Scientists achieve nuclear spin coherence in levitating microparticles

A new study in Physical Review Letters demonstrates the levitation of a microparticle using nuclear magnetic resonance (NMR), having potential implications from biology to quantum computing.

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

One of the main challenges with NMR is using it on small objects to control the quantum properties of levitating microparticles.

The researchers in this study wanted to address limitations previously encountered while studying this particular application, including the requirement of high magnetic fields, sub-Kelvin temperatures, and large volume sizes.

Phys.org spoke to the study’s first author, Julien Voisin, a Ph.D. student at LPENS (Laboratoire de physique de l’école normale supérieure) in France.

Speaking of the choice of the microparticle and what motivated them to use NMR, Voisin said, “A former Ph.D. student was able to measure electronic spins, but their short lifetime made it challenging to study them effectively. This led us to focus on nuclear spins, which we had already successfully measured on a fixed diamond outside the trap.”

How NMR works

Atomic nuclei containing an odd number of protons and/or neutrons have a property called spin.

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

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

When the oscillating field is turned off, the nuclei revert to their original energy states, emitting energy in the form of photons. These photons are detected as electromagnetic signals and are unique to each atom, serving as a fingerprint.

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

In quantum systems, especially those using nuclear spins 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, using NMR on small objects is a persistent challenge.

Diamonds are the solution

To address the problem, the researchers chose microdiamonds as their particles. However, these diamonds had a defect—nitrogen-vacancy (NV) centers.

NV centers are formed when a nitrogen atom replaces a carbon atom in the diamond lattice, and an adjacent lattice site remains vacant. NV centers have unique quantum properties, such as the ability to interact with magnetic fields and store and manipulate quantum information.

“Diamonds can host optically active crystalline defects, often called color centers. These color centers can have many interesting applications, with the NV center being widely used in physics because of its electronic spin and optical properties,” explained Voisin.

The microdiamonds had a diameter of 10–20 micrometers. The uniqueness of their study is the use of an electrical Paul trap to levitate these microdiamonds.

An electrical Paul trap consists of two sets of electrodes to create an oscillating electric field. This field produces a potential well, keeping the microdiamond confined in space, allowing them to levitate.

“The appeal of performing NMR with a levitated system is to access nuclear spins and leverage their properties, such as long coherence times,” explained Voisin.

The levitation offers other advantages, including less disturbance from the environment, and precise manipulation over the microparticles without any physical contact. These factors significantly enhance the reliability and precision of the NMR technique.

Using electronic spins to manipulate nuclear spins

The end goal was to manipulate and control the nuclear spins of the microdiamonds, thereby gaining control over the quantum state of the system. The researchers achieved this by gaining control over the electronic states in the NV centers.

The NV centers possess electronic spin states due to the free electron of the nitrogen. These electronic spin states can be manipulated using polarization, and this manipulation can then be transferred to the nuclear spins.

The researchers used a green laser light to polarize the electronic states in the NV centers. Following this, they leveraged the hyperfine interactions between electronic and nuclear spins using a method known as dynamic nuclear polarization or DNP.

This method allowed them to transfer the polarization from the electronic to the nuclear spins, enabling the manipulation of the nuclear spins and thereby, the quantum state of the system.

Improved coherence times and potential applications

The researchers’ approach allowed them to achieve nuclear spin coherence for levitating microdiamonds in the range of a few hundred microseconds (approximately 120 microseconds). This was an improvement of three orders of magnitude from previous studies.

While the results indicate a step forward compared to previous studies, Voisin noted, “The goal of this experiment was not to compete with NMR studies but to show that NMR can be achieved in a levitating system together with the foreseeable applications in spin-mechanics and fast rotation applications.”

While Voisin doesn’t see immediate applications in biology and quantum computation with the current experimental setup just yet, two promising applications include cooling macroscopic particles and gyroscopy.

For cooling, current feedback cooling in optical tweezers doesn’t work for diamonds in a vacuum because they graphitize and break. However, the spin cooling using nuclear spins could enable ground-state cooling due to their longer coherence times compared to electronic spins.

In gyroscopy, the smaller gyromagnetic ratio of nuclear spins makes them ideal for measuring the pseudo-magnetic fields generated by fast-rotating levitated particles. This small ratio can improve precision in gyroscopic applications by enhancing sensitivity to rotational movement.

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