  We introduce a primary thermometer which measures the temperature of a
Bose-Einstein Condensate in the sub-nK regime. We show, using quantum Fisher
information, that the precision of our technique improves the state-of-the-art
in thermometry in the sub-nK regime. The temperature of the condensate is
mapped onto the quantum phase of an atomic dot that interacts with the system
for short times. We show that the highest precision is achieved when the phase
is dynamical rather than geometric and when it is detected through Ramsey
interferometry. Standard techniques to determine the temperature of a
condensate involve an indirect estimation through mean particle velocities made
after releasing the condensate. In contrast to these destructive measurements,
our method involves a negligible disturbance of the system.
