Vibration squeezing and its detection in a single molecular junction

How to characterize the electron-vibration interactions at the single-molecule level is a fundamental challenge of molecular electronics. Electron tunneling spectroscopy has so far emerged as one of the powerful tools for witnessing the vibration-mediated electron transport process, while the fluctuation properties of nonthermal vibrations cannot be revealed by simple transport signatures. Here, we study the fluctuation properties of nonthermal vibrations in a current-carrying single molecular junction with or without an external laser radiation. Without the laser drive, the fluctuations of the nonthermal vibrations with sub- and super-Poissonian statistics remain above the zero-point level. When the laser field is turned on, the nonthermal vibrations can be squeezed due to the presence of molecular coherence, where the fluctuations are shown to be reduced below the zero-point level. It is demonstrated that the squeezing and the sub- or super-Poissonian vibration statistics do not always go together. We also propose a cavity engineering to detect the predicted vibration squeezing, where the vibrations and cavity photons are entangled indicated by the two-mode squeezing. This enables optical access to vibration fluctuations in single molecular junctions.