A laser-based technique for controlling qubit vector direction could advance practical quantum technology. 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 is intrinsically resilient to noise, as well as to variations in the strength or duration of the control. The work was based on a geometric concept known as the Berry phase and implemented through entirely optical means within a single electronic spin in diamond. An 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. Courtesy of Peter Allen. While a classical bit found in conventional electronics exists only in binary 1 or 0 states, the more-resourceful qubit (quantum bit) 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 a qubit's vector, which is generally done using fine-tuned and noise-isolated procedures. "We tend to view quantum operations as very fragile and susceptible to noise, especially when compared to conventional electronics," said professor David Awschalom. "In contrast, our approach shows incredible resilience to external influences and fulfills a key requirement for any practical quantum technology." When a quantum mechanical object, such as an electron, is cycled along some loop, it retains a memory of the path that it travelled: the Berry phase. By means of comparison, a pendulum, like those in a grandfather clock, typically oscillate back and forth within a fixed plane; a Foucault pendulum, however, oscillates along a plane that gradually rotates over the course of a day due to Earth's rotation, and in turn knocks, over a series of pins encircling the pendulum. The number of knocked-over pins is a direct measure of the total angular shift of the pendulum's oscillation plane, representing its acquired geometric phase. Essentially, the shift is directly related to the location of the pendulum on Earth's surface as the rotation of Earth transports the pendulum along a specific closed path, its circle of latitude. While this angular shift depends on the particular path traveled, it does not depend on the rotational speed of Earth or the oscillation frequency of the pendulum, Awschalom said. "Likewise, the Berry phase is a similar path-dependent rotation of the internal state of a quantum system, and it shows promise in quantum information processing as a robust means to manipulate qubit states," he said. In their experiment, the researchers manipulated the Berry phase of a quantum state within a nitrogen-vacancy (NV) center, an atomic-scale defect in diamond. Over the past decade and a half, its electronic spin state has garnered great interest as a potential qubit, the researchers said. They developed a method with which to draw paths for the defect's spin by varying the applied laser light. To demonstrate Berry phase, they traced loops similar to that of a tangerine slice within the quantum space of all of the potential combinations of spin states. "Essentially, the area of the tangerine slice's peel that we drew dictated the amount of Berry phase that we were able to accumulate," said postdoctoral scholar Christopher Yale. The approach using laser light to fully control the path of the electronic spin is in contrast to more-common techniques that control the NV center spin through the application of microwave fields. The Chicago said their approach may one day be useful in developing photonic networks of these defects, linked and controlled entirely by light, as a way to both process and transmit quantum information. 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 postdoctoral scholar Brian Zhou. "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, said Guido Burkard, professor of physics at the University of Konstanz who served as a 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. The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.278).