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Register a new userQuantum Consensus Dynamics by Entangling Maxwell Demon
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We introduce a Maxwell demon which generates many-body-entanglement robustly against thermal fluctuations, which allows us to obtain quantum advantage. Adopting the protocol of the voter model used for opinion dynamics approaching consensus, the demon randomly selects a qubit pair and performs a quantum feedback control, in continuous repetitions. We derive a lower bound of the entropy production rate by demons operation, which is determined by a competition between the quantum-classical mutual information acquired by the demon and the absolute irreversibility of the feedback control. Our finding of the lower bound corresponds to a reformulation of the second law of thermodynamics under a stochastic and continuous quantum feedback control.
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We consider a feedback control loop rectifying particle transport through a single quantum dot that is coupled to two electronic leads. While monitoring the occupation of the dot, we apply conditional control operations by changing the tunneling rates between the dots and its reservoirs, which can be interpreted as the action of a Maxwell demon opening or closing a shutter. This can generate a current at equilibrium or even against a potential bias, producing electric power from information. While this interpretation is well-explored in the weak-coupling limit, we can address the strong-coupling regime with a fermionic reaction-coordinate mapping, which maps the system into a serial triple quantum dot coupled to two leads. There, we find that a continuous projective measurement of the central dot would lead to a complete suppression of electronic transport due to the quantum Zeno effect. In contrast, a microscopic model for the quantum point contact detector implements a weak measurement, which allows for closure of the control loop without inducing transport blockade. In the weak-coupling regime between the central dot and its leads, the energy flows associated with the feedback loop are negligible, and the information gained in the measurement induces a bound for the generated electric power. In contrast, in the strong coupling limit, the protocol may require more energy for opening and closing the shutter than electric power produced, such that the device is no longer information-dominated and can thus not be interpreted as a Maxwell demon.
We propose and analyze Maxwells demon based on a single qubit with avoided level crossing. Its operation cycle consists of adiabatic drive to the point of minimum energy separation, measurement of the qubit state, and conditional feedback. We show that the heat extracted from the bath at temperature $T$ can ideally approach the Landauer limit of $k_BTln 2$ per cycle even in the quantum regime. Practical demon efficiency is limited by the interplay of Landau-Zener transitions and coupling to the bath. We suggest that an experimental demonstration of the demon is fully feasible using one of the standard superconducting qubits.
The Second Law of Thermodynamics states that the entropy of a closed system is non-decreasing. Discussing the Second Law in the quantum world poses new challenges and provides new opportunities, involving fundamental quantum-information-theoretic questions and novel quantum-engineered devices. In quantum mechanics, systems with an evolution described by a so-called unital quantum channel evolve with a non-decreasing entropy. Here, we seek the opposite, a system described by a non-unital and, furthermore, energy-conserving channel that describes a system whose entropy decreases with time. We propose a setup involving a mesoscopic four-lead scatterer augmented by a micro-environment in the form of a spin that realizes this goal. Within this non-unital and energy-conserving quantum channel, the micro-environment acts with two non-commuting operations on the system in an autonomous way. We find, that the process corresponds to a partial exchange or swap between the system and environment quantum states, with the systems entropy decreasing if the environments state is more pure. This entropy-decreasing process is naturally expressed through the action of a quantum Maxwell demon and we propose a quantum-thermodynamic engine with four qubits that extracts work from a single heat reservoir when provided with a reservoir of pure qubits. The special feature of this engine, which derives from the energy-conservation in the non-unital quantum channel, is its separation into two cycles, a working cycle and an entropy cycle, allowing to run this engine with no local waste heat.
It is shown that the possibility of using Maxwell demon to cheating in quantum non-locality tests is prohibited by the Landauers erasure principle.
We study the reduction in total entropy, and associated conversion of environmental heat into work, arising from the coupling and decoupling of two systems followed by processing determined by suitable mutual feedback. The scheme is based on the actions of Maxwells demon, namely the performance of a measurement on a system followed by an exploitation of the outcome to extract work. When this is carried out in a symmetric fashion, with each system informing the exploitation of the other (and both therefore acting as a demon), it may be shown that the second law can be broken, a consequence of the self-sorting character of the system dynamics.
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