In situ subwavelength microscopy of ultracold atoms using dressed excited states

In this work, we implement a new method for imaging ultracold atoms with subwavelength resolution capabilities and determine its regime of validity. It uses the laser driven interaction between excited states to engineer hyperfine ground state population transfer in a three-level system on scales much smaller than the optical resolution. Subwavelength imaging of a quantum gas is atypical in the sense that the measurement itself perturbs the dynamics of the system. To avoid induced dynamics affecting the measurement, one usually measures "rapidly" the wavefunction in a so-called strong imaging regime. We experimentally illustrate this regime using a thermal gas ensemble, and demonstrate subwavelength resolution in quantitative agreement with a fully analytical model. Additionally, we show that, counter-intuitively, the opposite weak imaging regime can also be exploited to reach subwavelength resolution. As a proof of concept, we demonstrate that this regime is a robust solution to select and spatially resolve a 30 nm wide wavefunction, which was created and singled out from a tightly confined 1D optical lattice. Using a general dissipation-included formalism, we derive validity criteria for both regimes. The formalism is applicable to other subwavelength methods.