The efficiency of a quantum heat engine is maximum when the unitary strokes of the quantum Otto cycle are adiabatic. On the other hand, this may not be always possible due to small energy gaps in the system, especially at the critical point (CP) where the gap between the ground state and the first excited state vanishes and the system gets excited. With the aim to regain this lost adiabaticity, we modify one of the unitary strokes of the Otto cycle by allowing the system to first evolve with a time dependent Hamiltonian as in the case of a usual Otto cycle, followed by an additional evolution with a different time independent Hamiltonian so that the system reaches a less excited state. This will help in increasing the magnitude of the heat absorbed from the hot bath so that the work output and efficiency of the engine can be increased. We demonstrate this method using an integrable model and a non-integrable model as the working medium and discuss the generality and limitations of this method. In the case of a two spin system, the optimal value for the time till which the system needs to be freely evolved is calculated analytically in the adiabatic limit. The results show that implementing this modified unitary stroke significantly improves the work output and efficiency of the engine, especially when it crosses the CP.

中文翻译：

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使用改进的奥托循环提高量子热机的性能

当量子奥托循环的单一冲程是绝热时，量子热机的效率最大。另一方面，由于系统中的能隙较小，这可能并不总是可能的，特别是在基态和第一激发态之间的间隙消失并且系统被激发的临界点（CP）处。为了重新获得这种失去的绝热性，我们修改了奥托循环的单一行程之一，允许系统首先像通常的奥托循环一样，用时间相关的哈密顿量演化，然后用不同的哈密顿量进行额外的演化。与时间无关的哈密顿量，使系统达到较低的激发态。这将有助于增加从热水浴中吸收的热量，从而提高发动机的功输出和效率。我们使用可积模型和不可积模型作为工作介质来演示该方法，并讨论该方法的通用性和局限性。在双自旋系统的情况下，系统需要自由演化的时间的最佳值是在绝热极限下分析计算的。结果表明，实施这种改进的单一冲程可显着提高发动机的工作输出和效率，特别是在穿过 CP 时。