System Bits: Dec. 20

Removing quasiparticles from superconducting quantum circuits improves lifetime
Given that an important prerequisite for the realization of high-performance quantum computers is that the stored data should remain intact for as long as possible, an international team of scientists at European interdisciplinary research institute Forschungszentrum Jülich has succeeded in making further improvements to the lifetime of superconducting quantum circuits.

The researchers, including Jülich physicist Dr. Gianluigi Catelani, have developed and tested a technique that removes unpaired electrons from the circuits known to shorten the qubit lifetime, the institute reported.

Quantum computers promise to one day achieve significantly higher computing speeds than conventional digital computers in performing certain types of tasks, and superconducting circuits belong to the most promising candidates for implementing quantum bits, known as qubits, with which quantum computers can store and process information, the team reminded.

Illustration of the filtering of unwanted quasiparticles (red spheres) from a stream of superconducting electron pairs (blue spheres) using a microwave-driven pump. (Source: Forschungszentrum Jülich)

High error rates associated with previously available qubits have up to now limited the size and efficiency of quantum computers but Dr. Gianluigi Catelani of the Peter Grünberg Institute in Jülich, together with colleagues has now found a way to prolong the time in which the superconducting circuits are able to store a “0” or a “1” without errors. The team includes researchers from MIT, Lincoln Laboratory, U.C. Berkeley, RIKEN in Japan, and Chalmers University of Technology in Sweden.

The researchers explained that when superconducting materials are cooled below a material-specific critical temperature, electrons come together to form pairs; then current can flow without resistance. However, so far it has not been possible to build superconducting circuits in which all electrons bundle together. Single electrons remain unpaired and are unable to flow without resistance. Due to these so-called quasiparticles, energy is lost and this limits the length of time that the circuits can store data.

The new technique can temporarily remove unpaired electrons away from the circuit; with the help of microwave pulses, they are in effect “pumped out,” which results in a three-fold improvement in the lifespan of the qubits.

Accelerating quantum bit switching
An international collaboration among physicists at the University of Chicago, Argonne National Laboratory, McGill University, and the University of Konstanz recently demonstrated a new framework for faster control of a quantum bit – — the basic unit of information in yet-to-be created quantum computers. Their experiments on a single electron in a diamond chip could create quantum devices less prone to errors when operated at high speeds.

Intricately shaped pulses of light pave a speedway for the accelerated dynamics of quantum particles, enabling faster switching of a quantum bit. (Source: University of Chicago)

The research team explained that to understand the experiment, one can look to the ultimate setting for speed in classical dynamics — the oval racetracks at the Indianapolis 500 or Daytona 500. To enable the racecars to navigate the turns at awesome speeds, the racetrack pavement is “banked” by up to 30 degrees. That inward slope of the pavement allows the normal force provided by the road to help cancel the car’s centrifugal acceleration, or tendency to slide outward from the turn. The greater the speed of the racecar, the greater the bank angle required. “The dynamics of quantum particles behave analogously,” said Aashish Clerk, professor of theoretical physics at McGill University. “Although the equations of motion are different, to accurately change the state of a quantum particle at high speeds, you need to design the right track to impart the right forces.”

Clerk, together with McGill postdoctoral fellows Alexandre Baksic and Hugo Ribeiro said they formulated a new technique to enable faster quantum dynamics by deftly absorbing detrimental accelerations felt by the quantum particle. Unless compensated, these accelerations would divert the particle from its intended trajectory in the space of quantum states, similar to how the centrifugal acceleration deflects the racecar from its intended racing line on the track.

Brian Zhou, postdoctoral scholar at the Institute for Molecular Engineering at the University of Chicago, aligns the lasers onto the diamond chips used in the experiments. (Source: University of Chicago)

David Awschalom, professor in spintronics and quantum information at University of Chicago’s Institute for Molecular Engineering, realized the new theory could be used to speed up the diamond-based quantum devices in his labs, following discussions with members of his own group and Clerk’s group.

The researchers expect their methods could also be applied for fast and accurate control over the physical motion of atoms or the transfer of quantum states between different systems, and convey benefits to quantum applications, such as secure communications and simulation of complex systems.