&Bullet; physics 14, 69
Researchers have prepared and manipulated subradiant states in a dense atomic cloud in which collective effects slow the decay of excited atoms.
What goes up has to come down, and for an atom in an excited state, “coming down” means decay through spontaneous emission of a photon. This emission, which is triggered by vacuum fluctuations, represents an unavoidable source of dissipation and is therefore a problem for quantum applications. The moment of decay can, however, be prevented by collective effects: if many atoms are separated from one another by a distance that is smaller than the emission wavelength, they can become “dark” and prevent each other from decaying into the ground state. This phenomenon, known as subradiance, has already been observed in dilute atomic ensembles  . Giovanni Ferioli and colleagues from the Institute of Optics in Palaiseau, France, have now created subradiant states in a dense ensemble of atoms in which a significant proportion of the atoms occupy a volume that is smaller than the emission wavelength  . The researchers also show that they can turn off the subradiance with a laser. Such an ability to control the collective optical response of atoms in real time has implications ranging from improving the accuracy of quantum information logs to studying out-of-equilibrium many-body physics in open quantum systems.
Subradiance is an emergent phenomenon that arises from photon-mediated dipole-dipole interactions between atoms, and it is the opposite of the more well-known superradiance introduced by Robert Dicke in 1954  . Photons emitted by atoms in a superradian state constructively interfere and increase the overall emission (and thus the rate of decay). In a subradiant state, however, the interference is destructive and the emission from the atomic ensemble is suppressed. Recent theoretical work has suggested the possibility of using such dark states to implement more efficient protocols for storing and retrieving photons  , Metrology  and nonlinear quantum optics  . So far, most of the research has focused on ordered arrays with the small lattice constants necessary to produce a collective atomic response to light. But despite impressive developments such as the realization of a subradiant, two-dimensional atomic mirror  Achieving these small distances between the particles in ordered arrays remains a challenge.
Ferioli and colleagues are able to achieve small distances between the particles by working with a very dense, disordered atomic cloud. You use a laser pulse to excite a cloud of rubidium atoms in an optical tweezer trap, and then record the intensity of the light emitted by the cloud. The laser mostly couples to over-radiating states, and at first the atoms disintegrate quickly. Subradiant states are very weakly excited, but because of their long lifespan they survive super radiant states and therefore dominate the emission at late times. In addition, a fraction of the super-radian states also leak into the sub-radiant states. These two processes contribute to the creation of a long emission tail in which the cloud slowly emits photons. The researchers examine clouds of different geometries and show that the development of this subradiant emission is determined exclusively by the number of atoms in the cloud: the longer the atomic number, the longer the lifespan (at later times). This atomic number dependence, which would not be observed in a dilute system, is an important confirmation of theoretical predictions for this dense regime.
The team also investigates multi-body subradiant states in which not just one, but several excitations are shared between atoms. They do this by increasing the intensity of the incident light and generating many overexposure stimuli that stochastically “cascade” towards the ground state through photon emission. This cascade can take place in a super-radiant manner in which photons are emitted in rapid succession. However, it can “get stuck” when the system reaches a dark state that takes a long time to break down. These dark states can consist of several stimuli, but still cannot radiate due to (multi-body) destructive interference. By examining the still mysterious structure of these many-body dark states, the experiment provides the first confirmation of the theoretical prediction  that they are “built up” from superimpositions of individual excitation states. This prediction was made for ordered arrays, but the results of Ferioli and colleagues indicate its validity for disordered clouds as well, although further research is needed in this direction.
Interesting as these results are, dark states become more useful when the subradiant stimuli can be released at will. The researchers achieve this control through a position-dependent detuning of the transition, which is achieved via an alternating current-Stark shift induced by the optical tweezer trap. This detuning disrupts the atomic resonance and brings the atoms out of their dark state, which leads to a sudden burst of radiation.
So what’s next? There are several adventures waiting for you – both for experimenters and theorists. Important experimental milestones would be the excitation of a single sub-radiation state with 100% efficiency and the selective creation of states with specific lifetimes and spatial profiles. The method used by Ferioli and colleagues and previously used by other groups  reaches subradiant states by waiting for the system to break down into them. This approach implies a loss of efficiency as most of the energy (and information) is released when the system reaches the target state. Addressing this challenge may not be possible in disordered ensembles, but it can be more easily achieved in orderly arrays, which are efficient interfaces between light and matter.
There are also many problems that need to be understood theoretically. Going beyond Dick’s pioneering work and understanding superradiance, subradiance, and correlated decay in extended systems has been an open problem for decades. With few exceptions [5, 8] Most theoretical studies have focused on systems in which only a few atoms are excited, which makes the problem more manageable. Crossing this limit requires a new framework to cope with the growth in exponential complexity, but the potential rewards are well worth it: understanding these out of balance dynamics could lead to the deterministic preparation of highly entangled states with long lifetimes, which is useful could be for metrology or calculation. In addition, the correlated decay shapes correlations in the emitted photons, which lead to non-classical light states and can help in the characterization (and possibly control) of this many-body system.
- W. Guerin et al., “Subradiance in a large cloud of cold atoms” Phys. Rev. Lett.116083601 (2016).
- G. Ferioli et al., “Storage and release of subradiant excitations in a dense atomic cloud” Phys. Rev. X.11021031 (2021).
- RH thickness, “coherence in spontaneous radiation processes”, Phys. Rev.9399 (1954).
- A. Asenjo-Garcia et al., “Exponential improvement of photon storage fidelity using subradiance and” selective radiation “in atomic arrays” Phys. Rev. X.7th031024 (2017).
- L. Henriet et al., “Critical Dynamics of Open Systems in a One-Dimensional Optical Lattice Clock” Phys. Rev. A.99023802 (2019).
- R. Bekenstein et al., “Quantum metasurfaces with atom arrays”, Nat. Phys.16676 (2020).
- J. Rui et al.“A subradiant optical mirror formed by a single structured atomic layer” nature583369 (2020).
- SJ Masson et al., “Many-body signatures of the collective decay in atomic chains” Phys. Rev. Lett.125263601 (2020).