We demonstrate a coherence time of 2.1(1)~s for electron spin superposition states of a single trapped 40Ca+ ion. The coherence time, measured with a spin-echo experiment, corresponds to residual rms magnetic field fluctuations ≤~2.7×10−12~T. The suppression of decoherence induced by fluctuating magnetic fields is achieved by combining a two-layer μ-metal shield, which reduces external magnetic noise by 20 to 30~dB for frequencies of 50~Hz to 100~kHz, with Sm2Co17 permanent magnets for generating a quantizing magnetic field of 0.37~mT. Our results extend the coherence time of the simple-to-operate spin qubit to ultralong coherence times which so far have been observed only for magnetic insensitive transitions in atomic qubits with hyperfine structure.
@article{arxiv.1606.07220,
title = {A long-lived Zeeman trapped-ion qubit},
author = {Thomas Ruster and Christian T. Schmiegelow and Henning Kaufmann and Claudia Warschburger and Ferdinand Schmidt-Kaler and Ulrich G. Poschinger},
journal= {arXiv preprint arXiv:1606.07220},
year = {2016}
}