The invention provides systems, devices, and methods for heating and cooling chemical or biological samples, such as genetic materials during Polymerase Chain Reaction (“PCR”). The systems, devices, and methods comprise use of a fluid that performs repeated heating and cooling cycles, e.g., ‘thermal cycling’, on sample reactants with a phase changing fluid during evaporation and condensation. The systems, devices, and methods eliminate the need for a heating block as a means to obtain fast and uniform thermal cycling. The disclosure also describes the use of an optical system in conjunction with the thermodynamic cycler for real-time detection. Ultimately, uniformity and speed of the thermodynamic cycler provides for higher sensitivity and throughput of gene replication and detection.