Cheng Yang
Quantum optomechanical systems use radiation pressure of light to couple the optical field and the center-of-mass motion of micromechanical devices. Such systems provide powerful tools for generating and manipulating quantum mechanical states. In this thesis, a 8.3 mm long high finesse optical cavity coupled to a 1.5 mm × 1.5 mm × 50 nm stoichiometric silicon nitride membrane is used as the optomechanical system, placed at 400 mK inside a 3He fridge. The major goals of this research are: laser cooling the 261 kHz membrane vibrational mode to its quantum ground state; detecting the quantum fluctuation of radiation pressure, known as radiation pressure shot noise; and generating squeezed light.
The low mechanical frequency in this optomechanical system makes it susceptible to substantial laser phase noise. This large phase noise limits the lowest phonon number we can reach with laser cooling, and complicates the detection of mechanical motional state. In this thesis, based on Børkje’s calculations, a clear understanding of laser cooling and heterodyne detection spectra when the laser classical noise is non-negligible is presented and compared to measured results. Preliminary laser cooling results down to about 60 phonons are shown, and a method to observe radiation pressure shot noise is discussed. To reduce the laser phase noise, a filter cavity is built and is verified to have lowered the classical noise by a factor of over 560, paving the way for achieving ground state cooling and observation of radiation pressure shot noise.
The thesis begins with an overview of optomechanical systems and major efforts to achieve ground state cooling and observation of radiation pressure shot noise. The necessary theory is then presented, with a focus on the effects of laser classical noise. Experimental design and measurement methods are then discussed, highlighting our technical accomplishments by successfully implementing various feedback and feedforward schemes. A chapter is devoted to discussing the measured laser classical noise. Then measurements of optomechanical effects and laser cooling down to about 60 phonons are presented. Finally future directions using filtered lasers are discussed.