# Absorption Spectrum of a Two-Level System Subjected to a Periodic Pulse Sequence

## TL;DR

This paper explores how periodic pulse control can modify the photon absorption spectrum of a two-level quantum system, enabling precise tuning and stabilization of absorption features for quantum information applications.

## Contribution

It demonstrates that periodic Rabi pulse sequences can effectively shift and lock the absorption spectrum of a two-level system, providing a new method for quantum control.

## Key findings

- Absorption spectrum features a peak at the pulse frequency with satellite peaks.
- Spectral shape remains stable under moderate detuning.
- Quantum control enables spectral tuning and locking.

## Abstract

We investigate how the quantum control of a two-level system (TLS) coupled to photons can modify and tune the TLS's photon absorption spectrum. Tuning and controlling the emission and the absorption is of much interest e.g.\ for the development of efficient interfaces between stationary and flying qubits in modern architectures for quantum computation and quantum communication. We consider the periodic pulse control, where the TLS is subjected to a periodic sequence of the near-resonant Rabi driving pulses, each pulse implementing a 180$^\circ$ rotation. For small inter-pulse delays, the absorption spectrum features a pronounced peak of stimulated emission at the pulse frequency, as well as equidistant satellite peaks with smaller spectral weights. As long as the detuning between the carrier frequency of the driving and the TLS transition frequency remains moderate, this spectral shape shows little change. Therefore, the quantum control allows shifting the absorption peak to a desired position, and locks the absorption peak to the carrier frequency of the driving pulses. Detailed description of the spectrum, and its evolution as a function time, the inter-pulse spacing and the detuning, is presented.