&Bullet; physics 14, 56
A graphene-based nanoscale device takes advantage of the wave nature of electrons and provides a level of control useful for quantum computers.
Optical interferometers use the wave nature of light to make measurements with extremely high accuracy. Researchers have also developed interferometers that take advantage of the wave nature of electrons, including some that use graphene, an atom-thick layer of carbon that allows electron waves to travel without being interrupted by ambient noise. Now a team has demonstrated a fully adjustable electron interferometer on graphene  . Researchers expect the device to be useful for some types of quantum computers.
An optical interferometer splits a beam of light into two rays, sends those rays in different paths, and then recombines them to create interference effects that reveal subtle differences between the two paths of the rays. Electron interferometers are useful for quantum computers whose quantum bits contain electrons in certain quantum states in nanoscale 2D devices. Performing logic operations on these bits can be performed with interferometers.
The beam splitter is a critical piece of an interferometer. Graphene-based electron beam splitters have not historically provided complete control over the amplitudes of the two output beams, which would allow researchers more flexibility in designing interferometers for specific purposes. Spurred on by a new understanding of certain electron tunneling effects in graphs, Preden Roulleau of the University of Paris-Saclay and colleagues set out to build a beam splitter that would provide this higher level of control and then plug it into an interferometer.
The researchers applied voltages above, below, and around the edges of a graphene flake to create two adjacent regions with different electrical properties. Turning on a strong magnetic field and sending electrical current through the flake caused currents to circulate in opposite directions around the edges of the two regions. Electrons in these so-called edge states behave similarly to coherent light rays that are forced to move in loops. “These channels act like optical fibers, only for electrons,” says Roulleau.
A single spin-up electron stream circulated clockwise around the left area, and two electron streams – one spin-up and one spin-down – circulated counterclockwise around the right area. To create a beam splitter, the researchers focused on the boundary between the two regions, where spin-up electrons from both circuits could move in parallel along either side of the boundary. When the electrons approached the boundary on the right, they could either stay on their side of the split or tunnel into the left area. Using an applied voltage, the team could control this tunneling effect and determine the amount of current traveling along each side of the boundary, much like a tunable optical beam splitter divides a beam of light into sections that travel in two different ways. Tests showed that this electron beam splitter provided very precise control.
Next, the researchers applied a second voltage to the other end of the boundary, inducing an “inverted” beam splitter that can merge the two currents. With both beam splitters they demonstrated how an interferometer works. By varying the strength of the magnetic field or the voltage applied to the first beam splitter, they observed interference fringes – oscillatory variations in the electron current output – as expected for an interferometer.
“These experiments show a really fundamental component in the electron quantum optics toolbox,” says Peter Samuelsson from Lund University in Sweden, who studies mesoscopic physics. “This opens up new perspectives for precise control of electron behavior.”
In addition to the tunable beam splitter, the technology also enables the further miniaturization of an interferometer, says Roulleau. “We hope that this opens up a new field of experimentation with extremely compact interferometers that would not be possible with other standard approaches,” he says. An important step in the direction of quantum computation would be, for example, the demonstration of so-called “flying qubits”, in which electrons that carry quantum information can be made to perform calculations in free flight. Such a demonstration would require very small interferometers, says Roulleau.
Correction (April 13, 2021): An earlier version contained a false affiliation with Preden Roulleau.
Mark Buchanan is a freelance science writer who splits his time between Abergavenny (UK) and Notre Dame de Courson (France).
- M. Jo et al., “Quantum Hall Valley Splitter and a Tunable Mach-Zehnder Interferometer in Graphs” Phys. Rev. Lett.126146803 (2021).