Experimental study of the optical qubit on the $435$-nm quadrupole transition in the $^{171}$Yb$^{+}$ ion

Ultracold ions provide one of the most promising platforms for quantum computing and make it possible to reach record coherence times, the fidelity of preparation and readout operations, and single- and two-qubit operations. Encoding quantum information in an optical qubit based on the $\,^2S_{1/2} (F=0,m_F=0) \rightarrow \,^2D_{3/2}(F=2, m_F=0)$ quadrupole transition in the $^{171}$Yb$^{+}$ ion at a wavelength of $435.5$ nm, which has potential advantages over similar systems in the scaling of the number of qubits and their sensitivity to fluctuations of the magnetic field, is proposed and experimentally studied. The proposed optical qubit in ytterbium is compared to other most widespread types of ion qubits. Experimental results on the implementation of the Pauli-X single-qubit operation are given. The fidelity of the operation is $96\%$ after correction to the error of preparation and readout and is limited by the temperature of the ion.