Coherent processes in general and electromagnetically induced transparency (EIT), in particular, have been a major building block for protocols in quantum information, along with myriad of applications. The phenomenon of EIT has been explored in various realistic but dirty physical systems, including rare-earth doped materials, optical fibers and thermal ensemble of hot atoms. In such open quantum systems one can therefore, ask how many atoms are truly in quantum superposition and how such coherence develops. Is it optical pumping that first prepares a thermal ensemble, with coherent superposition developing subsequently or is it the other way round: coherently superposed atoms driven to steadystate via optical pumping? We explore these questions with a novel, stroboscopic technique, probing the changing coherence and population in time. We have also observed coherent half-cycle Rabi flop in room-temperature atoms. We have also observed overshoot in transparency, akin to lasing-without-inversion (LWI). Numerical simulations and toy-model predictions confirm our claims. These studies reveal new insights into a rich and complex dynamics associated with atoms in a thermal ensemble, which are otherwise absent in state-prepared, cold atomic ensembles.

Quantifying coherence in such a media is of great interest in order to use quantum superposition as a resource, but is a difficult feat to achieve using the traditional pump-probe spectroscopic measurements. We have proposed and experimentally explored a quanti.er that captures effective coherent superposition of states in an atomic ensemble at room-temperature.Our quantifier is based on single shot measurement of probe transmission in time domain and is easy to implement in a broad range of dirty, room-temperature systems that use electromagnetic fields to achieve coherent superposition of quantum states. The quantifier gives a direct measure of ground state coherence and also captures the signature of transition from Electromagnetically Induced Transparency(EIT) to Autler-Townes splitting(ATS) which is otherwise very subjective. The next obvious question is can we devise a method to freeze the system in the maximal coherent state akin to quantum Zeno effect. We try to implement this by introducing an extra controllable coherence in the medium via a highly off-resonant biphoton Raman coherent beams. Such Zeno-like state preparation significantly improves the coherent properties of hot atomic ensemble. We have characterized the evolution of coherence in such a system on a Bloch sphere and realized that the phase between the two Zeno beams act as a fine tuning parameter to move the Bloch vector along the sphere implying a very good control on the media coherence. Storage of light in such medium offers a better retrieval probability and phase information. Our results also provide a platform for comparing competing hypothesis along with possible new applications in pulse shaping, amplification and optical switching with controlled slew rates.

References

1. Probing, Quantifying and Freezing Coherence in a Thermal Ensemble of Atoms. Arif Warsi Laskar, Niharika Singh, Pratik Adhikary, Arunabh Mukherjee, and Saikat Ghosh. Optica 5, 11, 1462-1467 (2018). (pdf) (Supplementary Information)

2. Interplay of classical and quantum dynamics in a thermal ensemble of atoms. Arif Warsi Laskar, Niharika Singh, Arunabh Mukherjee, Saikat Ghosh. New Journal of Physics 18 (5), 053022 (2016) (pdf)