Twenty millisecond electron-spin coherence in an erbium doped crystal

Rare-earth-ions are interesting physical systems because they have long lived states and record coherence times for both the optical and nuclear transitions [1]. Rare-earths with an odd number of electrons are also paramagnetic, with an electron-spin transition at GHz frequencies in magnetic fields less than 1 Tesla. For such transitions, coherence times up to 2 ms have been measured at temperatures greater than 100mK [2]. Here we present our recent EPR results in 50 ppm and 10 ppb Er doped CaWO4. These measurements were recorded in a previously unstudied temperature regime for this material: sub-Kelvin temperature down to 10mK, using a superconducting micro-resonator and a superconducting parametric amplifier [3]. We observe the longest recorded Hahn-echo decay for an electronic spin transition in an Erbium doped material, up to 20 ms.
[1] Zhong, M. et al., Nature 517, 177-180 (2015).
[2] Li, P. et al., Phys. Rev. Appl 13, 024080 (2020)
[3] A. Bienfait et al., Nature nanotechnology, vol. 11, no 3, p. 253, (2016)