A research team from Stony Brook University, led by professor of physics Dominik Schneble, has uncovered a new regime, or set of conditions within a system, for cooperative radiative phenomena, casting new light on a 70-year-old problem in quantum optics. Spontaneous emission is a phenomenon in which an excited atom falls to a lower-energy state and spontaneously emits a quantum of electromagnetic radiation in the form of a single photon. When a single excited atom decays and emits a photon, the probability of finding the atom in its excited state falls exponentially to zero as time progresses. In 1954, Princeton physicist R. H. Dicke considered what happens when a second, unexcited, atom is put in its immediate vicinity. He argued that the probability of finding an excited atom would fall to only one-half. The excited system consists of two simultaneous scenarios, one in which the atoms are in phase, leading to stronger emission (called superradiance), and one in which they are opposite in phase, when no emission occurs (subradiance). When both atoms are initially excited, the decay always turns superradiant. Schneble and colleagues used a platform of ultracold atoms in a one-dimensional optical lattice geometry to implement arrays of synthetic quantum emitters that decay by emitting slow atomic matter waves. In contrast, conventional processes emit photons traveling at the speed of light. This difference enabled them to access collective radiative phenomena in novel regimes. A representation of an array of matter-wave emitters held in an optical lattice tube along with data for directional superradiant emission. In the tube, atomic excitations have been colored red, emitted matter has been colored blue, and effective vacuum coupling has been colored green. Courtesy of Alfonso Lanuza. By preparing and manipulating arrays of emitters hosting weak and strong interacting many-body phases of excitations, the researchers demonstrated directional collective emission and studied the interplay between retardation, super-, and subradiant dynamics. “Dicke’s ideas are of great significance in quantum information science and technology. For example, there are intense efforts to harness super- and subradiance in arrays of quantum emitters coupled to one-dimensional waveguides,” said Schneble. “In our work, we are able to prepare and manipulate subradiant states with unprecedented control. We can shut off spontaneous emission and observe where the radiation hides in the array. To our knowledge, this is a first such demonstration.” Schneble explained that in Dicke’s theory, the photons do not play an active role since they move quickly between nearby emitters on the time scale of the decay. However, there are situations that can break this assumption, such as in a channel of a long-distance quantum network, where a guided photon escaping from a decaying emitter might need a long time to reach the neighboring one. This unexplored regime is exactly what the researchers were able to access because the emitted matter waves in their system are billions of times slower than photons. “We see how collective decay from a superradiant state containing a single excitation takes time to form,” said co-author Youngshin Kim. “It only happens once neighboring emitters have been able to communicate.” The team points out that keeping track of slow radiation in a system of emitters is a daunting theoretical challenge, likening it to a game of catch and release. They explain that after being emitted, photons might find themselves being bound back to the atom permanently or bounce between temporary bindings before finally escaping. This becomes even more challenging when more atoms and photons are involved. Despite this complicated photon and atom interplay, they were able to find mathematical solutions for the case of two emitters with up to two excitations and arbitrary vacuum coupling. This aspect of the work may lead to uncovering other complicated or unexpected collective atomic decay behaviors in future experiments. “Overall, our results on collective radiative dynamics establish ultracold matter waves as a versatile tool for studying many-body quantum optics in spatially extended and ordered systems,” Schneble said. The research was published in Nature Physics (www.doi.org/10.1038/s41567-024-02676-w).