Emergence of second-order coherence in superfluorescence

We experimentally investigate the second-order quantum coherence function of a superradiant burst in a cascaded quantum system. We chirally (i.e. direction-dependently) couple roughly 900 cesium atoms to the forward propagating mode of an optical nanofiber. We then prepare the ensemble in the maximally inverted state, where the subsequent collective emission of a burst is known as superfluorescence. Here, we observe that second-order coherence emerges in the course of the decay. This is a clear feature of the underlying collective dynamics that is also at the origin of the superradiant burst itself. We furthermore study the dynamics of the second-order coherence function of the emission in dependence on the initial average dipole moment of the ensemble. In addition, by correlating the detection of early and late photon emission events, we obtain evidence for fundamental shot-to-shot fluctuations in the delay of the start of the burst emission. Our findings reveal that, despite the fundamentally different coupling Hamiltonian, superradiance in cascaded and symmetrically coupled systems feature a strikingly large number of similarities.

💡 Research Summary

In this work the authors investigate the second‑order quantum coherence function g^(2)(t₁,t₂) of a superradiant burst emitted by a cascaded ensemble of roughly 900 cesium atoms coupled chirally to the forward‑propagating mode of an optical nanofiber. The atoms are trapped in a one‑dimensional array around the nanofiber using a combination of blue‑detuned (760 nm) and red‑detuned (1064 nm) evanescent fields, yielding at most one atom per trapping site due to collisional blockade. By adjusting the loading time of a magneto‑optical trap they set the optical depth to OD≈40, corresponding to N≈900 atoms.

A short (4 ns) resonant Rabi pulse is launched through the fiber to invert the ensemble. The pulse area A=ΩT_p is varied around the value π, which determines the initial collective dipole moment. Because the nanofiber mode couples asymmetrically (β≈0.01) to the atoms, emission is essentially unidirectional: each atom influences only those downstream, realizing a cascaded quantum system. After the excitation pulse the emitted light is split into two arms of a Hanbury‑Brown‑Twiss interferometer; one arm is delayed by ~100 ns so that coincidences can be recorded with a time resolution of 3 ns. The normalized second‑order correlation function is obtained as g^(2)(t₁,t₂)=n_c(t₁,t₂)/