Manufacturing qubits that can move

Like any other manufactured chip, the wiring that connects the quantum dots is locked into place during the chip’s manufacture. Since different error correction schemes require different connections among the qubits, this forces us to commit to specific error-correction schemes during manufacturing. If a better scheme is developed after a chip is made, it’s probably not possible to switch to it. Less complex algorithms may benefit from simpler error-correction schemes that require less overhead, but we wouldn’t be able to switch schemes with these chips.

So, quantum dots appear to typify the trade-offs that we’re facing with quantum computing: it’s easier for us to make lots of quantum dots and all the hardware needed to manipulate them, but it’s seemingly not possible for them to benefit from the flexibility that other types of qubits have.

The whole point of this new paper is to show that this isn’t necessarily true.

Moveable dots

The new work was done in collaboration between researchers at Delft University of Technology and the startup QuTech. The team built a chip that had a linear array of quantum dots, and they started out with single electron spins at each end. Then, with the appropriate electrical signals, they could shift the spins into the net dot, gradually bringing them closer together. (And, by gradually, we mean a fraction of a second here, but relatively slowly compared to basic switching in electronics.)

Once the electrons were close enough, the spin wavefunctions overlapped, allowing the researchers to perform two-qubit gates on them. These manipulations can be used to entangle the two spins and are thus needed to build error-corrected logical qubits; these gates are also needed for performing calculations.

The researchers then confirmed that they could move the electrons back to their starting positions, after which measurements confirmed that their spins were entangled. And since quantum teleportation also requires a two-qubit gate, they showed that the process could be used for teleportation. Teleportation can enhance the sort of mobility provided by moving the qubits around, since it can be used to move states around after the qubits have been widely separated.