  Optical atomic clocks have already overcome the eighteenth decimal digit of
instability and uncertainty demonstrating incredible control over external
perturbations of the clock transition frequency. At the same time there is an
increasing demand for atomic and ionic transitions with minimal sensitivity to
external fields, with practical operational wavelengths and robust readout
protocols. One of the goals is to simplify clock's operation maintaining its
relative uncertainty at low 10-18 level. It is especially important for
transportable and envisioned space-based optical clocks. We proved earlier that
the 1.14um inner-shell magnetic dipole transition in neutral thulium possesses
very low blackbody radiation shift compared to other neutrals. Here we
demonstrate operation of a bi-colour thulium optical clock with extraordinary
low sensitivity to the Zeeman shift due to a simultaneous interrogation of two
clock transitions and data processing. Our experiment shows suppression of the
quadratic Zeeman shift by at least three orders of magnitude. The effect of
tensor lattice Stark shift can be also reduced to below 10-18 in fractional
frequency units. All these features make thulium optical clock almost free from
hard-to-control systematic shifts. Together with convenient cooling and
trapping laser wavelengths, it provides great perspectives for thulium lattice
clock as a high-performance transportable system.
