(Phys.org)—Quantum engines are known to operate differently than—and in some cases, outperform—their classical counterparts. However, previous research on the performance of quantum engines may be overestimating their advantages. In a new study, physicists have developed an improved method to compute the efficiency of quantum engines. They show that the ultimate efficiency of quantum systems is subject to tighter fundamental limits than those imposed by the second law of thermodynamics, which governs the efficiency of classical systems.