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Posted: Jun 07, 2012
Using synthetic diamond, collaboration sets a new quantum information record
(Nanowerk News) Element Six, the world leader in synthetic diamond supermaterials, working in partnership with academics in Harvard University, California Institute of Technology and Max-Planck-Institut für Quantenoptik, has used its Element Six single crystal synthetic diamond grown by chemical vapour deposition (CVD) to demonstrate the capability of quantum bit memory to exceed one second at room temperature.
This study demonstrated the ability of synthetic diamond to provide the read-out of a quantum bit which had preserved its spin polarisation for several minutes and its memory coherence for over a second. This is the first time that such long memory times have been reported for a material at room temperature, giving synthetic diamond a significant advantage over rival materials and technologies that require complex infrastructure which necessitates, for example, cryogenic cooling.
The versatility, robustness, and potential scalability of this synthetic diamond system may allow for new applications in quantum information science and quantum based sensors used, for example, in nano-scale imaging of chemical/biological processes.
Using synthetic diamond, Element Six and Harvard University have set a new room temperature quantum information storage record of more than one second -- a thousand times longer than previously recorded. These findings may lead to extremely powerful quantum computers in the future, and novel sensors based on quantum processes in the near term.
The synthetic diamond technical work was completed by the Element Six synthetic diamond R&D team based at Ascot in the UK who developed novel processes for growing synthetic diamond using chemical vapour deposition (CVD) techniques. Steve Coe, Element Six Group Innovation Director, explained the success of the collaboration:
"The field of synthetic diamond science is moving very quickly and is requiring Element Six to develop synthesis processes with impurity control at the level of parts per trillion – real nano-engineering control of CVD diamond synthesis. We have been working closely with Professor Lukin's team in Harvard for three years - this result published in Science is an example of how successful this collaboration has been."
Professor Mikhail Lukin of Harvard University's Department of Physics described the significance of the research findings:
"Element Six's unique and engineered synthetic diamond material has been at the heart of these important developments. The demonstration of a single qubit quantum memory with seconds of storage time at room temperature is a very exciting development, which combines the four key requirements of initialisation, memory, control and measurement. These findings might one day lead to novel quantum communication and computation technologies, but in the nearer term may enable a range of novel and disruptive quantum sensor technologies, such as those being targeted to image magnetic fields on the nano-scale for use in imaging chemical and biological processes."
The findings represent the latest developments in quantum information processing, which involves manipulating individual atomic sized impurities in synthetic diamond and exploiting the quantum property spin of an individual electron, which can be thought of classically as a bar magnet having two states: up (1) and down (0). However, in the quantum mechanical description (physics of the very small), this quantum spin (qubit) can be both 0 and 1 simultaneously. It is this property that provides a framework for quantum computing, but also for more immediate applications such as novel magnetic sensing technologies.
The quantum information research collaboration
The study was a collaboration between the following organisations:
Element Six, Ascot, UK
Department of Physics, Harvard University, Cambridge, MA, USA
Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA, USA
Max-Planck-Institut für Quantenoptik, Garching, Germany
School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
Funding for some of this research was provided by the DARPA QuASAR programme.
Key to the material aspect of achieving this result was producing synthetic diamond with essentially no spin impurities other than a very specific defect called the N-V (nitrogen vacancy) centre (a vacancy next to a nitrogen atom in the diamond lattice). This 'N-V centre' has very specific properties in that it can be spin polarised (similar to the magnetic difference of North-South or South-North of a bar magnet at room temperature) using a simple green light source. Subsequently, the state of the N-V centre can be read out again using simple techniques within a period limited by the quantum de-coherence time.
The isotope carbon-12 forms 98.9% of the carbon usually found in synthetic diamond, while carbon-13 forms the remaining 1.1%. Carbon 13 has a nuclear spin, which through random thermally driven interactions can interact with the electronic spin of the N-V impurity. Removing as many of these nuclear spins while still maintaining the general high purity of the material was a milestone result for the Element Six CVD R&D team.
Specific to Harvard's breakthrough was to use a carbon-13 nuclear spin (that was still present) to couple with the N-V electronic spin. While the electron spin has a good de-coherence time, it still fluctuates on the millisecond timescale. Once the electron spin changes its spin, the quantum information (qubit) is lost. A single flip in the electronic spin completely destroys the coherence of the carbon-13 nuclear spin. To prevent the electron flips from affecting the nucleus, the Harvard team reset the electron's spin with green laser light, essentially turning off the interaction between the electron and nucleus when that interaction is not needed. This had the result of creating very fast electron flips which do no interact with the nuclear spin – effectively a non-fluctuating average field.
In combination with this method, the Harvard research team used a sequence of radio-frequency pulses to suppress interactions with other carbon nuclei in the synthetic diamond. As a result, they were able to store quantum information at room temperatures for nearly two seconds, which was significantly more than anticipated when the research commenced. Previous experiments in quantum information have generally demonstrated single qubit memory storage times to be in microseconds.