  The prototype of a quantum random number generator is a single photon which
impinges onto a beam splitter and is then detected by single photon detectors
at one of the two output paths. Prior to detection, the photon is in a quantum
mechanical superposition state of the two possible outcomes with - ideally -
equal amplitudes until its position is determined by measurement. When the two
output modes are observed by a single photon detector, the generated clicks can
be interpreted as ones and zeros - and a raw random bit stream is obtained.
Here we implement such a random bit generator based on single photons from a
defect center in diamond. We investigate the single photon emission of the
defect center by an anti-bunching measurement. This certifies the "quantumness"
of the supplied photonic input state, while the random "decision" is still
based on the vacuum fluctuations at the open port of the beam-splitter.
Technical limitations, such as intensity fluctuations, mechanical drift, and
bias are discussed. A number of ways to suppress such unwanted effects, and an
a priori entropy estimation are presented. The single photon nature allows for
a characterization of the non-classicality of the source, and allows to
determine a background fraction. Due to the NV-center's superior stability and
optical properties, we can operate the generator under ambient conditions
around the clock. We present a true 24/7 operation of the implemented random
bit generator.

Quantum Physics