The study of heat transport has the potential to reveal new insights into the physics of mesoscopic systems. This is especially true of those that show the integer quantum Hall effect(1), in which the robust quantization of Hall currents limits the amount of information that can be obtained from charge transport alone(2). As a consequence, our understanding of gapless edge excitations in these systems is incomplete. Effective edge-state theory describes them as prototypical one-dimensional chiral fermions(3,4)-a simple picture that explains a large body of observations(5) and suggests the use of quantum point contacts as electronic beam splitters to explore a variety of quantum mechanical phenomena(6-8). However, this picture is in apparent disagreement with the prevailing theoretical framework, which predicts in most situations(9) extra gapless edge modes(10). Here, we present a spectroscopic technique that addresses the question of whether some of the injected energy is captured by the predicted extra states, by probing the distribution function and energy flow in an edge channel operated out-of-equilibrium. Our results show it is not the case and therefore that regarding energy transport, quantum point contacts do indeed behave as optical beam splitters. This demonstrates a useful new tool for heat transport and out-of-equilibrium experiments.
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