Diamond-based quantum physics frontier technology

In this article, Daniel Twitchen and Matthew Markham explain why the most attractive allotrope of carbon may become the best friend of quantum physicists.

Quantum defect

In the 20th century, many technologies that changed the world were based on quantum mechanics, including semiconductors, lasers, and other devices that are now ubiquitous. However, in the entire quantum revolution, a key feature of quantum physics—superimposed state—still exists in the laboratory to a large extent, rather than studying its possible applications. It is purely due to the curiosity of scientists.

However, this is about to change due to some important initiatives aimed at achieving the second quantum revolution. The key to the success of this revolution is the ability to design and control qubits “easily”. For the word "easy", we must be very cautious, because initializing the quantum state and staying in the superimposed state for a long time is a daunting task. Scientists are experimenting with many different methods, using a variety of materials to compete in related areas, such as superconductors, synthetic diamonds, cold atoms and quantum dots. However, whether it is for quantum computing or other applications, such as magnetic field induction, diamond does have some attractive advantages.

"short" to long

There is a defect in the diamond that attracts the quantum quasi-revolutionaries, which is located in the regular carbon atom lattice. This defect consists of a single nitrogen atom combined with a missing carbon atom or a vacancy. Among other properties, the nitrogen vacancy (NV) center has unique optical absorption and emission properties that give diamonds a red to pink color—a feature that has long been the focus of fundamental research in crystal structure.

In addition to its unusual optical properties, the NV center in the negative charge state also has an electron spin S = 1 in its ground state. It is worth noting that the electron spins in this state can be controlled and read out at room temperature. The reason is that, unlike most materials, the lattice in diamond forms a low-noise environment, so it does not lose fragile quantum properties and can store and detect information for longer periods of time. When the system is excited by microwave radiation, the spin state can be read by measuring the intensity of light emitted by the center of the NV. At the resonant frequency of 2.88 GHz at the NV center, the spin state will flip from 0 to +1 or -1, causing a dip in the intensity of the emitted red light.

The robustness and readability of this spin state make NV diamond a very promising platform for various quantum technologies. Potential applications include (quantum) secure communication, (quantum) computing, (quantum) imaging and Quantum) sensing and so on. One of the key areas of research in the diamond industry recently is the use of NV defects to measure magnetic fields. Due to the Zeeman interaction, the gap between the frequencies of the 0→1 and -1→0 microwave transitions in the NV diamond increases as the magnetic field increases. Therefore, in the simplest case, the magnitude of the magnetic field can be estimated by exposing the NV center to microwaves of different frequencies and measuring the (frequency) spacing between the two sag of the illuminance. It is worth noting that this type of measurement can be performed essentially at room temperature using a single NV center. For multiple NV centers, the geometrical nature of the diamond lattice means extremely sensitive measurements of the direction and magnitude of the magnetic field.

(NV diamond based) precision engineering technology

raw material

        Of course, there are many techniques for estimating magnetic fields. These technologies include superconducting quantum interference devices (SQUID), steam chambers, fluxgate sensors, and Hall effect sensors that make up the compass in modern smartphones. However, SQUID-based magnetometers must be cryogenically cooled (cooling equipment) making such magnetometers relatively bulky and increasing operating costs. Other sensor technologies require frequent recalibration and can only measure varying magnetic fields within a limited frequency bandwidth. In contrast, NV diamond-based sensors do not require recalibration and have a wide bandwidth that can be integrated into lightweight, low-power devices. Importantly, due to the high spatial resolution of the microprobe, the NV center can be used to construct a magnetic field map on the surface of the material. For these reasons, diamond-based magnetometers can be used as both a replacement for the prior art and a new technology.

However, to make these applications a reality, we need off-the-shelf high quality NV diamonds. NV centers are rare in natural diamonds, and if they are limited to a single sample, it is difficult to conduct extensive research. We can synthesize NV diamond by chemical vapor deposition (CVD). The process involves filling the microwave chamber with a mixture of hydrogen, methane and nitrogen and heating it to 2500-3000 K to create a plasma. The carbon atoms from the plasma are deposited layer by layer on the diamond "seed" surface in the chamber, which eventually becomes the core of the new diamond. Hydrogen stabilizes the surface, promotes the deposition of carbon atoms to form diamond rather than graphite, and nitrogen acts as a dopant, making it possible to form NV centers.

This process allows us to grow diamonds in a controlled and scalable manner with purity far exceeding that of natural diamonds. It also controls the number of NV centers. Under high purity conditions, a small amount of NV center is produced by chemical action during growth. Because these isolated vacancies can be detected separately in experiments, this type of NV diamond is well suited for quantum computing. Magnetic induction applications require more NV centers, a goal that can be achieved by increasing the nitrogen concentration in the synthesis process and then bombarding the crystal with high-energy electrons to create additional vacancies. Heating the diamond to 800 °C causes these vacancies to migrate through the crystal lattice until they encounter a nitrogen atom to stop migration; at this point, the NV center has a lower potential energy than the nitrogen and vacancies alone, so this structure will change It is very stable.

Potential application

Diamond quantum technology is very promising, and many applications are already in the conceptual verification phase. These include applications in material characterization, such as nanoscale imaging of write heads for next-generation magnetic hard disk drives, and bio-imaging. New pressure and temperature sensing methods, as well as the possibility of diamond-based quantum computing, have made this field of research exciting and fruitful.

We believe that diamonds will continue to be a useful tool for us to understand the quantum world. What is really exciting, however, is the technology that can be achieved based on this understanding. At the end of 2016, a group of researchers led by Ron Walsworth of Harvard University in the United States used NV centers in diamond to study neuronal activity in marine worms and measured tiny magnetic pulses of individual neurons with high spatial resolution. No other prior art can measure with such high sensitivity and resolution; the maximum spatial resolution of standard magnetic resonance imaging is about 1 cubic millimeter. Theoretically, diamond-based magnetic field induction can provide us with the cellular level during chemical reactions ( Translator's Note: Image between 1-100 microns). With this proof-of-principle experiment, we hope to achieve breakthroughs in understanding how the brain works, and to develop new diagnostic and therapeutic options.

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