Electrons Trapped in Liquid Helium Show Promise as Quantum Bits

A team of researchers from the University of Chicago, Argonne National Laboratory (CSA CSM) and Yale University has developed a method to trap and manipulate electrons, opening the door for using the particles as quantum bits. Electrons represent an ideal quantum bit, according to the team, with a “spin” that can represent a zero when pointing up and a one when pointed down. Such electron bits are small—even smaller than an atom—and can remain quantum for long periods because they do not strongly interact.

The advance was made possible by coaxing the electrons to float above the surface of liquid helium at extremely low temperatures. Electrons in vacuum store quantum information almost perfectly, but in real materials they are disturbed by the jiggling of atoms around them. Electrons have a unique relationship with liquid helium, however, levitating above the surface, insensitive to atomic fluctuations below. This levitation occurs because electrons are attracted to their mirror—and oppositely charged—image across the surface of the helium. Quantum mechanical effects make the electrons jiggle and move away, but this dance of attraction and repulsion balances out at about 10 nanometers above the surface of the helium—quite far, by atomic standards—and that’s where the electrons stay.

The team used a superconducting resonator for the research. Electrons normally interact only very weakly with electrical signals, but the resonator acts like a hall of mirrors, allowing a signal to bounce back and forth more than 10,000 times, giving an electron more time to interact.

The resonator was based on others used at UChicago for quantum circuits, and building it, according to the team, was a delicate business. “The most challenging part was the size and the placement of all of the features with respect to each other that really requires specialized equipment,” says David Czaplewski, a scientist at Argonne’s Center for Nanoscale Materials who helped design and build the system. Its important features are around 100 nanometers, or 1,000 times smaller than the diameter of a human hair, and had to be placed with an accuracy of about 10 or 20 nanometers, the span of about 30 atoms, inside a channel that’s one micron deep and 500 nanometers wide.

“We couldn’t have done it without Argonne’s clean room facility and the fantastic staff scientists there,” says David Schuster, a University of Chicago assistant professor of physics. “The process involves a fair amount of chemistry and a number of specialized instruments, which requires deep technical know-how to get it to work. It wasn’t just one piece of equipment or another. It was the whole facility.”

The device is a circuit etched into a thick layer of niobium on a bed of sapphire, the same material used on the surface of Apple watches. Aluminum wires deposited on the bottom of the channel respond to applied electrical voltages and help keep the floating electrons trapped in place.

At the beginning of the experiment, the team first floods the sample with superfluid helium. This is the only element that remains a liquid even at a hundredth of a degree above absolute zero—the temperature at which the experiments are conducted.

The electrons themselves come from the tungsten filament of a miniature toy light bulb often used as streetlights in model train layouts. As the bulb heats up, electrons boil off and fly onto the surface of the cold superfluid helium.

In the first wave of experiments the scientists have been working with around 100,000 electrons—too many to count and too many to control quantum mechanically—but they are whittling the number down. The goal is a trap that would hold just a single electron, the behavior of which can be analyzed and controlled for use as a quantum bit.

“We’re not there yet, says Schuster. “But we’re pretty close.”