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Measuring Magnetic Monopoles Using Quantum Metrology in Diamond

Periodic Report Summary 1 - QUANTUM METROLOGY (Measuring Magnetic Monopoles Using Quantum Metrology in Diamond)

Summary Description of Project Objectives
This proposal seeks to create novel quantum states of nitrogen vacancy (NV) centres in diamond to develop a state of the art ‘quantum magnetometer’ and to use this to probe individual magnetic monopoles in spin ice samples. This provides an exciting interdisciplinary synergy of a innovative magnetometer, with an unmatched combination of sensitivity and spatial resolution, with samples that offer a rich and unexplored physics.

Description of Work Performed
NV0 - Train in state of the art diamond processing and NV centre control

NV1 - Develop further control of diamond samples

NV2 & NV3 - Create entangled states and Create spin squeezed states
During the course of the project, it has become clear that the NV center is a magnetometer of sufficient precision that developing entangled states and creating spin squeezed states is unnecessary to detect monopoles in spin ice (assuming the T2 can be preserved at an approximately bulk value). In fact, the increased complexity of generating these complex quantum states is to the active detriment of important measurements sort in this project.

However, there are other problems that were not appreciated at the start of the project, namely bringing condensed matter systems close to an NV center. Initially, it was thought that it would be possible to use an NV center in a diamond AFM tip. However, it now seems that it is difficult to preserve the T2 of the NV center during the complex tip fabrication procedure.

The solution to this problem is to re-engineer the set-up, effectively inverting the diamond and sample. By placing the magnetic sample on a tip, and leaving the diamond unprocessed, the T2 of the NV center is unaffected, and the sensitivity is retained.

Clearly, achieving this a more relevant step to making measurements of condensed matter magnetic systems. Therefore, the NV2 and NV3 goals have been substituted by two alternative goals NV2a and NV3a.

NV2a – Create quartz tips capable of bringing complex magnetic systems to bulk NV centers
This phase has sought to create a method whereby arbitrary condensed matter magnetic systems can be deposited onto a quartz tip, and patterned using standard lithographic techniques. Pulled quartz tips were encased in resin, polished down and deposited on. This is a completely new method to make these samples (and has already been adopted by other groups requiring similar functionality), and it enables a host of new experiments.

NV3a – Demonstrate feasibility using model magnetic systems
The feasibility of magnetic measurements can be demonstrated with a model sample, in this case permalloy. This allows us to understand the response of an NV center in the field of a nanomagnet, and determine its behavior.

- MM0 - Train in 3He confocal microscopy
- MM1 - Magnetic characterisation of monopoles.

Description of Main Results
The main result of this Fellowship remains the publication in Nature Nanotechnology in 2014: “Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins”.

Here, we demonstrated an MRI technique that provides subnanometre spatial resolution in three dimensions, with single electron-spin sensitivity. Our imaging method works under ambient conditions and can measure ubiquitous ‘dark’ spins, which constitute nearly all spin targets of interest. In this technique, the magnetic quantum-projection noise of dark spins is measured using a single nitrogen-vacancy (NV) magnetometer located near the surface of a diamond chip. The distribution of spins surrounding the NV magnetometer is imaged with a scanning magnetic-field gradient.

To evaluate the performance of the NV-MRI technique, we image the three-dimensional landscape of electronic spins at the diamond surface and achieve an unprecedented combination of resolution (0.8 nm laterally and 1.5 nm vertically) and single-spin sensitivity. Our measurements uncover electronic spins on the diamond surface that can potentially be used as resources for improved magnetic imaging. This NV-MRI technique is immediately applicable to diverse systems including imaging spin chains, readout of spin-based quantum bits, and determining the location of spin labels in biological systems.

The Expected Final Results and Their Potential Impact
The final results of this Fellowship are anticipated to be two-fold. The work has already added significantly to the state of the art in imaging with NV centers. The work published in Nature Nano demonstrated that it was possible to get the resolution necessary to image individual protein molecules. Additionally, the Fellowship seeks to contribute to the fundamental understanding of complex magnetic systems. This goal will be the driver of the next year of work.