  We introduce a general framework for thermometry based on collisional models,
where ancillas probe the temperature of the environment through an intermediary
system. This allows for the generation of correlated ancillas even if they are
initially independent. Using tools from parameter estimation theory, we show
through a minimal qubit model that individual ancillas can already outperform
the thermal Cramer-Rao bound. In addition, due to the steady-state nature of
our model, when measured collectively the ancillas always exhibit superlinear
scalings of the Fisher information. This means that even collective
measurements on pairs of ancillas will already lead to an advantage. As we find
in our qubit model, such a feature may be particularly valuable for weak
system-ancilla interactions. Our approach sets forth the notion of metrology in
a sequential interactions setting, and may inspire further advances in quantum
thermometry.
