Observation of short-lived laser-dressed quantum states in the frequency plane

A transient quantum state can be modified in a controlled way by coupling it to one or several of the neighboring states with a laser. Coupling of a pair of states leads to the so-called Autler-Townes doublet, with a characteristic energy splitting. A direct result of this splitting is reduced target absorption in the spectral gap of the doublet where the transmission is also modulated by the quantum interference effect known as electromagnetically induced transparency. These phenomena have a rich variety of applications including dissipation-free light transmission and fast low-power optical switching. They enable a control of decay branching ratios, measurements of transition dipole moments, electromagnetic field characterization, and offer a framework for single-qubit operations relying solely on optical fields.

The simplest manifestation of the laser coupling is a change in the transmission or reflection of the excitation pulse upon tuning the laser wavelength. The coupling can be explored in greater detail by measuring a so-called frequency map by changing the detuning of both the excitation laser and the coupling laser. Here, we present an approach to constructing a two-dimensional frequency map and demonstrate its applicability on the case of selectively coupled atomic resonances with femtosecond lifetimes and tens of eV excitation energies. We show that the complete information about the optical coupling and pulse properties may be extracted from the map measured by scanning the frequency and delay of the excitation pulse, while keeping the settings of the laser coupling pulse fixed. An essential ingredient of the proposed method is a nonzero laser chirp: when combined with the delay, different frequencies of the laser pulse overlap with the excitation pulse frequencies at different times to imitate the laser frequency scan.

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