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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.
Quantum computing and quantum information processing are next revolutionary technology expected to have immense impact. Quantum computers will be able to perform tasks too hard for even the most powerful conventional supercomputer and have a host of specific applications, from code-breaking and cyber security to medical diagnostics, big data analysis and logistics.
Currently 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. Each technical solution has different pros and cons. Trapped ions have exquisite quantum properties but are challenging to integrate, whereas circuits of superconductors can be fabricated but can only operate at cryogenic temperatures. This is where materials like diamond come into play as they offer a compromise by being solid-state – making it easier to integrate into devices – and operational at room temperature.
Diamond has recently emerged as a unique material for quantum information processing. In particular, Nitrogen-Vacancy (NV) centers in diamond exhibit quantum behavior up to room temperature. 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.
Quantum physicists at Harvard University are currently developing synthetic diamond-based quantum computer technology that could enable faster data processing and secure communication. Delft University in the Netherlands have also established that diamond spin qubits are a prime candidate for the realization of quantum networks. An experiment in China using diamonds has put quantum code-breaking a step closer to reality, threatening to one day break the digital encryption technologies that safeguard banks, governments and the military.
As part of a current project, Ronald Hanson’s group at the Delft University of Technology in the Netherlands is using the NV defect in diamond as a “quantum repeater node” in a 100% secure quantum internet. In such a network, the nodes are quantum mechanically entangled to build up a chain from the source to the receiver so that quantum information can be transmitted over large distances. Such a demonstration is a challenging target, but there are also many nearer-term applications using the fragility of the quantum states.
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.
Building on these achievements, a 10-qubit register that can store quantum information for up to 75 s has recently been demonstrated. As part of a current project, Ronald Hanson’s group at the Delft University of Technology in the Netherlands is using the NV defect in diamond as a “quantum repeater node” in a 100% secure quantum internet. In such a network, the nodes are quantum mechanically entangled to build up a chain from the source to the receiver so that quantum information can be transmitted over large distances. Such a demonstration is a challenging target, but there are also many nearer-term applications using the fragility of the quantum states.
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.
Diamonds for Quantum computing
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 reasons why diamond provides such a wonderful host to quantum defects come from its crystal structure. For example, diamond is a wide band-gap material, meaning that it can host a range of defects with transition energies in the optical regime, enabling the defects to be manipulated with readily available lasers. As carbon has a low atomic mass and very stiff interatomic bonds, it has a high Debye temperature (the temperature of a crystal’s highest normal mode of vibration), which makes the interaction of the NV centre with the vibrational modes of the surrounding lattice unusually weak, even at room temperature. Diamond also has a naturally low concentration of nuclear spins (carbon-12 has a nuclear spin of 0 and there is only 1.1% carbon-13 (spin –1/2) in diamond). This reduces the likelihood of the quantum states “decohering”, when the spin is no longer in the desired state. Quantum states can also decohere via spin-orbit coupling – a relativistic effect where the spin of a charge interacts with its orbital motion. But because the NV defect has weak spin-orbit coupling, there is limited decoherence and the spin state lasts for longer. Together, these properties mean that it is possible to fabricate a diamond with a spin decoherence time of milliseconds at room temperature. And as well as diamond being a good host for spin defects, the NV centre is also particularly special in that its electronic energy-level structure means that the electronic spin associated with it can be manipulated simply by shining green light on it.
Nitrogen Vacancy (NV) color centers exhibit remarkable and unique properties, including long coherence times at room temperature (~ ms), optical initialization and readout, and coherent microwave control. NV centers have several advantages over other candidate qubits. They’re an intrinsic feature of a physical structure, so they dispense with the complex hardware for trapping ions or atoms that other approaches require. And the diamond provided other advantages, according to Dr Xu Kebiao, first author of the paper, whose team’s diamond device could factor certain types of numbers of six digits or even higher. Existing prototype quantum computers are extremely sensitive to disturbance from outside environments such as heat and electromagnetic interference. They needed to be kept in liquid helium for extremely low temperature, or heavily shield rooms. “Our device just sits out in the open in the laboratory. It works in room temperature. We do not even bother to turn off the Wi-fi,” Xu said.
