  Entanglement is the central yet fleeting phenomena 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 providing speed
up to quantum computers, a path towards long distance quantum cryptography and
increased sensitivity in quantum metrology. Consequently, there have been a
number of experimental demonstration of entanglement between photons, atoms,
ions as well as solid state systems like 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 \pm 0.04) is proven by
quantum state tomography. Moreover, the lifetime of electron spin entanglement
is extended to ms by entanglement swapping to nuclear spins, demonstrating
nuclear spin entanglement over a length scale of 25 nm. 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.
