30 June 2009 New ICFO PhD Graduate

Dr. Ana Predojević

Thesis Committee

Dr. Ana Predojević obtained
her Doctoral Degree with a project on rubidium resonant squeezed light supervised by Prof. Morgan Mitchell.
Dr. Ana Predojević holds a degree of \"Diplomirani Fizičar\" since 2003, from University of Novi Sad, Serbia. Since 2003 she has been working in quantum optics.

Dr. Predojević’s research project is called “Rubidium resonant squeezed light from a diode-pumped optical-parametric oscillator”.

This thesis describes experimental and theoretical studies of generation of quadrature- and polarization-squeezed light suitable for interaction with rubidium atoms. Special attention is paid to phase noise, its effects on squeezing, and methods to achieve squeezing in the presence of diode laser phase noise.

Squeezed light is an important component of quantum memories experiments. Efficient storage of (squeezed) light in atomic ensembles requires the light to be resonant to the respective atomic transition. Diode lasers can access many atomic transitions as they cover significantly broader wavelength range than other classes of lasers. Consequently, employing diode-laser-based squeezed light sources would broaden the range of possible quantum memories experiments. Furthermore, diode lasers posses many attractive features like robustness, simplicity, compactness, and low price. The drawback of the diode laser is it’s excess phase noise, which results in a relatively large linewidth. This forms an obstacle for detection of phase-sensitive quantum states such as quadrature squeezing.

The thesis is structured as follows:

The first chapter presents the general ideas on parametric downconversion in an optical parametric oscillator. Here we derive the theoretical description of squeezing of the light field in a subthreshold optical parametric oscillator. The second chapter describes the experimental apparatus. First, we give a detailed description of the design of the optical parametric oscillator cavity and summarise the properties of the nonlinear crystal. In continuation, we describe the laser system and the locking systems used for the laser system and the optical parametric oscillator cavity stabilisations. Third, we discuss the amplification gain and the detection efficiency. Finally, we give a full overview of the experiment and we present the squeezing results.

The last chapter analyses the effects of phase noise on quadrature squeezing and describes a technique to eliminate its effect. First, we discuss the origin of the phase noise for diode laser systems. Second, we derive the observable squeezing taking into account the effects of quasi-static frequency fluctuations. Third, we show how the effects of the phase noise can be eliminated and, last but not least, we compare the theoretical prediction with our experimental results.

The outcome of this project is a rubidium resonant source of non-classical light. We characterised the output squeezed vacuum state. The maximum squeezing achieved in the experiment was 2.5dB below shot-noise level. Moreover, we performed an analysis of the effect the phase noise has on the squeezing. The results of this analysis showed that in presence of phase noise we expected that the squeezing level would depend on the relative delay between squeezing and local oscillator path. We experimentally tested this statement performing a measurement of squeezing as a function of the delay between the squeezed light and the local oscillator. The experimental results were consistent with the theory. Apart form building a source of rubidium resonant squeezed light we have proven that the diode laser is a source suitable for production of squeezed light. We provided a theory which treats the effect of phase noise on squeezing in optical parametric oscillator.

The experimental squeezing apparatus presented here uses standard techniques which could be applied to a variety of other wavelengths.