Moving electrons around loops with light: A quantum device based on geometry

Researchers at the University of Chicago and the University of Konstanz have demonstrated the ability to generate a quantum logic operation, or rotation of the qubit, that is intrinsically resilient to noise as well as to variations in the strength or duration of the control.

This artist's rendition shows a laser light guiding the evolution of an electronic spin within an atomic-scale defect in diamond. These light-driven loops give rise to a geometric phase, a quantum logic operation that shows remarkable resilience to noise. Source: Peter Allen

While a classical bit found in conventional electronics exists only in binary 1 or 0 states, the more resourceful quantum bit, or 'qubit' is represented by a vector, pointing to a simultaneous combination of the 1 and 0 states. To fully implement a qubit, it is necessary to control the direction of this qubit's vector, which is generally done using fine-tuned and noise-isolated procedures.

Researchers at the University of Chicago's Institute for Molecular Engineering and the University of Konstanz have demonstrated the ability to generate a quantum logic operation, or rotation of the qubit, that - surprisingly -- is intrinsically resilient to noise as well as to variations in the strength or duration of the control. Their achievement is based on a geometric concept known as the Berry phase and is implemented through entirely optical means within a single electronic spin in diamond.

Their findings were published online Feb. 15, 2016, in Nature Photonics and will appear in the March print issue. "We tend to view quantum operations as very fragile and susceptible to noise, especially when compared to conventional electronics," remarked David Awschalom, the Liew Family Professor of Molecular Engineering and senior scientist at Argonne National Laboratory, who led the research. "In contrast, our approach shows incredible resilience to external influences and fulfills a key requirement for any practical quantum technology."

A noisy path

A key feature of Berry phase that makes it a robust quantum logic operation is its resilience to noise sources. To test the robustness of their Berry phase operations, the researchers intentionally added noise to the laser light controlling the path. As a result, the spin state would travel along its intended path in an erratic fashion. However, as long as the total area of the path remained the same, so did the Berry phase that they measured.

"In particular, we found the Berry phase to be insensitive to fluctuations in the intensity of the laser. Noise like this is normally a bane for quantum control," said Brian Zhou, a postdoctoral scholar in the group, and co-lead author.

"Imagine you're hiking along the shore of a lake, and even though you continually leave the path to go take pictures, you eventually finish hiking around the lake," said F. Joseph Heremans, co-lead author, and now a staff scientist at Argonne National Laboratory. "You've still hiked the entire loop regardless of the bizarre path you took, and so the area enclosed remains virtually the same."

These optically controlled Berry phases within diamond suggest a route toward robust and fault-tolerant quantum information processing, noted Guido Burkard, professor of physics at the University of Konstanz and theory collaborator on the project.

"Though its technological applications are still nascent, Berry phases have a rich underlying mathematical framework that makes them a fascinating area of study," Burkard said.

Soure
University of Chicago