By now, a handful of technologies are leading contenders for producing a useful quantum computer. Companies have used them to build machines with dozens to hundreds of qubits, the error rates are coming down, and they've largely shifted from worrying about basic scientific problems to dealing with engineering challenges.
Yet even at this apparently late date in the field's development, there are companies that are still developing entirely new qubit technologies, betting the company that they have identified something that will let them scale in ways that enable a come-from-behind story. Recently, one of those companies published a paper that describes the physics of their qubit system, which involves lone electrons floating on top of liquid helium.
Trapping single electrons
So how do you get an electron to float on top of helium? To find out, Ars spoke with Johannes Pollanen, the chief scientific officer of EeroQ, the company that's done the new work. He said that it's actually old physics, with the first demonstrations of it having been done half a century ago.
"If you bring a charged particle like an electron near the surface, because the helium is dielectric, it'll create a small image charge underneath in the liquid," said Pollanen. "A little positive charge, much weaker than the electron charge, but there'll be a little positive image there. And then the electron will naturally be bound to its own image. It'll just see that positive charge and kind of want to move toward it, but it can't get to it, because the helium is completely chemically inert, there are no free spaces for electrons to go."
Obviously, to get the helium liquid in the first place requires extremely low temperatures. But it can actually remain liquid up to temperatures of 4 Kelvin, which doesn't require the extreme refrigeration technologies needed for things like transmons. Those temperatures also provide a natural vacuum, since pretty much anything else will also condense out onto the walls of the container.
Liquid helium is also a superfluid, meaning it flows without viscosity. This allows it to easily flow up tiny channels cut into the surface of silicon chips that the company used for its experiments. A tungsten filament next to the chip was used to load the surface of the helium with electrons at what you might consider the equivalent of a storage basin.
Channels then connect that to individual devices on the chip. Each of these has a superconductive plate beneath it, allowing the control electronics to create an electromagnetic trap that can hold electrons in place at the site of the device. Lowering the "walls" of this trap allows electrons to flow in from the storage basin, filling up the trap with dozens of them. The EeroQ team then showed it could gently lower the walls of the trap to allow the electrons to gradually drain back out until there were just a handful of possible states: zero, one, or two trapped electrons.
To distinguish among these, the chip had a pair of electrodes flanking the trap. These could be used to set up a resonator with the electrons, moving them horizontally between the two electrodes. The presence of electrons would, in turn, affect the resonance frequency of this setup, so tracking the frequency allowed the researchers to distinguish among these three states. So, the researchers showed that they could successfully drain the trap down to one electron and keep it there indefinitely by raising the walls of the trap again.
From physics to qubits
And that's where the paper ends. The trapped single electron, floating at the surface of liquid helium, is the foundation of a qubit. But the paper simply notes that they have crafted a "promising candidate for exploring mobile qubit architectures." So, what's left to be done, and why do the people at EeroQ think it could scale quickly enough to matter?
The plan is to store qubits in the spin of the electrons. That's been tested before in things like silicon impurities and quantum dots. But in those cases, the coherence of that spin—the ability to keep it isolated from its environment—is complicated in those materials, since the electrons are occupying orbitals and/or moving about a complicated electronic environment. None of that is true for an isolated electron floating between liquid helium and a vacuum. "The spin coherence of the electron is going to be fantastic," Pollanen told Ars. "I should say that no one knows experimentally what the spin coherence is, but it can't be worse than what's in silicon."
Another key benefit is that the individual devices can be manufactured using standard CMOS technology, and don't require cutting-edge capabilities to create the needed features. "With relatively dated CMOs, one can build scaling architectures that can host tens of thousands, hundreds of thousands, millions of qubits on chips that are quite small, Pollanen said. These chips would contain all the control circuitry and hardware needed to read the spin state. And, thanks to the compact wiring, the chips would enable digital control to extend to individual devices, potentially limiting the amount of wiring needed to run between the qubits and the outside world.
Ultimately, the company plans on using pairs of electrons in a single trap to store information. "Ultimately, a qubit is actually going to be encoded in two electrons with opposing spin," Pollanen told Ars. "If we move electrons around, they'll experience some inhomogeneous magnetic fields from the magnet that we need for the spin quantization. But if you create a qubit out of two electrons with opposing spin, any decoherence that happens to one will be canceled in the other."
That's essential because moving the electrons around will play a key role in any future quantum processor. You can entangle qubits by moving one electron from two pairs together and allowing them to interact before returning them to their original locations. EeroQ also plans on moving qubits to dedicated operations and measurement locations within the chip. All of this may sound challenging, but Pollanen said that earlier work had demonstrated moving a single electron around a device for long enough that it covered over a kilometer of total distance.
Are all those advantages enough to get the technology to take its next steps, which would involve both showing working qubits and scaling the number rapidly? There's no way to tell right now. But the physics behind it is pretty neat and could be an interesting experimental system even if the qubits don't work out.
Physical Review X, 2025. DOI: 10.1103/vcl7-73ms (About DOIs).
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