Storing Light in the Dark

Abstract

Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes. Here, we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model. DOI: 10.1103/PhysRevLett.116.083601 Despite its many applications, ranging from astrophysics [1] to mesoscopic physics [2,3] and quantum information technology [4], light interacting with a large ensemble of N scatterers still bears many surprising features and is at the focus of intense research. For N ¼ 2 atoms placed close together, the in-phase oscillation of the induced dipoles produces a large, superradiant dipole, whereas the out-ofphase oscillation corresponds to a subradiant quadrupole. Generalizing for N ≫ 2, Dicke has shown that, for samples of a small size compared to the wavelength of the atomic transition, the symmetric superposition of atomic states induces superradiant emission, scaling with the number of particles N, whereas the antisymmetric superpositions are decoupled from the environment, with vanishing emission rates, which corresponds to subradiance [5].Dicke superradiance has been extensively studied in the 1970s [6][7][8] but the observation of its counterpart, subradiance, has been restricted to indirect evidence of modified decay rates in one particular direction [9] or in systems of two particles at very short distance [10][11][12]. One challenge for the observation of subradiance by a large number of particles is the fragile nature of these states, which require protection from any local nonradiative decay mechanism [13]. Furthermore, contrary to the two-atom case, for which the distance between atoms has to be small compared to the wavelength, for N ≫ 2, the retarded, long-range resonant dipole-dipole interaction [14] gives rise to super-and subradiant effects ("cooperative scattering") also in dilute samples, with interatomic distances much larger than the wavelength, and corresponding large system sizes. Since, for N > 2, the Hamiltonians for short and long-range interactions do not commute, the collective eigenstates due to the long-range interactions are suppressed by short-range interactions [8]. These short-range or near-field effects (or "van der Waals dephasing") thus need to be avoided in t...