When Flaws Become Features: Diamonds in Quantum Tech

By Emily Ayshford, University of Chicago October 21, 2024

Collected at: https://scitechdaily.com/when-flaws-become-features-diamonds-in-quantum-tech/

Researchers at the University of Chicago have figured out how to improve the performance of diamond-based quantum sensors by studying defects at the atomic level.

These sensors, which use nitrogen vacancy (NV) centers in diamonds to measure magnetic fields, often struggle with interference from nearby defects. The new method isolates these disruptive elements, promising better sensors for future use in navigation and healthcare technologies.

Quantum Defects and Their Applications

Quantum defects hold great potential as ultra-sensitive sensors, with possible applications in advanced navigation systems and biological sensing technologies.

A key example of these systems is nitrogen vacancy (NV) centers in diamonds, which are capable of measuring nanoscale magnetic fields. These NV centers are individual defects in the diamond, where nitrogen atoms replace carbon atoms. While scientists can control the quantum spin of these NV centers, they still face challenges in isolating this spin from the spins of other nearby defects in the material. These neighboring spins can disrupt the NV center’s quantum memory, or coherence.

To unlock the full potential of these sensors, researchers need to understand the behavior of the material at the atomic level. In a recent study conducted at the University of Chicago’s Pritzker School of Molecular Engineering (PME), Prof. David Awschalom’s team has developed a new method to use the NV center’s spin to measure the behavior of other single electron defects in the diamond.

This new insight, published in Physical Review Letters as an Editors’ Suggestion, is expected to lead to improved quantum sensors with longer coherence times, enhancing their overall performance.

“We have devised a way to see certain behavior of single quantum spin states that have otherwise proven elusive to standard measurements,” Awschalom said. “This will impact both how we engineer quantum systems and how we think about charge in many materials.”

Enhancing Measurement Techniques in Quantum Systems

Led by PME PhD graduate and current Argonne postdoctoral researcher Jonathan Marcks, the research team synthesizes these NV centers in facilities at Argonne National Laboratory. They grow diamond layer-by-layer through chemical vapor deposition, and can introduce only a few nanometers of nitrogen dopants to create single NV centers.

These single spin defects are highly coherent, but their spin is still sensitive to the behavior of other defect spins in the material. That’s because no matter how carefully the diamond is grown, it always ends up with unintended nitrogen defects that have their own spin. That causes decoherence in the system, affecting its usefulness as a sensor.

“Even if we have good control over where we put nitrogen, we always end up with this background noise,” Marcks said. “Because we want to grow highly coherent nitrogen vacancy centers, we wanted to better understand how these surrounding defects behave and couple with each other.”

This will impact both how we engineer quantum systems and how we think about charge in many materials.”

Prof. David Awschalom

New Insights Into Electron Charge Dynamics

To better understand these single nitrogen electronic defects, the team used a laser and a home-built microscope system to measure the NV center. The number of photons that the NV center emits depends on the NV center’s spin state. Because these centers interact with other spins, the team realized they could use the NV center as a nanoscale sensor of the nearby nitrogen electron charge, which is otherwise invisible.

The process gave them the first-ever observation of coupled spin and charge dynamics within this material — right down to single defects.

“We expected the nitrogen defects would all just have a single charge state, but they actually flip back and forth, and they are not always in the same charge state,” Marcks said. “That’s different from what we assumed from solid-state physics.”

The team joined forces with Prof. Aashish Clerk and Prof. Giulia Galli, whose teams provided the theoretical and computational tools that allowed the researchers to better understand their observations.

Ultimately, the team will use this knowledge to not only better understand how these material systems behave but to also build better quantum sensors.

“By combining experiment, theory, and computation, we have ideas on how to create extremely clean materials for emerging quantum technologies and control some of these internal noise sources,” Galli said.

Reference: “Quantum Spin Probe of Single Charge Dynamics” by Jonathan C. Marcks, Mykyta Onizhuk, Yu-Xin Wang, Yizhi Zhu, Yu Jin, Benjamin S. Soloway, Masaya Fukami, Nazar Delegan, F. Joseph Heremans, Aashish A. Clerk, Giulia Galli and David D. Awschalom, 25 September 2024, Physical Review Letters.
DOI: 10.1103/PhysRevLett.133.130802