May 18, 2016

If biochemists had access to a quantum computer, they could perfectly simulate the properties of new molecules to develop novel drugs in ways that would take the fastest existing computers decades.

Electrons represent an ideal quantum bit, with a “spin” that when pointing up can represent a 0 and down can represent a 1.  Such bits are small (even smaller than an atom), and because they do not interact strongly they can remain quantum for long periods. However, exploiting electrons as qubits also poses a challenge in that they must be trapped and manipulated.  Which is exactly what David Schuster, a University of Chicago assistant professor of physics and his collaborators at UChicago, Argonne National Laboratory, and Yale University have done.  

“A key aspect of this experiment is that we have integrated trapped electrons with more well-developed superconducting quantum circuits” said Ge Yang, lead author of a paper in Physical Review X that reported the group’s findings. The team captured the electrons by coaxing them to float above the surface of liquid helium at extremely low temperatures.

“It’s a very important step along the way to being able to study single electrons and make those electrons work as quantum bits,” Schuster said.

While electrons in vacuum store quantum information nearly perfectly, in real materials they are disturbed by the jiggling of atoms around them.  However, electrons have a unique relationship with liquid helium. They levitate above the surface, insensitive to the frothy atomic fluctuations below.  This occurs because electrons see their own mirror image across the surface of the helium.

Because their image has opposite charge, they are attracted to their own reflection, like Narcissus of the Greek myth. But quantum mechanical effects make them jiggle and move away. Attraction and repulsion balance out at about 10 nanometers above the surface of the helium — quite far, by atomic standards — and that’s where the electrons stay.

“We can trap the electrons and hold them for basically as long as we want,” said co-author Gerwin Koolstra, a graduate student in physics at UChicago. “We’ve left them there for 12 hours, and then we got bored.”

“Electrons levitating, who would have guessed that? It’s just a crazy thing,” Schuster said. While this effect has been known, he said, “we’re holding them in a superconducting structure that allows us to interact with them, on much faster timescales, and much more sensitively.”

That structure is a “resonator” of a type Schuster’s lab developed for other work with quantum circuits, but incorporating the helium and trapped electrons.  Because they are so small electrons normally interact only very weakly with electrical signals.  A resonator works like a hall of mirrors, allowing the signal to bounce back and forth more than 10,000 times, giving the electron more time to interact. It is this setup that makes it possible to build a qubit, while making the measurement extremely sensitive.

The scientists look at the microwave photons emerging from the resonator and monitor that signal as they slowly let electrons leak from the trap.

Building the device 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,“ said David Czaplewski, a scientist at Argonne’s Center for Nanoscale Materials, who helped design and build it. The important features are around 100 nanometers, or 1,000 times smaller than the diameter of a human hair. And they 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.

Argonne’s specialized equipment

“We couldn’t have done it without Argonne’s cleanroom facility and the fantastic staff scientists there,” Schuster said. “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 helps 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 whose behavior can be analyzed and controlled for use as a quantum bit.

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

Citation: “Coupling an Ensemble of Electrons on Superfluid Helium to a Superconducting Circuit,” by Ge Yang, A. Fragner, G. Koolstra, L. Ocola, D.A. Czaplewski, R.J. Schoelkopf, and D.I. Schuster,”published March 21, 2016, in Physical Review X, DOI: 10.1103/PhysRevX.6.011031.

Funding: National Science Foundation, David and Lucile Packard Foundation, U.S. Department of Energy.