

The joint measurement of the two qubits serves two purposes: it changes the state of the entangled qubit at the receiving end so that it is in one of four possible states, and it gives the sender two binary values (1 or 0) which are effectively instructions for the receiver. The sender now has to get those two values to the receiver using normal, non-quantum communication methods, like sending a message over the internet. The receiver follows the instructions: for each value, a “1” tells the receiver to perform a specific measurement on the receiving qubit, and a “0” tells them not to do that measurement. As a result of those measurements (or no measurements at all if both values are “0”), the receiving qubit is in the state that the sending (non-entangled) qubit was originally in.

The particles have not moved around in space, but the information represented by the independent qubit has been teleported to the receiver.

Daniel Llewellyn, a quantum researcher at the University of Bristol, UK, said this is the biggest misconception about quantum teleportation.

“Quantum teleportation is not about the transfer of physical matter between systems, it is about the remote transfer of quantum states between systems,” he said.

Teleporting secret codes

Instantaneously sending information would be a huge improvement on today’s non-quantum communication channels, but using teleportation for communication actually doesn’t do away with traditional methods entirely. For a quantum state to be teleported, a sender still has to transmit two non-quantum bits of information—the instructions for the receiver.

But it’s this element that makes quantum teleportation an ideal method of transmitting secret information, with applications for cryptography and security.

In the example above, the sender doesn’t actually know the information (quantum state) they are teleporting. Unlike most encryption methods to date, it is impossible to know the information at the point of origin, and it is impossible to work out the information just from the instructions sent the receiver. The instructions only unlock the information when applied to the entangled receiving particle.

Whether or not the sender is able to know the information they are teleporting, the value is in the fact that the instructions can be broadcast publicly, and the teleported information would still be secure. In practice, the sender need not even know who the receiver is, or where they are; as long as the sender does the measurement and teleportation publishes the resulting values, the receiver, and only the receiver, will be able to get their entangled particle into the teleported state.

Llewellyn said that, while there are already attempts at commercial cryptography solutions using quantum teleportation, the approach will only become economically feasible when information states can be teleported using “chip-scale systems”, i.e. small enough to be done by consumer devices rather than research labs And this has been a huge engineering challenge—until very recently.

Teleportation at scale

Llewellyn, with a team of 18 other researchers from around the world, recently published breakthrough results for quantum teleportation in practice.

Photons—the quantum particle form of light—are often used as the basis for the quantum states to be teleported. Generating identical, isolated photons of the right kind is hard enough, and creating entangled photons on a chip, which was first achieved in 2015, was already a significant breakthrough. But this approach only generated two photons simultaneously (creating just one entangled pair on the chip). At this rate, teleportation wouldn’t scale-up for commercial applications.

The key innovation in Llewellyn’s research is that photons generated on different chips were successfully entangled, which gets around the limits of generating one pair at a time on a single chip. What’s more, the team built a “converter” on each chip which could switch between reading or storing information from one entangled photon property to another. In this way, the number of entangled pairs increases, since the same photon might be entangled with respect to its location property with one photon, and entangled with respect to its polarization with a different photon.

This approach enabled a number of key results—including the ability to program and control different operations on entangled photons, and the ability to send quantum information between two devices. The novelty, Llewellyn said, is in demonstrating all these features together.

“Quantum teleportation utilizes some of the most peculiar quirks of quantum physics and quantum information,” he said. “It has become a building block for potential complex applications of quantum physics, so it’s crucial that it can be demonstrated reliably with high fidelity [a measure of accuracy] in a sensible platform.”

Akira Furusawa, a quantum physics professor at the University of Tokyo who is not connected with Llewellyn’s research, put it in even stronger terms. Creating large-scale quantum computers which we can control in spite of the inherent uncertainties in quantum phenomena, referred to as “fault-tolerant” computers, depends on teleportation, he says.

“Quantum teleportation methodology is the only way for the realization of fault-tolerant universal quantum computers,” he said.

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