Quantum optical characterization of rare earth ion in superconductor quantum memory*

There is a need for the implementation of quantum entangled memory devices that can interface to computational qubit devices via efficient transfer of the low coherence time qubit states to the high coherence spin states of optically active ions. We present characterization in the cryogenic regime of a rare earth ion quantum memory we formed from ion implantation of Neodymium ions in superconducting Niobium Nd³⁺:Nb. We examine via spectroscopy in a cryo-magneto-optical probe station, the 4F3/2 to 4I 9/2 transition in the NIR at 903.4 nm as a function of magnetic and microwave field. We quantify the spin echoes to evaluate T1 and T2 coherence times. We find an improvement when entering the superconducting regime of Niobium at 7.5 K. We study the effects of varying the laser excitation intensity and the microwave and magnetic field strengths on key parameters. The results are consistent with an increase of the spin relaxation and are discussed in accordance with theoretical models.