And NV centers are natural light emitters, which makes it relatively easy to read information from them. Indeed, the light particles emitted by an NV center may themselves be in superposition, so they provide a way to move quantum information around. 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.
Diamond devices
One of the benefits of a diamond-based quantum device is its simplicity. A basic device can be fabricated from a green light source, a diamond, a small microwave source and a photodetector. This is because effective optical initialization and the readout process of NV spins do not require specialized narrow-linewidth lasers – even a simple green LED can be used. Furthermore, because of the wavelength of light being detected (637–800 nm), low-cost, off-the-shelf silicon photodetectors can be used. An offset magnet is also used to provide a field of a few millitesla, which separates the energy levels of the four different possible crystallographic orientations of the NV defect, allowing them to be probed independently. Lastly, the microwave frequencies that are used are roughly 2880 MHz. All of these components can be bought off the shelf for a few thousand pounds, and instructions to build such a room-temperature quantum device are readily available and suitable for a first-year physics degree demonstration (American Journal of Physics 86 225). However, while this set-up shows the principles of operation, it clearly does not show the enhancements in performance you would expect from a quantum device.
Professor Duan Changkui, another researcher involved in the experiment, said many technical challenges had to be overcome before the device could be used to break a code. These problems ranged from precise control of particles to better diamonds. The artificial diamonds must be extremely pure, and their nitrogen-vacancy centres perfectly aligned. The manufacturing process is very difficult,” he said.
Despite these desirable features, the NV defect in diamond is not perfect. In an ideal world, all the photons emitted would be at 637 nm for them to be quantum mechanically indistinguishable. However, most of the photons emitted are at different wavelengths, from 637 nm to 800 nm, due to phonon interactions, which provides a challenge for some applications. Other difficulties are that diamond is not as easily processed as materials such as silicon – it is much harder to etch structures in diamond to improve optical collection from the defects, and high-purity single-crystal diamond has only been made up to around 10 mm2. These limitations of diamond and the NV defect have led to scientists looking for alternative defects in diamond and in other wide band-gap materials such as silicon carbide (SiC) and zinc oxide (ZnO). However, while a few other defects in diamond and other materials have been identified to have useful properties, no-one has found one that rivals diamond’s NV centres.
Chinese diamond experiment may help crack one of the world’s toughest code
Quantum physicists in Hefei, Anhui province, reportedly broke down the number 35 into its factors – the numbers five and seven – on a new type of quantum computing device built inside a diamond. The process, known as factorisation, is the key to cracking the most popular digital algorithm used in encryption today. The research was led by quantum physicist Professor Du Jiangfeng at the University of Science and Technology of China, and details of the results were published in the journal Physical Review Letters in March.
In the experiment, laser and microwave beams were fired at particles trapped inside the diamond’s “nitrogen-vacancy centre”, a tiny space ideal for subatomic interaction. The particles came up with the solution in two microseconds, less than half of the time it takes for lightning to strike. Speed is key to code-cracking and quantum computers have the potential to dramatically cut the time needed to break an encryption thanks to a phenomenon called entanglement. Dr Xu Kebiao, first author of the paper, said the team’s diamond device could factor certain types of numbers of six digits or even higher. “And it is scalable, which is a huge advantage of our system,” he said. By summoning more entangled particles and creating more nitrogen-vacancy centres in the diamond, the quantum device may eventually harness enough capacity to outperform conventional computers.
The work in Hefei has caught the attention of cryptographers like Gao Jundao. Gao is an associate professor of cryptography at Xidian University in Xian, Shaanxi, and writes algorithms for the defence industry. “[The finding] is code-breaking, strictly speaking, albeit still in its infancy,” Gao said. “It is no doubt a breakthrough.” “[The finding] is code-breaking, strictly speaking, albeit still in its infancy,” Gao said. “It is no doubt a breakthrough.”
In 2012, Du’s team set a record by factorising the number 143, but it was achieved with nuclear magnetic resonance technology in a liquid, a medium not easy to scale up for practical applications.
