Scalable Radio-Frequency Magnetometer with Trapped Ions Achieves 13 Sensitivity Via Dynamical Decoupling

Precise measurement of magnetic fields underpins many areas of science and technology, and researchers continually seek more sensitive and practical magnetometry techniques. Yuxiang Huang, Wei Wu, and Qingyuan Mei, from the University of Science and Technology of China, alongside Yiheng Lin, now present a promising advance in scalable radio-frequency magnetometry using trapped ions. Their work demonstrates a method that combines established techniques with a novel approach to suppressing noise and magnetic field variations, achieving a sensitivity that could significantly improve existing measurements. The team’s simulations reveal the potential to extend coherence times to several minutes, paving the way for robust and practical magnetometry systems with broad applications in fields ranging from materials science to fundamental physics.

Trapped Ions Detect Picotesla Magnetic Fields

Quantum magnetometry is a fundamental area of modern physics, with applications spanning materials science, biomedical imaging, and tests of fundamental physics. This work details an experimental approach to building a scalable radio-frequency magnetometer using trapped ions, aiming for high sensitivity and spatial resolution. The magnetometer exploits the spin states of individual ions, held and controlled within an electromagnetic trap, to detect weak radio-frequency magnetic fields. By precisely manipulating and measuring the ion’s spin resonance, the system promises to resolve magnetic field variations with unprecedented accuracy, potentially reaching sensitivities exceeding 10 picotesla per root Hertz.

The approach centres on employing a chain of individually addressable ions, each acting as a miniature magnetic sensor, and utilising radio-frequency fields to drive transitions between their spin states. The ions are cooled to their lowest energy state to minimise disturbances and enhance measurement precision, and their spin states are initialised and read out using laser-induced fluorescence. A key innovation lies in the development of a multiplexed readout scheme, allowing simultaneous detection of magnetic fields at multiple spatial locations, significantly increasing the magnetometer’s effective sensing area and scalability. This research introduces a novel architecture for a scalable quantum magnetometer, overcoming limitations of single-sensor designs, and offering a pathway towards high-resolution, three-dimensional magnetic imaging.

With its potential for parallel readout and enhanced sensitivity, the proposed system promises to advance quantum sensing, enabling new investigations into a wide range of scientific and technological challenges, including the detection of weak biomagnetic signals and the characterisation of nanoscale magnetic materials. Furthermore, the demonstrated scalability represents a crucial step towards building practical, large-scale quantum sensors for real-world applications. A key component of quantum metrology involves trapped-ion systems, which have achieved picotesla per root Hertz sensitivity in single-ion radio-frequency magnetic field measurements via dressed states and dynamical decoupling. This work proposes a scalable trapped-ion magnetometer utilising a mixed dynamical decoupling method, combining dressed states with periodic sequences to suppress disturbances and spatial magnetic field inhomogeneity.

The method involves carefully engineered radio-frequency pulses applied to the ions, creating a superposition of quantum states sensitive to external magnetic fields. By combining the strengths of both dressed states and periodic decoupling sequences, the magnetometer aims to improve sensitivity and scalability compared to existing approaches. This technique effectively mitigates the effects of environmental noise and imperfections in the magnetic field, enabling more precise measurements over larger volumes. Trapped ions, held in place using electromagnetic fields, allow for precise control and isolation of individual ions or large crystals of ions.

Quantum metrology utilises quantum phenomena, such as superposition and entanglement, to enhance the precision of measurements beyond classical limits. Mixed dynamical decoupling is a technique to protect qubits from environmental noise, crucial for maintaining coherence and improving sensor sensitivity. Ions arrange themselves into ordered structures due to electrostatic repulsion, providing a stable environment for quantum operations. Micromotion, a residual oscillatory motion of the ions within the trap, can be harnessed for improved control. Sympathetic cooling cools ions of one species by coupling them to another, colder species, achieving very low temperatures essential for reducing noise and improving coherence.

Optical clocks utilise the precise frequencies of optical transitions in ions to create highly accurate clocks, while microwave clocks utilise microwave transitions. The team is also developing sensors based on trapped ions to detect weak magnetic fields, potentially for applications in magnetoencephalography and magnetomyography, which detect brain and muscle activity with high spatial and temporal resolution, and for fundamental physics research. Extending the time for which qubits maintain their quantum state is a major challenge, and this research explores techniques like mixed dynamical decoupling and careful trap design to minimise disturbances. The team reports achieving coherence times exceeding one hour in some cases.

Increasing the number of qubits in a trapped ion system is crucial for building more powerful sensors and quantum computers, and this research addresses challenges related to controlling and entangling large numbers of ions. The team is also investigating ways to mitigate or exploit micromotion to improve the performance of trapped ion systems, and using molecular dynamics simulations to understand and optimise the behaviour of trapped ion crystals. The research has reported coherence times exceeding one hour for single ion qubits, and the development of clocks with fractional frequency stability approaching 10^-18. High-fidelity quantum gates have been demonstrated using trapped ions, and the effective use of mixed dynamical decoupling to protect qubits from noise.

Precise measurement of hyperfine splitting in cadmium ions, crucial for clock development, has also been achieved, along with successful cooling of ions using sympathetic cooling techniques. These advancements have potential applications in advanced timekeeping, biomedical imaging, fundamental physics research, quantum computing, and geophysics. This research demonstrates a scalable trapped-ion magnetometer capable of highly sensitive radio-frequency magnetic field measurements. By combining dressed states with a mixed dynamical decoupling method, scientists achieved extended coherence times and robust resilience against magnetic field drift and spatial inhomogeneity.

Numerical simulations, utilising realistic experimental parameters, indicate a sensitivity of 13 for radio-frequency field detection, representing a significant advancement in magnetometer technology. The team successfully demonstrated improved sensitivity in both single-ion and larger, scalable systems containing up to 10,000 ions. Notably, the mixed dynamical decoupling method achieved a sensitivity of 13 fT/√Hz in the scalable system, highlighting its potential for practical applications. Beyond enhanced sensitivity, the large spatial extent of the ion crystal offers a unique capability for probing magnetic field gradients by partitioning the crystal or resolving signals from distinct regions.

This work establishes a strong foundation for future advancements in magnetometry and may benefit diverse fields, including the search for interactions between spins and dark matter, and the improvement of coherence in other quantum systems, such as rare-earth ions and neutral atom arrays. The authors acknowledge that coherence times are slightly reduced in larger systems, a factor that will be addressed in future work. Further research will focus on optimising the method for even greater sensitivity and exploring its application to more complex magnetic field environments.