Scientists spin diamonds at a billion RPM to test the limits of physics

Fluorescent nanodiamonds may be smaller than small but they pack a punch big enough to reverberate across many industries

As scientists’ understanding of the basic properties of matter has improved over time, they have been able to engineer materials with the best properties for specific applications. Such bespoke materials have revolutionised various sectors, including medical diagnostics, spaceflight, cryptography, commercial electronics, and computing. One such material is the fluorescent nanodiamond (FND).

FNDs are nanometre-sized diamonds made of carbon nanoparticles. They are produced in a high temperature and high pressure process. FNDs are stable under light and aren’t toxic to living things, so they have many applications in high-resolution imaging, microscale temperature sensing, and correlative microscopy, among others. In biology, scientists use FNDs to track cells and their progeny over long periods.

Fluorescence is the property of some materials to emit light of lower frequency when irradiated with light of a higher frequency. But unlike many other nano-scale fluorescent materials, FNDs don’t blink when irradiated for a long time. Their fluorescence lifespan is greater than 10 nanoseconds (ns) — a relatively long duration — which makes them better than quantum dots, whose inventors won the chemistry Nobel Prize last year.

In a recent study published in Nature Communications, physicists from Purdue University in the U.S. reported levitating FNDs in a high vacuum and spinning them very fast. It sounds like a simple, even comical, feat but is actually quite difficult. And now that it has been successfully demonstrated, it paves the way for multiple applications in industry, especially as sensors, and in fundamental research.

Quantum spin

One of the basic features of the building blocks of matter, like electrons and nuclei, is a property called spin. At any given moment, its value is a combination of two states called up and down. For a simplistic illustration, the spin of an electron can be 30% up and 70% down. If the down component is zero, the spin will be up, and vice versa. A computer can map these values to 0s and 1s and use the electrons to encode information. This is how a magnetic hard drive in a computer stores your data.

When a quantum computer manipulates the spin of some particles to perform its operations, each particle is called a spin qubit of the computer.

The Berry phase

The Purdue University team made some FNDs and spun them at an ultra-fast rate, making multiple notable findings.

For one, the team was able to record the Berry phase of the spin qubits due to the rotation.

Depending on the context, an electron can be a particle or a wave. When it’s a wave, it will have properties like frequency, wavelength, and phase. The phase of a wave tells us how much of a wave is completed in a given amount of time. This is like checking how much of an eye-blink has been completed in, say, 2 milliseconds, how far up the Sun has risen by 11 am or what fraction of an F1 race has been completed after 30 minutes.

There are some simple ways to control the state of an electron inside a material, like changing its energy by varying the strength and direction of an applied magnetic field. Say we cycle the electron through multiple states before bringing it back to its original state. If the electron wave’s phase in the final state is different from the original one, the phase difference is called the Berry phase.

It’s named for Michael Berry, a physicist who provided a generalised description of this attribute in 1986. (Indian physicist S. Pancharatnam had discovered a particular form of it 30 years prior.) The Berry phase is important for us to understand certain quantum effects and the properties of strange materials called topological insulators. By showing they could measure the Berry phase of the spin qubits due to the rotation, the Purdue team’s work opens the door for using FNDs in new contexts.

Testing the limits

Reconciling quantum physics with the classical physics of gravity is one of the biggest open problems in modern science. In the past, physicists have proposed that rapidly rotating FNDs containing spin qubits can be used to “test the limit of quantum mechanics and the quantum nature of gravity,” per the statement. But they hadn’t been able to put together a functional version of the setup required until now.

The Purdue researchers confined the FNDs in a cage made of electric and magnetic fields, and used the electric fields to set them spinning at up to 20 million times per second.

“With this method, the rotation frequency of a levitated nanodiamond is extremely stable and easily controllable,” the team wrote in its paper.

Tongcang Li, a professor of physics, astronomy and electrical and computer engineering at Purdue and the study leader, said in a statement, “In the past, experiments with these floating diamonds had trouble in preventing their loss in vacuum and reading out the spin qubits. However, in our work, we successfully levitated a diamond in a high vacuum … For the first time, we could observe and control the behaviour of the spin qubits inside the levitated diamond in high vacuum.”

Applications in industry

When the FNDs were irradiated with lasers, they emitted light of different colours in different directions. As the statement put it, it was as if the diamonds were throwing the world’s smallest disco party.

But beyond such simple pleasures, levitated FNDs are also sensitive to acceleration and electric fields, which means they can be used as sensors in many high-value industries and strategic sectors. The researchers also wrote in their paper that “the effect of the Berry phase generated by rotation … will be useful for creating a gyroscope for rotation sensing”.

FNDs can also be doped to enhance their electrical, magnetic, thermal, and/or optical properties. For instance, some carbon atoms in an FND can be replaced with nitrogen atoms. The substitution creates points in the atomic lattice called nitrogen vacancy (NV) centres. These NV centres host the electron spin qubits. When they are illuminated by green light, they emit red light, and vice versa.

The nitrogen atom has three valence electrons that can form bonds with three of the four valence electrons of carbon. When a neutral nitrogen vacancy centre (denoted NV⁰) accepts one more electron from the donor carbon atoms in the lattice, it forms a negatively charged centre called NV⁻. Physicists expect that FNDs containing NV⁻ centres can be used to produce the macroscopic version of the quantum superposition of electrons.

In sum, FNDs may be smaller than small, but they can pack a punch to reverberate across both theoretical and applied physics.

Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.

Published - October 03, 2024 05:30 am IST