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. Credit: Peter Allen
The quantum bit ie ‘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 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.
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. To better understand this concept, the Foucault pendulum, a common staple of science museums helps to give some intuition. A pendulum, like those in a grandfather clock, typically oscillates back and forth within a fixed plane. However, a Foucault pendulum 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, its acquired geometric phase. ie this 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 remarkably does not depend on the rotational speed of Earth or the oscillation frequency of the pendulum.
“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 this experiment, they 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. In their experiments, the team developed a method to draw paths for this 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 Christopher Yale.
This 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. Such an 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, they 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.
These optically controlled Berry phases within diamond suggest a route toward robust and fault-tolerant quantum information processing.
http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2015.278.html http://phys.org/news/2016-02-electrons-loops-quantum-device-based.html http://news.uchicago.edu/article/2016/02/19/moving-electrons-around-loops-light-quantum-device-based-geometry
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