Diamonds are quantum computing's best friend
Quantum information processing exploiting quantum entanglement, superposition and qubits will break the barrier to computational power and pave the way to exciting new devices. EU-funded scientists initiated the project 'Exploring quantum entanglement using spins in diamond' (EQESD) to develop a novel experimental system for the study of long-distance quantum entanglement, a field still in its infancy at the time of the project proposal. The experimental paradigm will enable rapid advancements toward the large-scale quantum information protocols of the future.Two-state quantum systems can be used to represent information much as bits of 0 and 1 do. However, with superposition of the two states, the system can be in two states at the same time, a defining characteristic of the qubit. An infinite number of possible states together with quantum entanglement (subtle, non-local correlations among the parts of the system) forms the basis for much more powerful information processing than currently possible. EQESD exploited a special kind of defect, nitrogen-vacancy centres, in diamonds, whose electron spins can be controlled and that exhibit photoluminescence. The objective was to study quantum entanglement between a single spin and a single photon and between two spins.The team optically excited a nitrogen-vacancy centre in a superposition of spin states, causing it to spontaneously emit a photon entangled with the nitrogen-vacancy centre's spin. Creation and detection of the entanglement required exquisite control and measurement of single spins, techniques whose development resulted in two publications in peer-reviewed scientific journals.Perhaps the milestone of the one-year project was demonstration of the potential for qubit control via initialisation, coherent manipulation and single-shot readout in a single experiment on a two-qubit register, a result published in Nature. A conventional two-bit register can represent only one of four states (00, 01, 10 and 11) at any given time whereas a two-qubit register can store all four numbers simultaneously. The real power comes in as the qubit number increases because storage capacity increases exponentially with the qubit number. In its short one year of activity, EQESD successfully demonstrated that nitrogen-vacancy centres in diamonds are potential building blocks of large-scale quantum information processing protocols.
