Emulating a quantum Maxwell’s demon with nonseparable structured light

Maxwell’s demon (MD) has proven to be an instructive vehicle for exploring the relationship between information theory and thermodynamics. A long-standing debate has been the concern of entropy violation, now resolved by the introduction of a quantum MD that can enact reversible operations on a system. However, implementing it experimentally is challenging, as it demands precise control over multi-particle entangled states and the execution of entangling and disentangling operations with high accuracy. Here, we show how this can be emulated using vectorial structured light that is nonseparable in the spin and orbital angular momentum (OAM) internal degrees of freedom of each photon in a classical laser beam. We experimentally demonstrate that the demon’s classical entropy, linked to the uncertainty in spin degree of freedom of each photon, increases during the process while that in the system’s state (represented by OAM per photon) decreases. This is achieved by entangling the demon’s memory with the system, allowing the demon to acquire quantum information and utilize it to control the OAM states of the system after a disentangling operation. As a result, we demonstrate that the quantum demon can emulate the extraction of useful work from the system in the form of OAM, thereby opening a path to information-driven optical spanners for the mechanical rotation of objects using light. Our demonstration can easily be extrapolated to other degrees of freedom, for robust and scalable implementations of MDs at both the classical and quantum realms, enlightening the role of a structured light as a tool for exploiting principles in thermodynamics to control and measure information.