Transition metal ion ensembles in crystals as solid-state coherent spin-photon interfaces: The case of nickel in magnesium oxide

TL;DR

This paper explores transition metal ions in crystals as potential solid-state spin-photon interfaces, demonstrating coherence and optical control in nickel-doped magnesium oxide, with implications for quantum memory applications.

Contribution

It provides guidelines for identifying solid-state systems suitable for coherent spin-photon interfaces and experimentally demonstrates coherence and optical control in nickel-doped magnesium oxide.

Findings

Electron spin coherence lasts several microseconds at liquid-helium temperatures.

Existence of well-isolated excited states enabling optical transitions.

Proposed schemes for optical initialization and quantum memory implementation.

Abstract

We present general guidelines for finding solid-state systems that could serve as coherent electron spin-photon interfaces even at relatively high temperatures, where phonons are abundant but cooling is easier, and show that transition metal ions in various crystals could comply with these guidelines. As an illustrative example, we focus on divalent nickel ions in magnesium oxide. We perform electron spin resonance spectroscopy and polarization-sensitive magneto-optical fluorescence spectroscopy of a dense ensemble of these ions and find that (i) the ground-state electron spin stays coherent at liquid-helium temperatures for several microseconds, and (ii) there exists energetically well-isolated excited states which can couple to two ground state spin sub-levels via optical transitions of orthogonal polarizations. The latter implies that fast, coherent optical control over the electron spin is possible. We then propose schemes for optical initialization and control of the ground-state electron spin using polarized optical pulses, as well as two schemes for implementing a noise-free, broadband quantum-optical memory at near-telecom wavelengths in this material system.