Diamonds are well known gems but this material has been used in industry, as a tool for machining the latest smartphones, as a window in high-power lasers used to produce automotive components, and even as a speaker-dome material in high-end audio systems. However new applications od diamond are emerging in diamond quantum technologies. Research into nitrogen-vacancy centers in diamond has exploded in the last decade due to its well-behaved quantum properties, and the ability to monitor and manipulate the quantum state with a combination of microwave fields and laser light.
Nitrogen is the most common foreign impurity in diamond. It can be easily incorporated into the diamond lattice due to the similarity of atomic sizes and valence shells between these two elements. Nearly all diamonds contain more or less the nitrogen: indeed, about 98% natural diamond contains about tens to several hundred parts per million (ppm) nitrogen atoms while the rest could have nitrogen concentration at 10 ppm.
A pure diamond consists of carbon atoms arranged in a regular latticework structure. If a carbon nucleus is missing from the lattice where one would be expected, that’s a vacancy. If a nitrogen atom takes the place of a carbon atom in the lattice, and it happens to be adjacent to a vacancy, that’s a nitrogen-vacancy (NV) center. It is also called the Nitrogen-Vacancy (NV) defect.
Associated with every NV center is a group of electrons from the adjacent atoms, which, like all electrons, have a property called spin that describes their magnetic orientation. When subjected to a strong magnetic field—from, say, a permanent magnet positioned above the diamond—an NV center’s electronic spin can be up, down, or a quantum superposition of the two. It can thus represent a quantum bit, or “qubit,” which differs from an ordinary computer bit in its ability to take on not just the values 1 or 0, but both at the same time.
The diamond has many properties that fairly isolates the qubit from the surrounding environment including rigid structure, excellent heat conduction, and conducting electricity not at all. That spin state can then be used as a sensor of external environment, since its energy levels are sensitive to external magnetic and electric filed.
The original motivations for academic work on diamond-based quantum systems were to investigate fundamental physics and to consider using diamond in a quantum computer. This is the most demanding of all the diamond quantum applications as it requires the most stringent performance of the defects. Specifically, each defect needs to behave in exactly the same way, emitting light at precisely the same wavelength. Unfortunately, imperfections – such as dislocations in diamond’s crystal structure – create strain, which shifts the emission wavelength of the light enough to make two NV defects distinguishable. This can be countered by applying an electric field near a defect that can be “Stark tuned” such that the emission wavelength is the same – however, changes in the local charge configuration surrounding the NV can still change during a measurement causing a wavelength shift. Despite these potential hurdles, several breakthrough results have been achieved with diamond, including the first successful “loophole-free Bell’s inequality test” in 2015 (Nature 526 682), and the longest spin lifetime without the use of any cryogens.
NV− center is most intensively studied and applied, owing to its outstanding properties such as strong fluorescence, long-lived ground state electron spin coherence, optical spin polarization or readout, state-selective intersystem crossing probabilities, etc. It is thus could be used to detect the electric field of a single electron at a distance of ∼25 nm within 1 s of averaging, absorb and re-emit single photon for quantum computing and information processing, provide robust quantum memories, and sense magnetic field, stress, temperature or current, etc
Diamond is now well established as a major player in quantum materials, with more than 200 academic groups around the world working on applications of its quantum properties. There is also a growing number of companies developing diamond quantum technology, including large firms such as Lockheed Martin, Bosch and Thales, as well as many start-ups such as Quantum Diamond Technologies, NVision and Qnami. The material is at the heart of all of this technology, but lots of time-consuming engineering is required to make optimized devices. Even so, in many cases, potential customers are already testing prototype systems.
Applications for masers — the microwave equivalent of lasers — have been hindered by their extreme operating conditions and the inability to produce continuous emissions. A diamond maser overcomes these limitations. Using a laser to ‘pump’ the NV centers into the 0 state. The defects can relax to the −1 state, and in doing so produce the microwave radiation associated with a maser. Most previous maser technologies have required cryogenic refrigeration and high-vacuum systems and been restricted to niche applications. While a room-temperature maser based on NV centers in diamond, which features the longest known solid-state spin lifetime (∼5 ms) at room temperature, high optical pumping efficiency (∼106 s −1) and material stability has been realized.
