May 16 2016
Quantum computers could allow biochemists to simulate new molecular properties to create novel drugs in ways that would take the fastest existing computers decades.
This image shows 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)
Electrons represent a perfect quantum bit, having a "spin" which when facing up represents a 0 and down represents a 1. Such bits are minute (even tinier than a single atom), and as they do not interact firmly they remain in a quantum state for a long time. Conversely, using electrons as qubits also presents a problem, as they should be confined and manipulated. This is precisely what David Schuster, Assistant Professor of Physics from the University of Chicago and his collaborators at UChicago, Yale University, and Argonne National Laboratory have done.
A key aspect of this experiment is that we have integrated trapped electrons with more well-developed superconducting quantum circuits.
Ge Yang, Lead Author
The team caught the electrons by making them to float beyond the liquid helium surface at very 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.
David Schuster, Assistant Professor of Physics, University of Chicago
Even though electrons under vacuum are capable of storing quantum data almost perfectly, in actual materials they are interrupted by the movement of atoms surrounding them. Nevertheless, electrons have an exceptional association with liquid helium. They ascend beyond the surface, remaining insensitive to the foamy atomic oscillations below. This occurs as electrons view their mirror image through the helium surface.
Since the electrons’ image has reverse charges, they are pulled towards their own reflection, similar to Narcissus of the Greek myth. However, the mechanical quantum effects enable the electrons to jiggle and move out. Repulsion and attraction maintain a balance at around 10 nm beyond the helium surface - quite distant, by atomic standards - and that is where the electrons remain.
We can trap the electrons and hold them for basically as long as we want. We've left them there for 12 hours, and then we got bored.
Gerwin Koolstra, Graduate Student in Physics, UChicago
"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 configuration is a "resonator" of a kind Schuster's lab invented for further work with quantum circuits, but adding the helium and entrapped electrons. As they are so tiny, electrons usually interact weakly with electrical signals only. The resonator works similar to a hall of mirrors, permitting the signal to bounce to and fro for over 10,000 times, allowing the electron to interact for more time. It is this arrangement that makes it likely to build a qubit, while ensuring that the measurement is highly sensitive.
The scientists view the microwave photons arising from the resonator and check that signal as they allow the electrons to gradually escape from the trap.
However, building the device proved to be a major challenge.
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.
David Czaplewski, Scientist, Argonne's Center for Nanoscale Materials
The most important features is that they are about 100 nm or 1,000 times smaller than the diameter of a human hair, and they also had to be kept with a precision of about 10 or 20 nm, the distance of about 30 atoms, within a channel which is 1 µm deep and 500 nm wide.
Argonne's specialized equipment
We couldn't have done it without Argonne's cleanroom facility and the fantastic staff scientists there. 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.
David Schuster, Assistant Professor of Physics, University of Chicago
This instrument is a circuit patterned into a dense niobium layer onto a bed of sapphire, which is exactly the same material utilized on the exterior of Apple watches. Aluminum wires casted on the base of the channel react to the applied electrical voltages and secures that the floating electrons remind trapped in their place.
At the start of the experiment, the researchers flooded the given sample with superfluid helium. Helium is the sole element which can remain a liquid even at temperatures hundred degree beyond absolute zero -- the usual temperature at which the research is carried out.
The electrons arise from the tungsten filament of a small toy light bulb commonly utilized as streetlights in model train layouts. As the bulb gets heated up, electrons "boil" and rise above the surface of the cold superfluid helium.
In the first series of experiments, the researchers have been working with about 100,000 electrons – which are difficult to count, and also too big a quantity to monitor quantum mechanically. However, the researchers are reducing the count. The objective is a setup that would accommodate a single electron whose characteristics can be tested and monitored for usage as a quantum bit.
We're not there yet, but we're pretty close.
David Schuster, Assistant Professor of Physics, University of Chicago