Lord Kelvin's formulation of the second law of thermodynamics prohibits any cyclic process  with he sole result that heat is absorbed by a system from a ``bath'' and  transformed into work; i.e., here can be no single-bath engine. An exception to this rule is that an observer acting as ``Maxwell's demon'' can commute information acquired by measuring the system into work in a single-bath engine [2]. Here we show that  frequent measurements of the energy of  a  quantum system immersed in a single bath enable the system to do work in a  cycle even if the measurement  results are unread (or unknown). This finding  cannot be attributed to  ``Maxwell's demon'' operation, since unread quantum measurements provide no information. Nor can it  be ascribed to  quantum  coherence in the system, which is the source of work in recently explored quantum heat engines (QHEs)[6], since unread measurements destroy such coherenceInstead, unread measurements are shown to enable the extraction of work from the system-bath quantum-correlation energy [8-10], which constitutes  a hitherto unexploited work resource.  It becomes available only when the cycle is shorter than the bath memory time. Strikingly,  this resource allows more work to be  obtained  by Maxwell's demon than  anticipated by the standard relation of information and work [3,4] and by the standard formulation of the second law for open quantum systems[14].  The second law is therefore reformulated to account for this resource. The predicted work resource may be the basis of novel nanomechanical or spin-based QHEs   embedded in a single solid-state bath. 

[1]Szilard, L. über die entropieverminderung in einem thermodynamischen system bei eingriffen intelligenter wesen. Zeitschrift für Physik A 53, 840–856 (1929).

[2]Scully, M. O., Zubairy, M. S., Agarwal, G. S. & Walther, H. Extracting Work from a Single Heat Bath via Vanishing Quantum Coherence. Science 299, 862–864 (2003).

[3] Erez, N., Gordon, G., Nest, M. & Kurizki, G. Thermodynamic control by frequent quantum measurements. Nature 452, 724 (2008).

[4] Gordon, G., Rao, D. D. B. & Kurizki, G. Equilibration by quantum observation. New Journal of Physics 12, 053033 (2010).

[5]Álvarez, G. A., Rao, D. D. B., Frydman, L. & Kurizki, G. Zeno and anti-zeno polarization control of spin ensembles by induced dephasing. Phys. Rev. Lett. 105, 160401 (2010).

[6]Landauer, R. Irreversibility and heat generation in the computing process. IBM Journal of Research and Development 5, 183 –191 (1961).

[7] Bennett, C. H. The thermodynamics of computation: a review. International Journal of Theoretical Physics 21, 905–940 (1982). 10.1007/BF02084158.

[8] Lindblad, G. Non-Equilibrium Entropy and Irreversibility (D. Reidel, Holland, 1983).