Their bright luminescence, combined with their readily modifiable surface and biocompatibility, makes diamond nanoparticles containing fluorescent NV− centers, which are extremely promising for biomedical applications. Depending on the correlated magnetic plus the fluorescence, cancer biomarkers expressed by rare tumor cells in a large population of healthy cells was quantified
Nano-diamond biomarkers containing NV− have a number of advantages over competing luminescent probes, such as quantum dots, fluorescent proteins or organic dyes. It has extremely small size which could be implanted into tissues, no toxicity to most cell types and the wavelength range 625–800 nm which is easy for penetrating through tissues. Understanding the functioning of the brain is one of the most significant challenge, it was shown that sensitivity of upcoming generation of NV magnetic sensors may not be enough for the measurement of single neuron action potential, while monitoring electromagnetic signals in brain slices or cardiac tissues seems very promising.
“Diamond is an extraordinary material with such diverse properties that it is used in a wide range of applications, including smartphone processing, high-power lasers for automotive manufacturing and high-end audio systems. Now, thanks to continued technological advances, synthetic diamond materials with engineered levels of qubits made up of nitrogen-vacancy (NV) defects are paving the way for the next quantum magnetic sensing devices” said Daniel Twitchen, chief technologist at Element Six (E6).
Dirk Englund of the Massachusetts Institute of Technology, who was not involved in this work says that the new study is an “amazing advance” in the field of quantum sensing. “It takes magnetic field sensing to a new extreme that now allows for resolution of chemical shift spectra at the micron-scale, which matches the lengths of interest in cells. Just a few years ago, this still seemed far off and progress has been tremendous.”
Potential applications include NMR studies of single-cell metabolomics and NMR fingerprinting of protein expression in tumour cells, for example, say Walsworth and colleagues. “It may even help in the development of new drugs through the study of very small, hard-to-manufacture samples.”
Temperature and vibration also can be detected by NV center in diamond. The intensity and linewidth of the ZPL of NV centers are conformed to be highly depended on the environmental temperature, and the energy level shifts of NV centers in diamond follow the modified Varshni model very well. Accordingly, the NV color center shows the ability in temperature measurement with a high accuracy of up to 98%. Besides, the 10 nm-scale faint machinery vibration has been measured based on the spin magnetic resonant effect of NV centers, although the theoretical detection limit of mechanical vibration is as high as 5.7 nm.
Almost every biological system of interest is linked to electrical signals, from neurons firing to the heart beating. These electrical signals have an associated magnetic field which, unlike them, is not screened by water and skin. In the field of biological sciences, NV− centers can detect the magnetic field in tiny amounts of magnetic nanoparticles for future biomedical applications. The NV− quantum spin states can be optically probed to form rapidly reconstructed images of the vector components of the magnetic field created by chains of magnetic nanoparticles (magnetosomes) produced in the bacteria. Their bright luminescence, combined with their readily modifiable surface and biocompatibility, makes diamond nanoparticles containing fluorescent NV− centers, which are extremely promising for biomedical applications.
Diamond Magnetic Sensor
In 2008 work taking place in the group of Wrachtrup – who was now at the University of Stuttgart, Germany – and in Mikhail Lukin and Ron Walsworth’s groups at Harvard University in the US, proposed and showed that diamond could be used to make a magnetic sensor, in which the brightness of the NV defects’ optical output depends on the strength of the magnetic field. If this defect is illuminated with a green laser, in response it will emit red light (fluoresce) with an interesting feature: its intensity varies depending on the magnetic properties in the environment. This unique feature makes the NV center particularly useful for measuring magnetic fields, magnetic imaging (MRI), and quantum computing and information.
In a diamond magnetic sensor, the luminescence collected does not have to be at a particular wavelength and therefore all emission in the phonon sidebands from 637 nm to 800 nm can be collected as signal. A diamond magnetic field sensor in principle has many advantages over other sensor technologies. For example, it is an intrinsic vector sensor by virtue of the fact that it is sensitive along the axis of the NV defect, which means that the four different NV orientations can be used to reconstruct a vector field. It also has a massive bandwidth, being sensitive to magnetic fields over several orders of magnitude, and – unlike other technologies such as vapour cells – it does not require special magnetic shielding.
There are also different modalities of NV magnetic sensor depending on how many defects are used in sensing. Due to the NV defects’ properties, strong electronic dipole luminescence from single ones can be easily measured, allowing magnetic fields to be measured on the nanometre scale. Competing technologies such as magnetic resonance force microscopy can also do this, but as they are intrinsically magnetic, they perturb the system they are trying to measure. Many groups are therefore using NV-based tools for material characterization, such as investigating magnetic materials that contain skyrmions. Start-ups such as Qnami, based in Switzerland, are trying to capitalize on these developments by selling ready-made diamond probes containing NV defects.
