| Squeezing is a purely quantum-mechanical effect, and an immediate result of Heisenberg uncertainty principle. Many proposed applications of squeezing involve precision measurement, gravitational wave detection, improvement of spectroscopic sensitivity, enhancement of signal-to-noise ratio in optical communication, and so on. There are many ways of generating squeezed states. Among them is by using squeeze operator. In this dissertation we mainly construct squeeze operator of atom and optical field through quantum reservoir. Squeezing has a close connection with entanglement. Entanglement is a type of nonclassical correlation between the subsystems of bipartite or multipartite systems. Nowadays entanglement is increasingly viewed as an indispensable resource, and plays a significant role in quantum information processing and quantum communication.First, we show that, in analogy to light fields, effective bosonic modes of atomic ensembles in the Holstein-Primakoff representation can be prepared in multimode squeezed states, and multipartite continuous-variable cluster and GHZ entangled states. Our scheme uses a cascade of optical cavities, in each of which one atomic ensemble is placed. Through Raman transitions between stable atomic ground states, the atomic ensembles are in the interactions with the cavity and laser fields. Because of the long-lived ground states, the atomic ensembles act as quantum network nodes, whose cluster entangled states are applicable in the one-way quantum computer, and whose GHZ entangled states have potential for quantum communication.Second, we show that it is possible to use an atom-cavity reservoir to prepare the two-mode squeezed and entangled states of a hybrid system of an atomic ensemble and an optical field, which do not directly interact with each other. The essential mechanism is based on the combined effect of a two-mode squeezing interaction and a beam-splitter interaction between the system and the reservoir. The reservoir mechanism is important for quantum networking in that it allows interface between a localized matter-based memory and an optical carrier of quantum information without direct interaction.Finally, in an electromagnetically induced transparency system, the atoms have long-lived coherence compared to the cavity lifetime and interact nonadiabatically with the laser fields. We show that the high frequency fluctuations of both the intensities and the intensity difference can be squeezed below the shot noise limit due to the nonadiabatic effects. This noise squeezing can be used to enhance the precision in the short time measurements based on the intensities or the intensity difference. |
