Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. To appreciate the deep connection between quantum mechanics and thermodynamics we need only recall that in 1901 Planck introduced the quantum of action to calculate the entropy of thermal light, and in 1905 Einstein's studies of the entropy of thermal light led him to introduce the photon. Then in 1917, he discovered stimulated emission by using detailed balance arguments. Half a century later, Scovil and Schulz-DuBois applied detailed balance ideas to show that maser photons were produced with Carnot quantum efficiency (see Fig. 1A). Furthermore, Shockley and Quiesser invoked detailed balance to obtain the efficiency of a photocell illuminated by "hot" thermal light (see Fig. 2A). To understand this detailed balance limit, we note that in the QHE, the incident light excites electrons, which can then deliver useful work to a load. However, the efficiency is limited by radiative recombination in which the excited electrons are returned to the ground state. But it has been proven that radiatively induced quantum coherence can break detailed balance and yield lasing without inversion. Here we show that noise-induced coherence enables us to break detailed balance and get more power out of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE (see Fig. 3A). Furthermore, this noise-induced coherence can be robust against environmental decoherence.

Two seemingly unrelated effects attributed to quantum coherence have been reported recently in natural and artificial light-harvesting systems. First, an enhanced solar cell efficiency was predicted and second, population oscillations were measured in photosynthetic antennae excited by sequences of coherent ultrashort laser pulses. Because both systems operate as quantum heat engines (QHEs) that convert the solar photon energy to useful work (electric currents or chemical energy, respectively), the question arises whether coherence could also enhance the photosynthetic yield. Here, we show that both effects arise from the same population-coherence coupling term which is induced by noise, does not require coherent light, and will therefore work for incoherent excitation under natural conditions of solar excitation. Charge separation in light-harvesting complexes occurs in a pair of tightly coupled chlorophylls (the special pair) at the heart of photosynthetic reaction centers of both plants and bacteria. We show the analogy between the energy level schemes of the special pair and of the laser/photocell QHEs, and that both population oscillations and enhanced yield have a common origin and are expected to coexist for typical parameters. We predict an enhanced yield of 27% in a QHE motivated by the reaction center. This suggests nature-mimicking architectures for artificial solar energy devices.photosynthesis | quantum biology | population oscillations | quantum coherence A ccording to the laws of quantum thermodynamics, quantum heat engines (QHEs) convert hot thermal radiation into low-entropy useful work (1, 2). The ultimate efficiency of such QHEs is usually governed by a detailed balance between absorption and emission of the hot pump radiation (3). The laser is an example of a QHE, which can use incoherent pump (heat) radiation to produce highly coherent (low-entropy) light ( Fig. 1 A and B). Moreover, it was demonstrated both theoretically and experimentally that noise-induced quantum coherence (4) can break detailed balance and yield lasers without population inversion (5) and/or with enhanced efficiency (Fig. 1C).Recently it has been shown that quantum coherence can, in principle, enhance the efficiency of a solar cell or a photodetector (6-10). This photocell QHE (Fig. 1D) can be described by the same model as the laser QHE (Fig. 1E) and obeys similar detailed balance physics. To use the broad solar spectrum and eliminate phonon loss, we separate solar flux into narrow frequency intervals and direct it onto a cell array where each of the cells has been prepared to have its band gap equal to that photon energy (7). In particular, Shockley and Queisser (11) invoked detailed balance to show that the open-circuit voltage of a photocell is related to the energy input of a "hot" monochromatic thermal light by the Carnot factor. However, just as in the case of the laser, we can, in principle, break detailed balance by inducing coherence (Fig. 1F), which can enhance the photocell efficiency (9, 10).Other recent p...