Targeting the spin of individual atoms

New research indicates that it may now be possible to measure the states of individual atoms — opening up the possibility of analysing atomic structure in incredible detail.

To measure the precession a carbon nucleus, the ETH Zurich researchers used the spin of a neighbouring defect in the crystal lattice as a sensor. ( ETH Zurich / Jan Rhensius, Kristian Cujia)

Christian Degen, a Professor of Solid-State Physics at ETH Zurich, and his team of researchers have developed a new approach to Nuclear magnetic resonance spectroscopy — NMR — that allows them to directly track the precession of single nuclear spins. The new method represents a huge improvement on current NMR measurements, which require1000s of atomic nuclei in order to register a measurement signal.

The team analysed the behaviour of carbon-13 atoms in diamonds. using the spin of an adjacent electron within an imperfection in the diamond’s crystal lattice — known as an N-V centre — as a rudimentary sensor.

Kristian Cujia, a doctoral student in Degen’s group, summarises the principle:

“We use a second quantum system to study the behaviour of the first quantum system. In this way, we created a very sensitive way of measurement.”

What is Nuclear magnetic resonance spectroscopy — NMR?

NMR spectroscopy is an important method of physicochemical analysis, as it can be used to precisely determine molecular structures and dynamics. ETH Zurich’s two latest Nobel laureates — Richard Ernst and Kurt Wüthrich — were recognised specifically for their work with the method, emphasising its importance as a developing field of investigation.

The technique is based on nuclear magnetic resonance — which takes advantage of the fact that certain atomic nuclei interact with a magnetic field. A key factor in the analysis is the nuclear spin, which the researchers say can be compared with the spinning of a child’s top.

Just as the top begins to slow and ‘wobble’ — a movement known as precession — nuclear spins that are exposed to a magnetic field begin to precess. This generates an electromagnetic signal that can be measured using an induction coil.

Why is this new method so significant?

When dealing with a quantum system, any measurement will also influence the state of that system. This makes them extremely hard to ‘pin down’.

This means that, thus far, physicists have been unable to track precession continuously — its movement being changed too drastically by measurement.

To solve this problem, Deagan and his team developed a special measurement method to capture the spin of the carbon atom through a series of weak measurements in quick succession.

This minimises the influence of their observation, thus not causing a measurable perturbance on the system, leaving the original circular motion perceptible.

Degan explains: “Our method paves the way for remarkable advances in NMR technology.

“This potentially enables us to directly record the spectra of individual molecules and analyse structures at the atomic level.”

As a preliminary test run, the physicists identified the three-dimensional position of the carbon nuclei in the diamond lattice with atomic resolution.

Degan and his team see huge potential in this development. Such detailed NMR measurements open up the possibility of new insights in many areas of research — as has already been the case with conventional NMR spectroscopy in recent decades, concludes Degan.