  The superconducting proximity effect in semiconductor nanowires has recently
enabled the study of new superconducting architectures, such as gate-tunable
superconducting qubits and multiterminal Josephson junctions. As opposed to
their metallic counterparts, the electron density in semiconductor nanosystems
is tunable by external electrostatic gates providing a highly scalable and
in-situ variation of the device properties. In addition, semiconductors with
large $g$-factor and spin-orbit coupling have been shown to give rise to exotic
phenomena in superconductivity, such as $\varphi_0$ Josephson junctions and the
emergence of Majorana bound states. Here, we report microwave spectroscopy
measurements that directly reveal the presence of Andreev bound states (ABS) in
ballistic semiconductor channels. We show that the measured ABS spectra are the
result of transport channels with gate-tunable, high transmission probabilities
up to $0.9$, which is required for gate-tunable Andreev qubits and beneficial
for braiding schemes of Majorana states. For the first time, we detect
excitations of a spin-split pair of ABS and observe symmetry-broken ABS, a
direct consequence of the spin-orbit coupling in the semiconductor.
