Optical coherence properties of Kramers' rare-earth ions at the nanoscale for quantum applications

TL;DR

This study characterizes the optical coherence properties of Kramers' rare-earth ions, specifically Er$^{3+}$ and Nd$^{3+}$, at the nanoscale, providing insights for their use in quantum technologies such as quantum memory and single-photon sources.

Contribution

The paper provides detailed spectroscopic measurements of Er$^{3+}$ and Nd$^{3+}$ ions at low temperatures, highlighting their coherence times and linewidths relevant for quantum applications.

Findings

Inhomogeneous linewidths are 10.7 GHz for Er$^{3+}$ and 8.2 GHz for Nd$^{3+}$.

Maximum homogeneous linewidths are 379 kHz for Er$^{3+}$ at 3 K and 62 kHz for Nd$^{3+}$ at 1.6 K.

Coherence times T$_2$ are 839 ns for Er$^{3+}$ and 5.14 μs for Nd$^{3+}$.

Abstract

Rare-earth (RE) ion doped nano-materials are promising candidates for a range of quantum technology applications. Among RE ions, the so-called Kramers' ions possess spin transitions in the GHz range at low magnetic fields, which allows for high-bandwidth multimode quantum storage, fast qubit operations as well as interfacing with superconducting circuits. They also present relevant optical transitions in the infrared. In particular, Er$^{3+}$ has an optical transition in the telecom band, while Nd$^{3+}$ presents a high-emission-rate transition close to 890 nm. In this paper, we measure spectroscopic properties that are of relevance to using these materials in quantum technology applications. We find the inhomogeneous linewidth to be 10.7 GHz for Er$^{3+}$ and 8.2 GHz for Nd$^{3+}$, and the excited state lifetime T$_1$ to be 13.68 ms for Er$^{3+}$ and 540 $\mu$s for Nd$^{3+}$. We study the dependence of homogeneous linewidth on temperature for both samples, with the narrowest linewidth being 379 kHz (T$_2$ = 839 ns) for Er$^{3+}$ measured at 3 K, and 62 kHz (T$_2$ = 5.14 $\mu$s) for Nd$^{3+}$ measured at 1.6 K. Further, we investigate time-dependent homogeneous linewidth broadening due to spectral diffusion and the dependence of homogeneous linewidth on magnetic field, in order to get additional clarity of mechanisms that can influence the coherence time. In light of our results, we discuss two applications: single qubit-state readout and a Fourier-limited single photon source.