  Quantum thermodynamics has emerged as a separate sub-discipline, revising the
concepts and laws of thermodynamics, at the quantum scale. In particular, there
has been a disruptive shift in the way thermometry, and thermometers are
perceived and designed. Currently, we face two major challenges in quantum
thermometry. First, all of the existing optimally precise temperature probes
are local, meaning their operation is optimal only for a narrow range of
temperatures. Second, aforesaid optimal local probes mandate complex energy
spectrum with immense degeneracy, rendering them impractical. Here, we address
these challenges by formalizing the notion of global thermometry leading to the
development of optimal temperature sensors over a wide range of temperatures.
We observe the emergence of different phases for such optimal probes as the
temperature interval is increased. In addition, we show how the best
approximation of optimal global probes can be realized in spin chains,
implementable in ion traps and quantum dots.
