Robust engineering of phonon squeezed states in optically excited solids has emerged as a promising tool to control and manipulate their properties. However, in contrast to quantum optical systems, detection of phonon squeezing is subtle and elusive, and an important question is what constitutes an unambiguous signature of it. The state of the art involves observing oscillations at twice the phonon frequency in time-resolved measurements of the out-of-equilibrium phonon fluctuation. Using the Keldysh formalism we show that such a signal is a necessary but not a sufficient signature of a squeezed phonon, since we identify several mechanisms that do not involve squeezing and yet produce similar oscillations. We show that reliable detection requires a time- and frequency-resolved measurement of the phonon spectral function.

中文翻译：

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泵浦光谱法中压缩声子的检测

光学激发固体中声子压缩态的鲁棒工程已经成为控制和操纵其性质的有前途的工具。但是，与量子光学系统相比，声子压缩的检测是微妙而难以捉摸的，一个重要的问题是什么构成了声子压缩。现有技术涉及在时间分辨的非平衡声子波动​​的测量中观察声子频率两倍的振荡。使用Keldysh形式主义，我们证明了这种信号是压缩声子的必要但非充分的特征，因为我们确定了几种不涉及压缩但仍会产生类似振荡的机制。我们表明，可靠的检测需要对声子谱函数进行时间和频率分辨的测量。