Comment on ‘‘Obtainment of thermal noise from a pure quantum state’’

Yurke and Potasek [Phys. Rev. A 36, 3464 (1987)] have shown that pure states of correlated pairs of quantum systems can lead to mixed-state behavior if only one of the systems is observed. We note that such behavior is well known and describe how the mixed-state properties of these states have led to their application in thermodynamical problems via the thermofield formalism. The correlations have been found to lead to manifestly nonclassical behavior (such as squeezing) if both systems are observed.