Gaining Nanoscale Control over Quantum Systems

Scientists at Oak Ridge National Laboratory (ORNL) and Vanderbilt University have developed methods to help control the leaky, dissipative behavior inherent in quantum systems and materials. To do so, they examined the quantum nature of nanostructures. ORNL researcher Eugene Dumitrescu had previously explored the need for precise control over nanoscale energy transfer to enable long-lived entanglement (Physical Review A, doi: 10.1103/PhysRevA.96.053826).

“Our goal is to develop experimental platforms that allow us to probe and control quantum coherent dynamics in materials,” said ORNL researcher Benjamin Lawrie. “To do that, you often have to be able to understand what’s going on at the nanoscale.”

One project focused on driving nitrogen-vacancy center defects in nanodiamonds with plasmons to investigate the defects for use in entanglement. Working with plasmons allowed researchers to examine electromagnetic fields at the nanoscale.

An electron beam (teal) hits a nanodiamond, exciting plasmons and vibrations in the nanodiamond that interact with the sample’s nitrogen-vacancy center defects. Correlated photons (yellow, left) are emitted from the nanodiamond, while uncorrelated photons (yellow, right) are emitted by a nearby diamond excited by surface plasmons (red). Courtesy of Raphael Pooser/Oak Ridge National Laboratory, U.S. Department of Energy.

Vanderbilt University researchers used a high-energy electron beam to excite nitrogen-vacancy centers in diamond nanoparticles, causing them to emit light. A cathodoluminescence microscope was used to collect the emitted photons and characterize high-speed interactions among nitrogen-vacancy centers, plasmons, and vibrations within the nanodiamond. The photon statistics that were collected could be used to calculate entanglement.

In a separate experiment, a cathodoluminescence microscope was used by ORNL researchers to excite plasmons in gold nanospirals. Researchers investigated how the geometry of the spirals could be harnessed to focus energy in nanoscale systems.

Nanospiral plasmon modes at low energies isolated with cathodoluminescence microscopy. Courtesy of Jordan Hachtel/Oak Ridge National Laboratory, U.S. Department of Energy.

“This work advances our knowledge of how to control light-matter interactions, providing experimental proof of a phenomenon that had previously been described by simulations,” Lawrie said.

Quantum information is considered fragile because it can be lost when the system in which it is encoded interacts with its environment, in a process called dissipation. Closed systems, in which quantum information can be kept away from its surroundings, theoretically can prevent dissipation. But real-world quantum systems are open to numerous influences that can result in information leakage.

(From left) Eugene Dumitrescu, Ben Lawrie, Matthew Feldman, and Jordan Hachtel have conducted investigations aimed at controlling the dissipative nature of quantum systems and materials. The cathodoluminescence microscope used in their work appears at right. Courtesy of Jason Richards/Oak Ridge National Laboratory, U.S. Department of Energy.

“The elephant in the room in discussions of quantum systems is decoherence. If we can model an environment to influence how a quantum system works, we can enable entanglement,” Vanderbilt researcher Matthew Feldman said.

“We know quantum systems will be leaky. One remedy is to drive them. The driving mechanisms we’re exploring cancel out the effects of dissipation,” Dumitrescu said.

Dumitrescu used the analogy of a musical instrument to explain the researchers' attempts to control quantum systems: “If you pluck a violin string, you get the sound, but it begins to dissipate through the environment, the air. But if you slowly draw the bow across the string, you get a more stable, longer-lasting sound. You’ve brought control to the system.”

The research was published in Physical Review B (doi:10.1103/PhysRevB.97.081404) and in Optics Letters (doi:10.1364/OL.43.000927).