See with radio waves

Scientists in the Physics Division of the University of Tsukuba have used the quantum effect called “rotation locking” to dramatically improve the resolution when radiofrequency imaging of nitrogen vacancy defects in diamond. This work may lead to faster and more precise analysis of materials, as well as a path to practical quantum computers.

Nitrogen vacancy centers (NVs) have long been studied for their potential use in quantum computers. An NV center is a type of defect in the lattice of a diamond, in which two adjacent carbon atoms have been replaced with a nitrogen atom and a void. This leaves an unpaired electron, which can be detected using radio frequency waves, since its probability of emitting a photon depends on its spin state. However, the spatial resolution of radio wave detection using conventional radio frequency techniques has remained far from optimal.

Today, researchers at the University of Tsukuba have taken resolution to its limits using a technique called “spin-locking”. Microwave pulses are used to put the spin of the electron in a quantum superposition from top to bottom simultaneously. Then, a driving electromagnetic field precedes the direction of rotation, like a flickering top. The end result is an electron spin which is shielded from random noise but strongly coupled to the detection equipment. “Rotation lock ensures high accuracy and sensitivity of electromagnetic field imaging,” says first author Professor Shintaro Nomura. Due to the high density of NV centers in the diamond samples used, the collective signal they produced could be easily picked up with this method. This allowed the detection of collections of NV centers at the micrometric scale. “The spatial resolution we got with RF imaging was much better than with similar existing methods,” continues Professor Nomura, “and it was only limited by the resolution of the light microscope we were using.”

The approach demonstrated in this project can be applied in a wide variety of application areas, for example, the characterization of polar molecules, polymers and proteins, as well as the characterization of materials. It could also be used in medical applications, for example as a new way to perform magnetocardiography.