  Topological superconductivity is an exotic phenomenon due to the
symmetry-protected topological surface state, in which a quantum system has an
energy gap in the bulk but supports gapless excitations confined to its
boundary. Symmetries including central and time-reversal (TRS), along with
their relations with topology, are crucial for topological superconductivity.
We report muon spin relaxation/rotation ($\mu$SR) experiments on a topological
noncentrosymmetric superconductor PbTaSe$_2$ to study its TRS and gap symmetry.
Zero-field $\mu$SR experiments indicate the absence of internal magnetic field
in the superconducting state, consistent with previous $\mu$SR results.
Furthermore, transverse-field $\mu$SR measurements reveals that the
superconducting gap of PbTaSe$_2$ is an isotropic three-dimensional
fully-gapped single-band. The fully-gapped results can help understand the
pairing mechanism and further classify the topological superconductivity in
this system.
