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.
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.
Diamonds for Quantum computing and Quantum Sensing
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.
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.
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.
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.
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.
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.
Now, 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.”
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
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.
Mass producing Diamonds
Scientists have developed a way to mass-produce tiny diamond crystals shaped like needles and threads, which may power next generation of quantum computing. Physicists from the Lomonosov Moscow State University in Russia have described structural peculiarities of micrometre-sized diamond crystals in needle and thread-like shapes, and their interrelation with luminescence features and field electron emission efficiency.
“The proposed technique involves determining formation of polycrystalline films from crystallites of elongate (columnar) shape,” Alexander Obraztsov, professor at the Lomonosov Moscow State University. For instance, ice on a surface of a lake often consists of such crystallites, which can be observed while melting,” said Obraztsov.
The field of synthetic diamond science is moving very quickly, requiring us to develop CVD techniques that produce exceptionally pure synthetic diamond material with nano-engineering control,” said Bruce Bolliger, head of sales and marketing at Element Six Technologies.