Quantum teleportation used to distribute a calculation

Performing complex algorithms on quantum computers will eventually require access to tens of thousands of hardware qubits. For most of the technologies being developed, this creates a problem: It's difficult to create hardware that can hold that many qubits. As a result, people are looking at various ideas of how we might link processors together in order to have them function as a single computational unit (a challenge that has obviously been solved for classical computers).

In today's issue of Nature, a team at Oxford University describes using quantum teleportation to link two pieces of quantum hardware that were located about 2 meters apart, meaning they could easily have been in different rooms entirely. Once linked, the two pieces of hardware could be treated as a single quantum computer, allowing simple algorithms to be performed that involved operations on both sides of the 2-meter gap.

Quantum teleportation is... different

Our idea of teleportation has been heavily shaped by Star Trek, where people disappear from one location while simultaneously appearing elsewhere. Quantum teleportation doesn't work like that. Instead, you need to pre-position quantum objects at both the source and receiving ends of the teleport and entangle them. Once that's done, it's possible to perform a series of actions that force the recipient to adopt the quantum state of the source. The process of performing this teleportation involves a measurement of the source object, which destroys its quantum state even as it appears at the distant site, so it does share that feature with the popular conception of teleportation.

(That's important, because the rules of quantum mechanics dictate that you can't simply copy a quantum state.)

It's easy to think of this as a way to exchange information between different quantum chips, performing part of a calculation on one chip before teleporting the answer-in-progress to the second for further work. But the possibilities are quite a bit more elaborate than that, since the operations needed to perform a teleportation consist of manipulations that can also perform an algorithmic operation, termed a gate. In other words, it's possible to do computation via teleportation.

With the right combination of teleportation operations, it's possible to perform the full set of logical quantum gates. In other words, you can make a universal quantum computer, capable of performing any quantum algorithm, purely by performing teleportation.

The quantum gate teleportation process may seem like a lot of effort to go through, given that it's possible to simply use the classical parts of quantum hardware to transfer information between distant hardware. But the interface between the classical and quantum world creates the risk of an error. Teleportation, by contrast, is lossless; its error rate should be the same as any operation performed on a single piece of local hardware.

Because of its advantages, people had already been looking at incorporating gate teleportation into algorithms. And a number of demonstrations have been done between hardware qubits located in different parts of a single system. But, to this point, there haven't been reports of it being used between physically separated pieces of hardware.

From theory to hardware

The Oxford team was simply interested in a proof-of-concept, and so used an extremely simplified system. Each end of the 2-meter gap had a single trap holding two ions, one strontium and one calcium. The two atoms could be entangled with each other, getting them to operate as a single unit. The calcium ion served as a local memory and was used in computations, while the strontium ion served as one of the two ends of the quantum network. An optical cable between the two ion traps allowed photons to entangle the two strontium ions, getting the whole system to operate as a single unit.

The key thing about the entanglement processes used here is that a failure to entangle left the system in its original state, meaning that the researchers could simply keep trying until the qubits were entangled. The entanglement event would also lead to a photon that could be measured, allowing the team to know when success had been achieved (this sort of entanglement with a success signal is termed "heralded" by those in the field).