Submillihertz magnetic spectroscopy performed with a nanoscale quantum sensor
Simon Schmitt, Tuvia Gefen, Felix M. Stürner, Thomas Unden, Gerhard Wolff, Christoph Müller, Jochen Scheuer, Boris Naydenov, Matthew Markham, Sébastien Pezzagna, Jan Meijer, Ilai Schwarz, Martin Bodo Plenio, Alex Retzker, Liam Paul McGuinness, Fedor Jelezko
Enhancing quantum sensing The quantum properties of the nitrogen vacancy (NV) defect in diamond can be used as an atomic compass needle that is sensitive to tiny variations in magnetic field. Schmitt et al. and Boss et al. successfully enhanced this sensitivity by several orders of magnitude (see the Perspective by Jordan). They applied a sequence of pulses to the NV center, the timing of which was set by and compared with a highly stable oscillator. This allowed them to measure the frequency of an oscillating magnetic field (megahertz bandwidth) with submillihertz resolution. Such enhanced precision measurement could be applied, for example, to improve nuclear magnetic resonance-based imaging protocols of single molecules. Science , this issue p. 832 , p. 837 ; see also p. 802
Enhancing quantum sensing The quantum properties of the nitrogen vacancy (NV) defect in diamond can be used as an atomic compass needle that is sensitive to tiny variations in magnetic field. Schmitt et al. and Boss et al. successfully enhanced this sensitivity by several orders of magnitude (see the Perspective by Jordan). They applied a sequence of pulses to the NV center, the timing of which was set by and compared with a highly stable oscillator. This allowed them to measure the frequency of an oscillating magnetic field (megahertz bandwidth) with submillihertz resolution. Such enhanced precision measurement could be applied, for example, to improve nuclear magnetic resonance-based imaging protocols of single molecules. Science , this issue p. 832 , p. 837 ; see also p. 802
Enhancing quantum sensing The quantum properties of the nitrogen vacancy (NV) defect in diamond can be used as an atomic compass needle that is sensitive to tiny variations in magnetic field. Schmitt et al. and Boss et al. successfully enhanced this sensitivity by several orders of magnitude (see the Perspective by Jordan). They applied a sequence of pulses to the NV center, the timing of which was set by and compared with a highly stable oscillator. This allowed them to measure the frequency of an oscillating magnetic field (megahertz bandwidth) with submillihertz resolution. Such enhanced precision measurement could be applied, for example, to improve nuclear magnetic resonance-based imaging protocols of single molecules. Science , this issue p. 832 , p. 837 ; see also p. 802
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.
Quantum technologies are expected to significantly change our world in the next decades and the possible applications are diverse – ranging from quantum computation and encryption to metrology with unprecedented sensitivity. This thesis focuses on quantum sensing and imaging based on single nitrogen-vacancy (NV) centres in diamond to measure magnetic fields. Its electronic spin qualifies this point defect as a highly sensitive quantum sensor and its atomic size offers nanoscale spatial resolution. The simple optical initialisation and readout, together with the relatively long coherence time at ambient conditions, make NV centres promising candidates for biological applications and in particular for nuclear magnetic resonance (NMR) on the nanoscale. Recording the spectral information on the nanoscale allows to determine the structure of single or just a few molecules. However, the current main problems and challenges include insufficient spectral resolution, artificial signals and overall poor performance. In this thesis new spectroscopic techniques which tackle these issues are presented. First, it is shown that by randomisation of control pulse phases spurious harmonic signals can be suppressed, which otherwise complicate spectrum interpretation and can lead to false signal identification. At the same time the effects of pulse imperfections are mitigated. Furthermore, a protocol allowing to measure with a coherence independent spectral resolution is presented. It is demonstrated that linewidths below 1mHz are possible, which is an improvement of more than seven orders of magnitude compared to traditional methods. Due to its simple implementation, the protocol is relevant for many applications. Initially developed to measure signals oscillating up to several megahertz, it is further shown that modifications allow to extend the frequency range into the gigahertz domain. Finally, quantum frequency discrimination with the task to differentiate between two frequency distributions using a quantum sensor is analysed. An optimised protocol is introduced and the influences of different readout strategies are discussed with respect to the probability to make an incorrect decision. These findings are useful for diagnostic tests in NMR.
Quantum sensors harness the high sensitivity of quantum systems to external perturbations to accurately measure various physical quantities. Among the quantum systems employed for sensing purposes, the nitrogen-vacancy (NV) defect in diamond has garnered considerable attention as a magnetometer. A single NV center can be integrated in a scanning probe microscope to build a highly sensitive imaging tool which is particularly adapted to the investigation of antiferromagnets. Two approaches are possible, either the measurement of small static stray fields or the detection of magnetic noise.For canted antiferromagnetic textures, like the cycloidal state in multiferroic bismuth ferrite, the NV center can probe the stray field produced by the uncompensated moments, revealing the presence of topological defects in the magnetic order at the surface of bulk single crystals [1].Alternatively, thermally activated spin waves confined in domain walls generate a magnetic noise to which the NV center is also sensitive. This property allows the localization of the domain walls, which was demonstrated in synthetic antiferromagnets [2].Besides the NV centers, new sensors are emerging as magnetic field probes like the boron vacancies in h-BN, which offer the possibilty to integrate a sensing layer inside a van der Waals heterostructure [3].[1] Finco et al., Physical Review Letters, 128, 187201 (2022)[2] Finco et al., Nature Communications 12, 767 (2021)[3] Kumar et al., Physical Review Applied, 18, L061002 (2022)
Wide-field quantum magnetometry using nitrogen-vacancy (NV) center in diamond can be a breakthrough for a nuclear magnetic resonance (NMR) in a small volume, which is important for biological applications. Although the coherence time of the electron spin of the NV center results in a limited frequency resolution for diamond magnetometry in the range 10–100 kHz, recent studies have shown that a phase-sensitive protocol can circumvent this limit using a confocal setup. We proposed a new measurement protocol, “iQdyne,” which facilitates an improved frequency resolution of wide-field imaging, unencumbered by the coherence limit imposed by the NV center. We demonstrated wide-field magnetometry with a frequency resolution of 238 mHz and a magnetic sensitivity of 65 nT/Hz1/2, which are superior to those obtained using a conventional XY8-based technique, and showed the potential of the iQdyne protocol for the wide-field NMR imaging.
Recent advances in engineering and control of nanoscale quantum sensors have opened new paradigms in precision metrology. Unfortunately, hardware restrictions often limit the sensor performance. In nanoscale magnetic resonance probes, for instance, finite sampling times greatly limit the achievable sensitivity and spectral resolution. Here we introduce a technique for coherent quantum interpolation that can overcome these problems. Using a quantum sensor associated with the nitrogen vacancy center in diamond, we experimentally demonstrate that quantum interpolation can achieve spectroscopy of classical magnetic fields and individual quantum spins with orders of magnitude finer frequency resolution than conventionally possible. Not only is quantum interpolation an enabling technique to extract structural and chemical information from single biomolecules, but it can be directly applied to other quantum systems for superresolution quantum spectroscopy.