Manufacturing qubits that can move

To get quantum computing to work, we will ultimately need lots of high-quality qubits, which we can tie together into groups of error-corrected logical qubits. Companies are taking distinct approaches to get there, but you can think of them as falling into two broad categories. Some companies are focused on hosting the qubits in electronics that we can manufacture, guaranteeing that we can get lots of devices. Others are using atoms or photons as qubits, which give more consistent behavior but require lots of complicated hardware to manage.

One advantage of systems that use atoms or ions is that we can move them around. This allows us to entangle any qubit with any other, which provides a great deal of flexibility for error correction. Systems based on electronic devices, in contrast, are locked into whatever configuration they’re wired into during manufacturing.

But this week, a new paper examined research that seems to provide the best of both worlds. It works with quantum dots, which can be manufactured in bulk and host a qubit as a single electron’s spin. The work showed that it’s possible to move these spin qubits from one quantum dot to another without losing quantum information. The ability to move them around could potentially enable the sort of any-to-any connectivity we see with atoms and ions.

Quantum trade-offs

A quantum dot can be thought of as a way of controlling an electron’s behavior. Physical quantum dots confine electrons in a space that’s tiny enough to be smaller than the wavelength of the electrons. Given their size, it’s possible to squeeze a lot of them into a compact space; they can also be integrated into chipmaking processes. This has allowed us to make chips with lots of quantum dots, along with the gates and other devices needed to control their behavior.

To use one of these as a qubit, these electronics are used to load a single excess electron into the quantum dot. Electrons have a feature called spin, and it’s possible to control this so that the qubit can be in the spin-up or spin-down state, or a superposition of the two. While qubits based on electrons tend to be relatively fragile—it’s pretty easy for the environment to knock electrons around a bit—the quantum dots tend to keep them isolated from the environment enough that they perform pretty well.