  We consider the entanglement between two internal states of a single atom and
two photon number states describing either the vaccum or a single photon and
thus containing, on average, less than one photon. We show that this intriguing
entanglement can be characterized through substantial violations of a Bell
inequality by performing homodyne detections on the optical mode. We present
the experimental challenges that need to be overcome to pave the way towards a
loophole-free Bell test in this setup.
