Power narrowing: counteracting Doppler broadening in two-color transitions

Abstract

Doppler broadening in thermal ensembles degrades the absorption cross-section and the coherence time of collective excitations. In two photon transitions, it is common to assume that this problem becomes worse with larger wavelength mismatch. Here we identify an opposite mechanism, where such wavelength mismatch leads to cancellation of Doppler broadening via the counteracting effects of velocity-dependent light-shifts and Doppler shifts. We show that this effect is general, common to both absorption and transparency resonances, and favorably scales with wavelength mismatch. We experimentally confirm the enhancement of transitions for different low-lying orbitals in rubidium atoms and use calculations to extrapolate to high-lying Rydberg orbitals. These calculations predict a dramatic enhancement of up to 20-fold increase in absorption, even in the presence of large homogeneous broadening. More general configurations, where an auxiliary dressing field is used to counteract Doppler broadening, are also discussed and experimentally demonstrated. The mechanism we study can be applied as well for rephasing of spin waves and increasing the coherence time of quantum memories. Doppler broadening is ubiquitous in atomic and molecular spectroscopy. Atoms and molecules with different thermal velocities experience different Doppler shifts, which broaden the absorption lines and reduce their contrast [1]. Increasing the light intensity typically causes further broadening due to saturation or other powerbroadening mechanisms, such as inhomogeneous light-shifts. These broadening or dephasing mechanisms are major limiting factors, particularly in the field of quantum optics with atomic ensembles [2][3][4][5][6]. For instance, the coherence time of collective excitations in atomic gasses is often limited by Doppler dephasing, hindering the performance of single-photon sources and memories [7-9]. As Doppler broadening is an inhomogeneous dephasing mechanism, one can potentially counteract it by introducing additional velocity-dependent shifts [10][11][12][13][14]. Here we study the counteraction of Doppler broadening by velocity dependent light-shifts and identify an important class of systems where such counteraction occurs naturally, without a need for additional auxiliary fields.Coherent two-photon processes, such as Raman transitions, two-photon absorption, and electromagnetically induced transparency (EIT), are at the heart of many quantum-optics protocols, ranging from quantum light sources and memories [15][16][17][18] to sensing and quantum nonlinear optics [19]. The canonical example of a three-level system employed for these processes is the Λ configuration, where two longlived ground states are coupled via an intermediate excited state. In these systems, two-photon transitions are usually characterized by negligible residual Doppler broadening, as the nearly-degenerate transition wavelengths experience opposite Doppler shifts [20]. Particularly in a degenerate Λ system under EIT conditions, atoms at all velocities cont...

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