We propose a new way of looking at the quantum Maxwell's demon problem in terms of conditional action. A "conditional action" on a system is a unitary time evolution, selected according to the result of a previous measurement, which can reduce the entropy of the system. However, any conditional action can be realized by an (unconditional) unitary time evolution of a larger system and a subsequent Luders measurement, whereby the entropy of the entire system is either increased or remains constant. We give some examples that illustrate and confirm our proposal, including the erasure of $N$ qubits and the Szilard engine, thus relating the present approach to the Szilard principle and the Landauer principle that have been discussed as possible solutions of the Maxwell's demon problem.

中文翻译：

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麦克斯韦恶魔的条件动作和量子版本

我们提出了一种从条件作用的角度看待量子麦克斯韦妖问题的新方法。系统上的“条件动作”是一种单一的时间演化，根据先前的测量结果选择，可以减少系统的熵。然而，任何有条件的动作都可以通过更大系统的（无条件）幺正时间演化和随后的 Luders 测量来实现，从而整个系统的熵要么增加，要么保持不变。我们给出了一些例子来说明和证实我们的提议，包括 $N$ 量子比特和 Szilard 引擎的擦除，从而将目前的方法与 Szilard 原理和 Landauer 原理联系起来，这些原理已经被讨论为麦克斯韦妖问题的可能解决方案。