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Entanglement is the central yet fleeting phenomenon of quantum physics. Once being considered a peculiar counter-intuitive property of quantum theory, it has developed into the most central element of quantum technology. Consequently, there have been a number of experimental demonstrations of entanglement between photons, atoms, ions and solid-state systems such as spins or quantum dots, superconducting circuits and macroscopic diamond. Here we experimentally demonstrate entanglement between two engineered single solid-state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground-state spin correlation. Entanglement (fidelity = 0.67±0.04) is proved by quantum state tomography. Moreover, the lifetime of electron spin entanglement is extended to milliseconds by entanglement swapping to nuclear spins. The experiments mark an important step towards a scalable room-temperature quantum device being of potential use in quantum information processing as well as metrology. [ABSTRACT FROM AUTHOR]

We investigate the effect of surface termination on the charge state of nitrogen-vacancy (NV) centers, which have been ion-implanted a few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV- centers convert into NV0 and, subsequently, into an unknown nonfluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the disappearance of NV- in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface conductive layer, leads to a depletion of electrons in the nitrogen-vacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way to control the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to electrostatic control of the charge state with the use of an external gate electrode. [ABSTRACT FROM AUTHOR]