New device steps toward isolating single electrons for quantum computing

May 11, 2016
The geometry of the microwave resonator and the DC bias pin (diagonal from upper left) that University of Chicago physicists used to control the electric field they used to trap approximately 100,000 electrons. Their work is aimed at helping to develop control of single electrons as qubits for quantum computing. The scale of the image is fewer than 20 microns across, much less than the diameter of a human hair. Credit: Ge Yang

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 " 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 . 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 —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 bit.

"We're not there yet, said Schuster. "But we're pretty close."

Explore further: Quantum dot replaces metallic 'island' to improve electronic turnstile

More information: Ge Yang et al, Coupling an Ensemble of Electrons on Superfluid Helium to a Superconducting Circuit, Physical Review X (2016). DOI: 10.1103/PhysRevX.6.011031

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1 / 5 (1) May 11, 2016
Why use quantum bits? You can define any instance in spacetime, I like using lambda as 3D dimension an 1/nu as the time scale. And one axiom, protons and electrons are never created or destroyed and are the maximum quanta, i.e. the center of the sphere from which 1 over the surface area, i.e ke/r^2 QM is so old fashion! So you're counting electrons or probability of electrons? I can construct my space to perfectly depict the field at any instant of time. My quanta is a set of attributes defined for any point in space time as charge or proximity to charge or proximity to a superimposed charge, i.e. field definition or defined viewport.. Everything update at the same time, priori defined set of attributes and whether to modify to fit a field of give the calculated update.

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