  We demonstrate quantum interference between indistinguishable photons emitted
by two nitrogen-vacancy (NV) centers in distinct diamond samples separated by
two meters. Macroscopic solid immersion lenses are used to enhance photon
collection efficiency. Quantum interference is verified by measuring a value of
the second-order cross-correlation function $g^{(2)}(0) = 0.35 \pm 0.04<0.5$.
In addition, optical transition frequencies of two separated NV centers are
tuned into resonance with each other by applying external electric fields.
Extension of the present approach to generate entanglement of remote
solid-state qubits is discussed.
