Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A long-lived Zeeman trapped-ion qubit   We demonstrate a coherence time of 2.1(1)~s for electron spin superposition
states of a single trapped $^{40}$Ca$^+$ ion. The coherence time, measured with
a spin-echo experiment, corresponds to residual rms magnetic field fluctuations
$\leq$~2.7$\times$10$^{-12}$~T. 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 to 30~dB for frequencies of
50~Hz to 100~kHz, with Sm$_2$Co$_{17}$ 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.

4
The categories of this paper are: cond-mat
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
36
The categories of this paper are: cmp-lg
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The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
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The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
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The categories of this paper are: eess
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The categories of this paper are: hep-lat
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-ex