A groundbreaking study recently published in Physical Review Letters demonstrates the levitation of microparticles using nuclear magnetic resonance (NMR), a technique traditionally associated with material analysis. This innovative approach could pave the way for advancements in diverse fields, including quantum computing, spin-mechanics, and gyroscopy. The researchers’ ability to levitate microdiamonds and manipulate their nuclear spins marks a significant leap in applying NMR to small-scale systems.
NMR is a widely used spectroscopic technique that provides insights into the internal structure, dynamics, and environment of materials. It works by analyzing the response of atomic nuclei to external magnetic fields. This response is influenced by the nuclei’s unique spin properties, which act as a “fingerprint” for each atom. Despite its widespread use, applying NMR to small particles has remained challenging due to the need for high magnetic fields, extremely low temperatures, and precise control over quantum properties.
Addressing Key Challenges with NMR in Small Systems
The study, conducted by Julien Voisin and his team at LPENS (Laboratoire de physique de l’école normale supérieure) in France, sought to overcome these limitations. Voisin explained that earlier research had focused on electronic spins, but their short coherence times made in-depth studies difficult. The team shifted their focus to nuclear spins, which exhibit longer coherence times and are therefore more suitable for such applications.
NMR relies on the Zeeman effect, where atomic nuclei with odd numbers of protons or neutrons align with or against an external magnetic field. By introducing a weak oscillating magnetic field, nuclei absorb energy and transition between discrete energy levels. When the field is removed, the nuclei return to their original states, emitting photons that serve as detectable signals. This technique enables scientists to probe the structure of materials and explore quantum systems.
However, for small objects like microparticles, these processes become increasingly difficult due to environmental disturbances and the complexity of maintaining coherence. Voisin and his team addressed these challenges by leveraging the unique properties of microdiamonds with nitrogen-vacancy (NV) centers.
Leveraging Microdiamonds and Nitrogen-Vacancy Centers
The researchers selected microdiamonds, each measuring 10–20 micrometers in diameter, as their subject of study. These diamonds were intentionally chosen for their NV centers, which are crystalline defects where a nitrogen atom replaces a carbon atom in the diamond lattice, leaving an adjacent vacancy. NV centers have remarkable quantum properties, including the ability to interact with magnetic fields and store quantum information.
Diamonds with NV centers are particularly advantageous because of their optical and spin properties. They can be used for a wide range of quantum applications, including sensors and information storage. Voisin explained that these properties made microdiamonds ideal candidates for NMR studies, especially when combined with levitation techniques.
Achieving Levitation with an Electrical Paul Trap
To levitate the microdiamonds, the researchers used an electrical Paul trap. This device creates an oscillating electric field using two sets of electrodes. The field produces a potential well, confining the particles in space without any physical contact. Levitation offers several advantages for NMR studies, including reduced environmental disturbances and enhanced control over the particles. This setup ensures precise manipulation and greater reliability in quantum experiments.
Levitation also allows researchers to access the nuclear spins of the microdiamonds, a critical factor in the study. Nuclear spins exhibit long coherence times, making them ideal for exploring quantum systems and performing dynamic manipulations.
Manipulating Nuclear Spins via Electronic Spins
The ultimate goal of the study was to control the nuclear spins of the levitating microdiamonds. This was achieved by manipulating the electronic spin states in the NV centers. These states, associated with the free electron of the nitrogen atom, can be polarized using green laser light. The team employed a process called dynamic nuclear polarization (DNP) to transfer polarization from the electronic spins to the nuclear spins.
DNP utilizes hyperfine interactions, which couple electronic and nuclear spins, to achieve this transfer. By controlling the electronic spins, the researchers gained precise control over the nuclear spins, thereby influencing the quantum state of the system. This capability represents a significant milestone in quantum control and spin-mechanics.
Improved Coherence Times and Potential Applications
The study achieved nuclear spin coherence times of approximately 120 microseconds for the levitating microdiamonds—an improvement of three orders of magnitude compared to previous studies. While this represents a significant advancement, the researchers emphasize that their primary goal was not to compete with traditional NMR but to demonstrate its feasibility in levitating systems.
Voisin highlighted two potential applications stemming from their findings: spin cooling for macroscopic particles and enhanced gyroscopy.
In spin cooling, the longer coherence times of nuclear spins could enable ground-state cooling for particles that would otherwise degrade under traditional optical feedback cooling. This approach is particularly useful for diamonds in a vacuum, where optical tweezers are ineffective due to graphitization risks.
In gyroscopy, the small gyromagnetic ratio of nuclear spins offers unique advantages. It enhances sensitivity to pseudo-magnetic fields generated by fast-rotating levitated particles, potentially improving precision in rotational measurements. This capability could revolutionize navigation systems and other technologies reliant on gyroscopes.
Implications for Quantum Computing and Beyond
Although immediate applications in biology and quantum computing are limited by the current experimental setup, the study provides a proof of concept for integrating NMR with levitation. The researchers believe this combination could lead to innovative tools for studying quantum systems, controlling decoherence, and exploring spin-mechanics.
The long coherence times of nuclear spins make them promising candidates for quantum information processing. Coupled with the precision of levitation techniques, this approach could enable new methods for manipulating and storing quantum information. Additionally, the ability to perform NMR in a levitated system opens doors for studying fast rotation dynamics and other complex quantum phenomena.