Scientists achieve coherence of nuclear spins in suspended particles

A new study published in Physical Review Letters demonstrates the suspension of microparticles using nuclear magnetic resonance (NMR), with potential implications from biology to quantum computing.

NMR is a spectroscopic technique commonly used to analyze various materials based on how their atomic nuclei react to external magnetic fields. This provides information about the internal structure, mechanics, and environment of the material.

One of the main challenges of NMR is the use of NMR on small objects to control the quantum properties of airborne particles.

The researchers in this study wanted to address limitations previously encountered while studying this particular application, such as the requirements for high magnetic fields, sub-Kelvin temperatures, and large capacitance sizes.

Phys.org spoke to the study’s lead author, Dr. Julien Voisin. Student at LPENS (Laboratoire de physique de l’école Normale Supérieure) in France.

Regarding the choice of particles and the motivation for using NMR, Voisin says: “Former PhD students were able to measure electron spin, but the short lifetime of electron spin made it difficult to study it effectively; The nuclear spin has already been successfully measured in a fixed diamond outside the trap. ”

How NMR works

Nuclei containing an odd number of protons or neutrons have a property called spin.

When placed in an external magnetic field, these spins align either with or against the magnetic field. This phenomenon, known as the Zeeman effect, causes an energy level to split into two or more discrete levels.

In NMR, a weak oscillating magnetic field is applied in addition to the previous magnetic field, causing the atomic nuclei to absorb energy and transition between these energy levels.

When the vibration field is turned off, the atomic nucleus returns to its original energy state and releases energy in the form of photons. These photons are detected as electromagnetic signals and are unique to each atom, acting as a fingerprint.

Therefore, NMR is a common method for studying the structure and properties of materials and can also be extended to the study of quantum systems.

In quantum systems, especially those that use nuclear spin for quantum information processing, NMR can be used to control and measure the spin states of particles, making it a valuable tool for studying decoherence.

However, as mentioned earlier, using NMR for small objects is a persistent challenge.

Diamonds are the solution

To address this issue, the researchers chose microdiamonds as the particles. However, these diamonds had a defect: nitrogen vacancy (NV) centers.

NV centers are formed when a carbon atom in the diamond lattice is replaced by a nitrogen atom, leaving adjacent lattice sites vacant. NV centers have unique quantum properties, including the ability to interact with magnetic fields and store and manipulate quantum information.

“Diamonds can contain optically active crystal defects, often called color centers. These color centers have many interesting applications, and NV centers are known for their electronic spin and optical It is widely used in physics due to its properties,” Voisin explained.

The diameter of the microdiamonds was 10–20 micrometers. What’s unique about their research is that they use electric pole traps to suspend these microdiamonds.

An electric pole trap consists of two sets of electrodes that generate an oscillating electric field. This field creates a potential well, trapping the microdiamonds in space and causing them to float.

“The appeal of performing NMR in levitated systems is the ability to access nuclear spins and take advantage of their properties, such as long coherence times,” Voisin explained.

Levitation has other advantages, including less interference from the environment and the ability to precisely manipulate particles without physical contact. These factors greatly improve the reliability and accuracy of NMR techniques.

Manipulating nuclear spin using electron spin

The ultimate goal was to manipulate and control the nuclear spin of microdiamonds and thereby control the quantum state of the system. The researchers achieved this by controlling the electronic state of the NV center.

The NV center has an electronic spin state due to the free electrons of nitrogen. These electronic spin states can be manipulated using polarization, and this manipulation can be transferred to the nuclear spins.

The researchers used green laser light to polarize the electronic states at the NV centers. Following this, they used a method known as dynamic nuclear polarization (DNP) to exploit the hyperfine interaction between electron and nuclear spins.

This method allowed polarization to be transferred from the electronic spin to the nuclear spin, making it possible to manipulate the nuclear spin and thus the quantum state of the system.

Improved coherence time and potential applications

The researchers’ approach allowed them to achieve nuclear spin coherence in suspended microdiamonds in the hundreds of microseconds (about 120 microseconds). This was a three-order-of-magnitude improvement over previous studies.

Although the results represent a step forward compared to previous studies, “the purpose of this experiment was not to compete with NMR studies, but rather to demonstrate how NMR can be used in conjunction with spin dynamics and predictable applications in levitated systems. It was about showing what can be accomplished,” Voisin said. High speed rotation applications. ”

Voisin doesn’t yet see direct applications in biology or quantum computing for the current experimental setup, but two promising applications include cooling macroscopic particles and gyroscopes.

Regarding cooling, current feedback cooling with optical tweezers does not work for diamonds in vacuum. This is because diamonds become graphitized and break down. However, spin cooling using nuclear spins may enable cooling of the ground state due to their long coherence times compared to electronic spins.

Gyroscopes have a small gyromagnetic ratio of nuclear spins, making them ideal for measuring pseudomagnetic fields produced by rapidly rotating suspended particles. This small ratio increases sensitivity to rotational motion and improves accuracy for gyroscope applications.

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