  Starting from Polchinski's thought experiment on how to distinguish between
pure and thermal states, we construct a specific system to study the
interaction between qubit and cavity quantum field theory (QFT) in order to
provide a more operational point of view. Without imposing any restrictions on
the initial states of qubit and cavity QFT, we compute the evolution of the
system order by order by the perturbation method. We choose Landauer's
principle, an important bound in quantum computation and quantum measurement,
as the basis for the determination of the thermal state. By backtracking the
initial state form, we obtain the conditions that must be satisfied by the
cavity QFT: the expectation value of the annihilation operator should be zero,
and the expectation value of the particle number operator should satisfy the
Bose-Einstein distribution. We also discuss the difference between the thermal
state and a possible alternative to the thermal state: the canonical thermal
pure quantum (CTPQ) state.
