A long-lived Zeeman trapped-ion qubit

We demonstrate unprecedentedly long lifetimes for electron spin superposition states of a single trapped 40 Ca+ ion. For a Ramsey measurement, we achieve a 1/root e coherence time of 300(50) ms, while a spin-echo experiment yields a coherence time of 2.1(1) s. The latter corresponds to residual effective rms magnetic field fluctuations <= 2.7 x 10(-12) T during a measurement time of about 1500 s. The suppression of decoherence induced by fluctuating magnetic fields is achieved by combining a two-layer mu-metal shield, which reduces external magnetic noise by 20-30 dB for frequencies of 50 Hz-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 trapped-ion spin qubit to ultralong coherence times which so far have been observed only for magnetic insensitive transitions in atomic qubits with hyperfine structure.