Physicists recently demonstrated a new framework for faster control of a quantum bit. Their experiments on a single electron in a diamond chip could create quantum devices that are less prone to errors when operated at high speeds.
To understand their experiment, consider the ultimate setting for speed in classical dynamics: the oval racetracks at the Indianapolis or Daytona 500. To enable the racecars to navigate the turns at awesome speeds, the racetrack’s pavement is “banked” by up to 30 degrees. A student in Newtonian mechanics could explain that this inward slope of the pavement allows the normal force provided by the road to help cancel the car’s centrifugal acceleration, or its tendency to slide outward from the turn. The greater the speed, the greater the bank angle that is required.
“The dynamics of quantum particles behave analogously,” says 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 and postdoctoral fellows Alexandre Baksic and Hugo Ribeiro formulated a new technique to enable faster quantum dynamics by absorbing detrimental accelerations felt by the quantum particle. These accelerations, unless compensated, 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.
Through conversations with members of his own group and the Clerk group, David Awschalom, professor in spintronics and quantum information at the Institute for Molecular Engineering at the University of Chicago, realized that the new theory could be used to speed up the diamond-based quantum devices in his labs. However, just as constructing the banked speedways presented challenges in civil engineering, experimentally executing the control sequences envisioned by Clerk and coworkers presented ones in quantum engineering.
Building the quantum fast track required shining intricately shaped, synchronized laser pulses on single electrons trapped at defects inside their diamond chips.
“We demonstrated that these new protocols could flip the state of a quantum bit, from ‘off’ to ‘on,’ 300 percent faster than conventional methods,” says Awschalom, also a senior scientist at Argonne National Laboratory. “Shaving every nanosecond from the operation time is essential to reduce the impact of quantum decoherence,” he explains, referring to the process by which quantum information is lost to the environment.
“What is promising for translating these techniques beyond the laboratory is that they are effective even when the system is not perfectly isolated,” says Guido Burkard, a professor at the University of Konstanz who worked with Awschalom and Clerk’s groups.
The researchers anticipate that their methods can be further 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.
The findings appear in Nature Physics.
Funding came from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division; the Air Force Office of Scientific Research; the National Science Foundation; and the German Research Foundation.
Source: McGill University