Chains of ultracold ions trapped with varying electric fields are one of the most promising platforms for quantum computations, which is being actively studied at the moment. It features long coherence time, well-developed and high-fidelity techniques for quantum state initialization and readout as well as a strong Coulomb interaction between particles, which allows to efficiently entangle them. One of the approaches to this platform further development is a search for more suitable ion species or new ways of encoding quantum information in their electronic structure.

In this letter, we experimentally investigate quantum information encoding in an optical quadrupole transition in ¹⁷¹Yb⁺ ion, which is already widely used for quantum computations but with microwave qubit encoding. Optical qubits are easier to individually address with laser beams than microwave ones as there is no need for bichromatic laser emission from different directions and only one beam is sufficient. Initialization and readout of optical qubits are also usually more accurate. These properties may help to overcome one of the major issues with ion quantum computers – scalability problems. On the other hand, optical qubits suffer from shorter coherence times.

We compare proposed optical qubit with microwave qubit in ¹⁷¹Yb⁺ ion as well as with the most widespread at the moment optical qubit in ⁴⁰Ca⁺. We also experimentally demonstrate and characterize fidelity of a single-qubit Pauli-X operation and fidelities of our preparation and detection schemes.

Level scheme of ¹⁷¹Yb⁺ ion, showing both microwave qubit in the ion as well as proposed optical qubit. States proposed to use for qubit encoding are shown as |0> and |1>.