Spin thermometry and spin relaxation of optically detected Cr3+ ions in ruby Al2O3

TL;DR

This study investigates the ultra-low-temperature spin properties of Cr3+ ions in ruby using optical and magnetic resonance techniques, revealing relaxation dynamics and population behaviors crucial for quantum applications.

Contribution

The paper provides new ultra-low-temperature optical and spin measurements of Cr3+ in ruby, including relaxation times and population dynamics, which were previously uncharacterized at these temperatures.

Findings

Spin relaxation T1 matches one-phonon model

Lattice temperature under laser excitation is 143 mK

Magnetically driven transitions observed at various fields

Abstract

Paramagnetic ions in solid state crystals form the basis for many advanced technologies such as lasers, masers, frequency standards, and quantum-enhanced sensors. One of the most-studied examples is the Cr3+ ion in sapphire (Al2O3), also known as ruby, which has been intensely studied in the 1950s and 1960s. However, despite decades of research on ruby, some of its fundamental optical and spin properties have not yet been characterized at ultra low-temperatures. In this paper, we present optical measurements on a ruby crystal in a dilution refrigerator at ultra-low temperatures down to 20 mK. Analyzing the relative populations of its 4A2 ground state spin levels, we extract a lattice temperature of 143(7) mK under continuous laser excitation. We perform spin lattice relaxation T1 measurements in excellent agreement with the direct, one-phonon model. Furthermore, we perform optically detected magnetic resonance measurements showing magnetically driven transitions between the ground state spin levels for various magnetic fields. Our measurements characterize some of ruby's low temperature spin properties, and lay the foundations for more advanced spin control experiments.