Two years later, a multinational team of researchers from Japan, Britain and Microsoft set a new record by breaking down the number of 56,153 using the same technology. But for the first time, the Chinese experiment factorised a number in a setting built entirely on solid material, making the system more stable.
Feedback technique used on diamond ‘qubits’ could make quantum computing more practical
MIT researchers describe a new approach to preserving superposition in a class of quantum devices built from synthetic diamonds. The work could ultimately prove an important step toward reliable quantum computers.
In the Nature paper, Cappellaro and her former PhD student Masashi Hirose, describe a feedback-control system for maintaining quantum superposition that requires no measurement. “Instead of having a classical controller to implement the feedback, we now use a quantum controller,” Cappellaro explains. “Because the controller is quantum, I don’t need to do a measurement to know what’s going on
Like electrons, atomic nuclei have spin, and Cappellaro and Hirose use the spin state of the nitrogen nucleus to control the NV center’s electronic spin. First, a dose of microwaves puts the electronic spin into superposition. Then a burst of radio-frequency radiation puts the nitrogen nucleus into a specified spin state. A second, lower-power dose of microwaves “entangles” the spins of the nitrogen nucleus and the NV center, so that they become dependent on each other.
Because the spins of the nitrogen nucleus and the NV center are entangled, if anything goes wrong during the computation, it will be reflected in the spin of the nitrogen nucleus. After the computation is performed, a third dose of microwaves—whose polarization is rotated relative to that of the second—disentangles the nucleus and the NV center. The researchers then subject the system to a final sequence of microwave exposures. Those exposures are calibrated, however, so that their effect on the NV center depends on the state of the nitrogen nucleus. If an error crept in during the computation, the microwaves will correct it; if not, they’ll leave the NV center’s state unaltered.
In experiments, the researchers found that, with their feedback-control system, an NV-center quantum bit would stay in superposition about 1,000 times as long as it would without it.
Single photon generation
To achieve the secure data transmission by quantum cryptography, individual photons of known wavelengths must be used but are difficult to generate. Pure diamonds are naturally colorless, but gaps in the crystal structure or impurities of other elements can create colors and even emit fluorescence. Recently, researchers have shown that the fluorescent lattice defects could be useful as single photon sources for quantum cryptography and as bright luminescent makers in living cells.
Now, Takayuki Iwasaki and co-workers at Tokyo Institute of Technology (Tokyo Tech), together with scientists across Japan and Germany, have demonstrated a new type of diamond crystal defect that fluoresces to produce single photons in a narrow, high energy wavelength band. The defects, which have been named germanium-vacancy (GeV) centres, are relatively easy to fabricate in a reliable, reproducible way
Quantum Entanglement
In 2013, the collaboration of Element Six and Researchers from Delft University of Technology led by professor Ronald Hanson, successfully entangled qubits in two separated synthetic diamonds over a large distance of 1.3 km. These diamonds contained a particular defect that can be manipulated using light and microwaves. This defect consists of a single nitrogen atom adjacent to a missing carbon atom, known as a nitrogen vacancy (NV) defect. The light emitted from the NV defect allows the quantum properties to be “read-out” using an optical microscope.
By forming small crystallographically aligned lenses around the NV defect and carefully tuning the optical emission through electric fields, the Delft team was able to make the two NV defects emit indistinguishable particles of light (photons). These photons contained the quantum information from the NV defects and via further manipulation, the team was able to quantum mechanically entangle the two defects over a distance of 1.3 km.
In the Delft experiment, two diamonds were placed in labs on opposite sides of the university campus, with each containing an electron trapped in the diamond’s nitrogen vacancy. The team then hit the diamonds with microwave pulses and laser light, causing each electron to emit a photon entangled with the electron’s magnetic spin. The photons then traveled to a third location in between the two labs where photo detection heralded generation of entanglement. In such cases, the distant electrons’ spins were independently measured in a randomly chosen direction. After 245 measurements, the labs detected more highly correlated spins than local realism would allow – closing the loopholes.
