# Optical coherence of $^{166}$Er:$^{7}$LiYF$_4$ crystal below 1 Kelvin

## TL;DR

This study investigates the spin coherence properties of isotopically purified $^{166}$Er:$^{7}$LiYF$_4$ crystals at sub-Kelvin temperatures and low magnetic fields, revealing potential for quantum memory and quantum circuit interfacing.

## Contribution

It demonstrates spin coherence of Er:LYF crystals at low magnetic fields and temperatures, suitable for quantum memory and superconducting circuit integration, which was not achieved before.

## Key findings

- Record-narrow optical linewidths of 16 MHz in Er:LYF crystals.
- Spin coherence observed at magnetic fields compatible with quantum applications.
- Potential for implementing quantum memory and interfacing with superconducting circuits.

## Abstract

We explore spin dynamics of isotopically purified $^{166}$Er:$^{7}$LiYF$_4$ crystal below 1 Kelvin and at weak magnetic fields $<$0.3 T. Crystals grown in our lab demonstrate record-narrow inhomogeneous optical broadening down to 16~MHz. Solid state atomic ensembles with such narrow linewidths are very attractive for the implementation of off-resonant Raman quantum memory and for the interfacing of superconducting quantum circuits and telecom C-band optical photons. Both applications require low magnetic field of $\sim10$ mT. However, at conventional experimental temperatures $T>1.5$ K and time scales of $\mu$s, spin coherence of Er:LYF crystal appears only at magnetic fields above 1 Tesla. In the present work, we demonstrate spin coherence of Er:LYF crystals at the field range compatible with ZEFOZ transitions of $^{167}$Er isotope and with working conditions of superconducting quantum circuits.