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synthetic diamond emerging as unique material for quantum information processing

In their experiment, 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.

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.”

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.

Chinese diamond experiment may help crack one of the world’s toughest code

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.

And the diamond provided other advantages, according to Xu. 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.


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

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.

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.

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 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.

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.


Diamonds might power the next generation of Quantum computing

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.


Quantum Entanglement

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.

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


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