Optical Investigations of Coherence and Relaxation Dynamics of a Thulium-doped Yttrium Gallium Garnet Crystal at sub-Kelvin Temperatures for Optical Quantum Memory

TL;DR

This study investigates the optical coherence and relaxation properties of thulium-doped yttrium gallium garnet at sub-Kelvin temperatures, demonstrating its potential for quantum memory applications through spectral hole burning and photon echo techniques.

Contribution

It provides the first detailed measurements of coherence times and hyperfine lifetimes of Tm:YGG at millikelvin temperatures, highlighting its suitability for quantum memory.

Findings

Hyperfine ground-level lifetimes of several minutes at <1000 G.

Optical coherence time exceeding one millisecond.

Spectral diffusion reduces effective coherence to a few microseconds over 200 seconds.

Abstract

Rare-earth ion-doped crystals are of great interest for quantum memories, a central component in future quantum repeaters. To assess the promise of 1$\%$ Tm$^{3+}$-doped yttrium gallium garnet (Tm:YGG), we report measurements of optical coherence and energy-level lifetimes of its $^3$H$_6$ $\leftrightarrow$ $^3$H$_4$ transition at a temperature of around 500 mK and various magnetic fields. Using spectral hole burning, we find hyperfine ground-level (Zeeman level) lifetimes of several minutes at magnetic fields of less than 1000 G. We also measure coherence time exceeding one millisecond using two-pulse photon echoes. Three-pulse photon echo and spectral hole burning measurements reveal that due to spectral diffusion, the effective coherence time reduces to a few $\mu$s over a timescale of around two hundred seconds. Finally, temporal and frequency-multiplexed storage of optical pulses using the atomic frequency comb protocol is demonstrated. Our results suggest Tm:YGG to be promising for multiplexed photonic quantum memory for quantum repeaters.