# Slow and fast light behavior of single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line

## TL;DR

This study demonstrates the manipulation of single photons emitted from a quantum dot using the hyperfine structure of Cesium D1-line, showing slow and fast light effects for potential quantum network synchronization.

## Contribution

It introduces a method to control single photon delays via hyperfine interactions, advancing quantum memory and synchronization techniques.

## Key findings

- Single photons from quantum dots can be delayed by 5 ns.
- Hyperfine structure causes observable slow and fast light effects.
- Proposes all-optical control for photon synchronization in quantum networks.

## Abstract

Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single In(Ga)As quantum dot with its excitonic transition precisely set relative to the Cesium D1 transition. The delay of spectral components of the single photon wave packet with almost Fourier-limited width is investigated in detail with a 200 MHz narrow-band monolithic Fabry-P\'erot resonator. Reflecting the excited state hyperfine structure of Cesium, "slow light" and "fast light" behavior is observed. As a step towards room-temperature alkali vapor memories, quantum dot photons are delayed for 5 ns by strong dispersion between the two 1.17 GHz hyperfine-split excited state transitions. Based on optical pumping on the hyperfine-split ground states, we propose a simple, all-optically controllable delay for synchronization of heralded narrow-band photons in a quantum network.