A Photo-Carnot engine is a Carnot cycle engine in which the working medium is a photon inside a cavity with perfectly reflecting walls. Radiation is the working fluid, and the piston is driven by radiation pressure.
The deep physics behind the second law of thermodynamics is not violated; nevertheless, the quantum Carnot engine has certain features that are not possible in a classical engine.
The internal energy of the photo-Carnot engine is proportional to the volume (unlike the ideal-gas equivalent) as well as the 4th power of the temperature (see Stefan-Boltzmann law).
The Radiation pressure is only proportional to this 4th power of temperature but no other variables, meaning that for this photo-Carnot engine an isotherm is equivalent to an isobar.
Using the First law of thermodynamics () we can determine the work done through an adiabatic () expansion by using the chain rule () and setting it equal to
Combining these gives us which we can solve to find
The efficiency of this reversible engine must be the Carnot efficiency, regardless of the mechanism and so
- Marlan O. Scully, M. Suhail Zubairy, G. S. Agarwal, and Herbert Walther (2003-02-07). "Extracting Work from a Single Heat Bath via Vanishing Quantum Coherence". Science 299 (5608): 862–864. Bibcode:2003Sci...299..862S. doi:10.1126/science.1078955. PMID 12511655.
- Zubairy, M. Suhail (2002). "The Photo-Carnot Cycle: The Preparation Energy for Atomic Coherence". QUANTUM LIMITS TO THE SECOND LAW: First International Conference on Quantum Limits to the Second Law. AIP Conference Proceedings 643. pp. 92–97. doi:10.1063/1.1523787.
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