Experimental demonstration of complementarity with single photons

We demonstrate the principle of complementarity in quantum mechanics in a single-photon interference experiment. Single photons are provided by isolated. optically pumped nitrogen-vacancy centers in diamond, which can be easily addressed by confocal microscopy. In order to observe the particle-like behavior of photons, we perform an elementary Welcher-Weg measurement, detecting photons behind a beam splitter. In contrast, if we dispense with this Welcher-Weg information, we observe interference fringes with a visibility of about 96%, revealing the wave nature of the photon.