Photons can suffer traffic jams in much the same way as other particles, although their merging technique is not classical. For one thing, they can only interact with each other through matter. By trapping a caesium atom in an optical cavity, Kevin Birnbaum and colleagues1 show in this week’s issue of Nature that photons bunch up outside the cavity and can only enter one at a time.

The atom–cavity system is considered a model two-state system. After emitting a photon from the first excited state the system lies in its ground state and is unable to emit a second photon for some time — a dead time that leads to non-poissonian statistics — because transitions in the second excited state are not allowed (being out of resonance). This 'photon blockade' is analogous to the electronic Coulomb blockade observed in mesoscopic systems. In a quantum dot, for example, it costs a certain amount of charging energy to overcome the Coulomb repulsion and add one more electron, so current flows in quantized steps as electrons are added one by one. If an electron does not have sufficient energy, it is blocked from tunnelling into the dot.

But how do you see a photon blockade? It can’t be measured by transport techniques, as it can for electrons. The authors study the temporal intensity correlation function g(2)(τ). For a laser light field, this function is equal to unity because the intensity is constant over time. Instead, Birnbaum et al. measured a g(2)(τ) that is both subpoissonian (less than unity) and antibunched — distinctly non-classical behaviour.

In reality, the atom–cavity system is more complicated than a two-state system. The multistate atom is strongly coupled to two cavity modes, and the theory presented by the authors doesn’t work for long timescales and will need improvement. Experimentally, however, this demonstration of a single two-level quantum emitter is promising for quantum information devices. Further work may even produce a single photon source 'on tap'.