  Realization of devices based on quantum laws might lead to building
processors that outperform their classical analogues and establishing
unconditionally secure communication protocols. Solids do usually present a
serious challenge to quantum coherence. However, owing to their spin-free
lattice and low spin orbit coupling, carbon materials and particularly diamond
are suitable for hosting robust solid state quantum registers. We show that
scalable quantum logic elements can be realized by exploring long range
magnetic dipolar coupling between individually addressable single electron
spins associated with separate color centers in diamond. Strong distance
dependence of coupling was used to characterize the separation of single qubits
98 A with unprecedented accuracy (3 A) close to a crystal lattice spacing. Our
demonstration of coherent control over both electron spins, conditional
dynamics, selective readout as well as switchable interaction, opens the way
towards a room temperature solid state scalable quantum register. Since both
electron spins are optically addressable, this solid state quantum device
operating at ambient conditions provides a degree of control that is currently
available only for atomic systems.