Using ensembles of NV defects can make the device more sensitive but lower its spatial resolution. As a compromise, a few microns of high-NV-defect-containing diamond on top of a high-purity diamond can be used to image magnetic fields where the thickness of the NV-containing layer determines the spatial resolution. This technique has been used to measure the magnetic signature from a meteorite, which could then be used to establish the magnetic field when the solar system was formed. Adding even more NV defects into the diamond to make a bulk diamond sample containing NV defects throughout can push sensitivities into the picotesla regime.
A diamond magnetometer with a handheld sensing head, with a sensitivity of 7 nT/Hz−−−√ and an ultimate noise floor of 3 nT/Hz−−−√ has newly developed
Lockheed Martin’s diamond magnetometer for a GPS does not rely on an external source that can be jammed. The system is currently the size of a shoebox but can be shrunk down to the size of hockey puck.
Using such a bulk magnetometer, a team at Lockheed Martin – the US-based aerospace company – has been developing a diamond magnetometer that can be used as an alternative GPS that does not rely on external signals. The technology works by using the vector capability of a diamond magnetometer to sense the strength and direction of Earth’s magnetic field. Given that Earth’s field varies depending on where you are on the surface, this fact can be used to position yourself without relying on an external source that can be jammed. Even early prototype systems have demonstrated this and, while not as accurate as satellite-based GPS, they will likely work alongside existing technology to provide redundancy.
The sensor can also be used in reverse to detect radio-frequency (RF) fields. In this configuration, a magnetic field gradient is placed across the NV-containing diamond in a controlled way, which then provides a known Zeeman shift of the energy levels. When a microwave signal of unknown frequency is applied, a magnetic resonance appears at the position that corresponds to that frequency. The big advantage of this approach is that you can measure across a whole frequency spectrum – more than tens of gigahertz – in one measurement and with high resolution. This technology could be used in 5G networks to prevent interference between neighbouring cell towers.
Student-designed experiment to measure Earth’s magnetic field at Space Station reported in Sep 2021
Oscar-Qube, short for Optical Sensors based on CARbon materials: QUantum Belgium, is an experiment developed by a group of students from the University of Hasselt, Belgium. Oscar-Qube’s mission is to create a detailed map of Earth’s magnetic field. It makes use of a new type of magnetometer that exploits diamond-based quantum sensing, meaning that it is highly sensitive, offers measurements to the nano scale, and has a better than 100-nanosecond response time.
These features combine to create a powerful experiment that, once in position, will allow it to map the Earth’s magnetic field to an unrivaled level of precision. Oscar-Qube is designed and built exclusively by the first student team to test a diamond-based quantum technology sensing device in space. They will go on to manage operations during its 10-month stay onboard the International Space Station.
Enhancing Magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) spectroscopy
Diamond-based quantum technology also has applications in the medical industry. A few groups around the world, as well as the start-up NVision based in Germany, are using diamond to enhance magnetic resonance imaging (MRI) – turning it from an anatomical to a molecular imaging modality similar to positron emission tomography (PET). The principle of the technology is to transfer the electronic spin from NV defects to the nuclear spin of a target molecule. The NV defects are placed in close contact with the target molecules and are then illuminated with green light, and a microwave source is also applied. Then, by using a series of microwave pulses, the spin can be transferred from the diamond to the target molecules’ nuclear spin. The nuclear spin lasts long enough to allow the molecules to be administered to the patient and the patient measured in an MRI, where now there is a high degree of spin polarization that gives high contrast in the MRI.
Diamond quantum sensor breaks new record
Thanks to a new measurement scheme that makes use of quantum sensors in diamond, the spectral resolution of nuclear magnetic resonance (NMR) spectroscopy has been increased by 100-fold in microscopic volumes. The breakthrough allows researchers to perform NMR chemical analysis at the scale of single biological cells for the first time.
NV centres act like tiny quantum magnets that are isolated from their surroundings and can be manipulated using laser pulses. They are ideal as biological probes because they are non-toxic, photostable and can easily be inserted into or placed adjacent to living cells and tissues. NV centres are capable of detecting the very weak magnetic fields from a single cell, molecule, or organism, as the intensity of the light they emit changes with the local magnetic field. They can thus be used as highly sensitive magnetic probes that can monitor local spin changes in a material over microscopic distances.
“This work reports the first experimental demonstration of NMR spectroscopy with full chemical specificity at the scale of a single biological cell, which has been a major scientific goal for the last 50 years,” says Ronald Walsworth of Harvard University, who led this research effort. “We use a new measurement scheme employing quantum sensors in diamond to realize a 100X improvement in NMR spectral resolution under ambient conditions for a sample volume comparable to that of a single cell – about 10 trillionths of a litre.” The quantum sensors used by Walsworth and colleagues are nitrogen vacancy (NV) colour centres in diamond. These defects occur when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site.
