The development of quantum technologies is driving fundamental and applied research to unforeseen limits. Diverse platforms, from atomic systems to integrated photonic circuits to nitrogen vacancy centers in diamond or superconducting qubits are investigated and pushed to the extremes of quantum mechanics due to the impressive degree of control and manipulation achieved. Some of the most promising technologies, e.g. quantum simulation, are based on ideal ‘closed’ quantum systems where only a handful of states are present with allowed transitions between them and no interaction with the rest of the universe. However, this ideal description is not real, and current technological applications are hindered by decoherence in the form of losses or decay. In this framework, the study of ‘open quantum systems’ where decoherence or dissipation is present becomes a necessity to further increase the efficacy and robustness of developing quantum technologies. A natural candidate to study the physics of open quantum systems are atom-light interfaces. Within the last two decades, research activity in atom-light interfaces, exploiting the unique properties of atomic systems and its interaction with light at the level of quantum mechanics allowed the development and demonstration of a wide range of quantum phenomena: from quantum protocols for Quantum Information, e.g. quantum memories of light, to Quantum Metrology protocols exploiting quantum nondemolition measurements, to the engineering of quantum states of matter for Quantum Simulation. This impressive progress was possible due to a thorough understanding of atom-atom and atom-light interactions. Yet, there are regimes that challenge our understanding of quantum atom-light interfaces and pose limitations to both technological and fundamental developments.
One example of these situations arises when a stream of photons interacts with a high-density atomic sample. The driving field mediates a dipole-dipole (DD) interaction among the atoms both in the form of real scattered photons and exchange of virtual photons. This DD interaction leads to the formation of collective states of matter, that is, states that show behavior governed by a collective property of the ensemble. Collective or cooperative scattering effects in high density media can lead to an optical behavior substantially different from the optical response of a single atom under the same driving conditions (see Figure 2). To date, collective scattering effects in ultracold atomic systems were studied primarily using alkaline atoms. The reported theoretical and experimental effects range from modified decay rates to resonance shifts, to vacuum Rabi splitting , to spectral broadening of the line, and the collective modes of these spatially disordered systems show interesting properties and phase transitions between diffusive transport and localization. Besides its implications for the scattered light properties, DD interactions were also proposed to generate non-classical states of light, to explore new quantum information protocols, or in many-body optical lattice systems as a tool to engineer the system Hamiltonian using alkaline-earth-like atoms such as Sr. More recently, several experiments started to investigate the collective scattering effects in ordered arrays, mainly using nanofibers in order to benefit from the increased directionality of the emitted fluorescence, and a lot of theoretical effort was devoted to study subradiant/superradiant atomic states in ordered arrays.
In this context, the project proposes to investigate the effect of the collective effects in driven-dissipative systems made of large, high-density clouds of ultra-cold atoms,to further our understanding of the light-mediated DD interactions. The ambition of this project is to exploit this knowledge to better control and enhance quantum metrology protocols for precision measurements.
