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Article By : Maurizio Di Paolo Emilio

The performance of quantum states offers excellent potential for highly sensitive sensors to measure a range of variables such as magnetism, temperature, and electric fields. This potential is limited because the quantum states (manifested as quantum bits, or qubits) need to be isolated and cooled to optimize the measurement, which poses a control challenge for engineers. Diamond is a material that, thanks to its unique properties, could be used to solve some of these challenges.

In an interview with EE Times, Daniel Twitchen, chief technologist at Element Six (E6), explained that advances in the company’s chemical vapor deposition (CVD) diamond growth process are paving the way for diamond to realize its potential as a solution.

Element Six, part of the De Beers Group, began offering a general-purpose quantum-grade CVD diamond in June 2020. E6 highlighted that this specialized diamond, which it calls DNV-B1, is an appropriate starting material for those interested in researching nitrogen-vacancy (NV) ensembles for quantum demonstrations, masers, detection of RF radiation, gyroscopes, sensing and further emerging applications, ranging from GPS-denied navigation to medical imaging.

“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 Twitchen.

Diamond technology
Advances in quantum mechanics have already led to innovations such as lasers and transistors. The next wave of quantum technology will result from the manipulation of quantum characteristics such as superposition and entanglement.

The extreme fragility of qubits creates challenges, however. Fragility is the fine line of interaction that needs to be mastered in order to control error occurrence and measurements, taking full advantage of this exciting technology. Ideally, quantum states would be isolated from their surroundings, but measurement requires some degree of external interaction.

A wide range of different technological solutions are being investigated for these new applications, such as trapped ions, superconductors, quantum dots, photons and defects in semiconductors. Trapped ions are challenging to integrate, while superconducting circuits only work at cryogenic temperatures. Twitchen pointed out that diamond, being in the solid-state, easily integrates, solving some of the quantum challenges, it is also biocompatible and delivers high spatial resolution magnetic imaging. Moreover, it can be used at room or body temperature, thus removing the need for vast cooling equipment.

“Scientists and engineers have addressed many of these challenges already, enabling these quantum spins to be used effectively as atomic scale magnetic compasses measuring fields more than 1000x lower than the earth’s, with spatial resolution than can approach nanometers,” Twitchen said. “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. Based on this biological premise, diamond’s biocompatibility is opening up new applications in pharmaceutical and medical science, from drug development to early disease diagnosis.

“The ability to now make synthetic diamond with exceptional purity has unlocked the intrinsic qualities that make it the perfect host material for solid-state qubits,” Twitchen continued. He added, “a series of pioneering academic studies, initially at the Universities of Stuttgart and Harvard, demonstrated that NV color centers have a quantum spin which can be manipulated and read out at room temperature using simple, low-cost optical techniques, which generate exceptional quantum properties. NV centers are created by removing two adjacent carbon atoms from a diamond molecule and replacing one of them with a nitrogen atom, leaving a vacancy in the center.”

The NV has an electron spin that is highly sensitive to magnetic fields, forming the basis for sensitive magnetometry. The electron spin can be detected and aligned by switching on a green LED on the material and measuring the intensity of the red fluorescence emitted. “It has been shown that NV electron spins can store quantum information for over 1s at room temperature,” said Twitchen.

Quantum Diamond Technologies, Inc (QDTI) is a start-up that has spun out from Harvard University to make point-of-care diagnostics possible for diseases which require ultrasensitive detection of proteins in the early stages, such as heart conditions, cancer and Alzheimer’s. QDTI is exploiting diamond-enabled quantum systems, leveraging NV centers as the engine that will power our novel approach to biomolecule detection.

 

Medical
Many current medical imaging solutions, such as the superconducting magnets used in magnetic resonance imaging (MRI), require cryogenic cooling systems, only viable at the largest research hospitals.

“Diamond magnetometry typically relies on a large ensemble of NV centers to increase magnetic sensitivity, offering higher spatial resolution, all while operating at room temperature. The reduction in size and the biocompatibility of diamond allow sensors to be moved closer to, or in contact with, the biological specimen (e.g. the patient’s skin). These unique properties are ideal for medical diagnostic techniques such as magnetocardiography (MCG), which measures the magnetic fields produced by electrical currents in the heart. Why is this important? Heart conditions are the leading cause of death worldwide. Low cost and fast response diamond-enabled quantum magnetometers could allow health professionals to detect heart diseases quicker and more accurately at the point-of-care, leading to faster diagnosis and reduced discharge times for patients” said Twitchen.

Moreover, Twitchen highlighted how, when seeking to diagnose illnesses in patients, it can be crucial to measure the level of biomarkers, such as proteins, nucleic acids and cells, in a fluid sample such as their blood or saliva. “These immunoassays are currently analyzed by labeling the biomarker of interest with a tag, such as a green fluorescent protein, followed by imaging the sample under a microscope to count the tagged biomarkers. This versatile and successful approach is behind a global market worth over $18bn annually. However, immunoassays with fluorescent tags suffer from unwanted background auto-fluorescence from biomolecules, and removing this auto-fluorescence requires a lengthy process of washing and sample processing before imaging can take place.”

As biological samples have a low magnetism, immunoassays can also be measured with a magnetic tag instead of a fluorescent tag. “Diamond containing NV centres (DNV) delivers excellent magnetic images from this method, with no need for washing. Single-cell resolution has been demonstrated by QDTI with wide-field DNV magnetic microscopy over a field-of-view of ~1 mm² with a one-minute imaging time used to spot the magnetically labeled cells. This enables new methodologies for isolating and counting key biomarkers at high sensitivity, in small systems, with faster processes” said Twitchen.

Twitchen emphasized that the user’s main interest lies not in the technology itself, but rather in its ease of use and the value it creates in making everyday life easier and better. The aim is to unleash the potential of diamond quantum sensors to make it simple and reliable to use. “Specifically, efficient and robust integration of diamond sensors with optical excitation, control electronics and read out integrated with the relevant existing technologies will need to evolve. A major challenge for diamond engineers is to integrate their solution into existing supporting technologies. Technological revolutions often require user behavior to change, but for that change to happen, the related benefits have to be sufficiently significant. It is exciting to see how, in all the potential applications discussed, the concepts have rapidly gone from an idea to prototypes, which can now be explored and tested by end-users,” said Twitchen.

Twitchen pointed out that many start-ups are working with quantum effects using diamond, including NVision, Qnami, QZabre, QDM.IO and QDTI.

A further barrier to device development is the learning curve needed to get the most out of the NV defect, which requires specialist knowledge in the field of materials, lasers, microwave and quantum.  The ultimate challenge is to make it available in the most practical markets by optimizing the manufacturing and engineering processes.

This article was originally published on EE Times.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.

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