Broad NMR spectral lines
“Over the last few years, we and other research groups have been able to apply NV sensors to NMR of nanometre and micrometre volumes,” explains Walsworth. “But until now the measurement techniques produced broad NMR spectral lines (typically greater than 100 Hz), due to both the short spin state lifetime of the NV centre (around 3 ms) and the fluctuating statistical spin polarization of the sample. This spectral resolution is too coarse to resolve molecular structure fingerprints important in chemistry, structural biology and materials research.”
Micro-NMR spectral resolution reaches 1Hz
Walworth’s team says that it has now overcome these problems by using an ensemble of NV centres combined with thermal spin polarization of the sample and a narrowband synchronized readout measurement that can sense NMR signals for as long as 103 seconds. The new technique produces an NMR spectral resolution of about 1Hz in the sample volume of a typical cell (about 10 trillionths of a litre), which allows observation of the key spectral features needed for chemical analysis. According to the researchers, with further improvements in sensitivity, it might even allow for NMR spectroscopy of small molecules and proteins at the single-cell level.
Scientists demonstrate the simultaneously imaging technique of magnetic field and temperature using a wide-field quantum diamond microscope, reported in March 2021
The techniques such as thermo-reflectance microscopy, infrared microscopy (IR), micro-Raman spectroscopy and so on are widely used to measure temperature meanwhile magnetic force microscopy (MFM), superconducting quantum interference devices (SQUIDs), Hall effect sensors and others for the magnetic field measuring. However, the techniques cannot measure the magnetic field and temperature at the same time. At present, it is of great significance to simultaneously image magnetic fields and temperature in many aspects, e.g. material dynamics, carrier transport (nanorobot) and the chip detection.
Nowadays, solid-state electronic spins in diamond have garnered increasing relevance in sensing. In particular, a negatively charged nitrogen-vacancy (NV) color center in diamond has demonstrated competitive sensing sensitivity in ambient room temperature environments and employed in various applications benefiting from its long spin coherence times at room temperature and the optical initialization and readout of NV spin states by precisely and optically detected magnetic resonance (ODMR). For magnetic fields and temperature sensing, techniques based on NV color centers have been shown outstanding sensitivity and spatial resolution and developed for biology, current wires, paramagnetic molecules and so on.
In addition, measuring not only the value of the field intensity but also the full intensity image of the sample is particularly valuable. The technique combined ODMR and imaging can not only support a combination of superior spatial resolution, sensing sensitivity, and wide field of view but also the ability to produce a full-field image of the sample. Also, the imaging technique using spins in diamond does not need prolonged data acquisition as MFM and can be performed at room temperature compared with electron and x-ray microscopy, SQUID, and so on. These advantages make it a powerful imaging tool for magnetic field and temperature detection especially in the situation that wide-field, high-spatial resolution and real-time are needed.
Enhancing the quantum sensing capabilities of diamond
In order to produce optimal magnetic detectors, the density of these defects should be increased without increasing environmental noise and damaging the diamond properties. Researchers have discovered that dense ensembles of quantum spins can be created in diamond with high resolution using an electron microscopes, paving the way for enhanced sensors and resources for quantum technologies. Scientists from the research group of Nir Bar-Gill at the Hebrew University of Jerusalem’s Racah Institute of Physics and Department of Applied Physics, in cooperation with Prof. Eyal Buks of the Technion – Israel Institute of Technology, have shown that ultra-high densities of NV centers can be obtained by a simple process of using electron beams to kick carbon atoms out of the lattice.
This work, published in the scientific journal Applied Physics Letters, is a continuation of previous work in the field, and demonstrates an improvement in the densities of NV centers in a variety of diamond types. The irradiation is performed using an electron beam microscope (Transmission Electron Microscope or TEM), which has been specifically converted for this purpose. The availability of this device in nanotechnology centers in many universities in Israel and around the world enables this process with high spatial accuracy, quickly and simply.
The enhanced densities of the NV color centers obtained, while maintaining their unique quantum properties, foreshadow future improvements in the sensitivity of diamond magnetic measurements, as well as promising directions in the study of solid state physics and quantum information theory. “This work is an important stepping stone toward utilizing NV centers in diamond as resources for quantum technologies, such as enhanced sensing, quantum simulation and potentially quantum information processing”, said Bar-Gill, an Assistant Professor in the Dept. of Applied Physics and Racah Institute of Physics at the Hebrew University, where he founded the Quantum Information, Simulation and Sensing lab. “What is special about our approach is that it’s very simple and straightforward,” said Hebrew University researcher Dima Farfurnik. “You get sufficiently high NV concentrations that are appropriate for many applications with a simple procedure that can be done in-house.”