Quantum Beats in Second-order Coherence: Principles and Applications

Abstract

This dissertation is based on a novel experimental discovery of what we have labeled as a “ghost frequency comb (GFC),” observed from the measurements of nonlocal correlations with a laser beam. Unlike a conventional frequency comb, the laser beam used in this work does not consist of a pulse train—instead it is in a continuous-wave (cw) operation. In addition, the laser beam is in a multi-longitudinal-mode coherent state, far from a thermal state. The intensity fluctuations of the laser beam are found to be correlated within periodically spaced, precise, narrow time windows, giving periodic sharp correlation peaks. However, as long as the relative delay of the correlation falls into the region between these peaks, the intensity fluctuations of the laser beam remain uncorrelated. These experimental observations lead one to speculate whether there is a light source with such a peculiar statistical behavior, and beg two important questions: (1) How could a cw laser beam, approximated to be in coherent state, produce nontrivial intensity correlations like thermal state? (2) How could a cw laser generate a frequency comb? It is not surprising that a mode-locked laser can generate a train of sharp pulses as frequency comb, but the observation of a frequency comb from a cw laser beam would be unexpected. This dissertation provides a conclusive answer to these fundamentally interesting questions: the GFC is the result of two-photon interference—a pair of distinguishable groups of indistinguishable photons interfering with the pair itself. It is the nonlocal two-photon beats that produce the comb-like intensity fluctuation correlation. Besides its fundamental importance, this dissertation also explores useful applications of the GFC in precision spectroscopy. Nontrivial correlations of light have been a subject of debate since the development of intensity interferometry by Hanbury Brown and Twiss (HBT) in the 1950s. Conventional theory suggests that the HBT correlation of distant stars is the intrinsic statistical property of thermal state. Questions such as “Are the photons ‘bunched’ or ‘anti-bunched’ at the light source?” have been raised for decades. This dissertation also attempts to address this fundamental issue surrounding the origin of second-order correlation. In addition to the work with multi-cavity-mode cw laser beam mentioned above, a study of the second-order correlation of a multi-color or multi-frequency thermal light source in interferometric settings is reported. It is concluded that, like the GFC, the second-order correlation in multi-frequency thermal light also results from two-photon beats. Thus, this dissertation provides solid experimental evidence and theoretical analysis for exploring the nonlocal quantum interference as the origin of nontrivial correlations of light in general. The findings presented here should further corroborate the view that the two-photon correlation picture provides an accurate result in second-order coherence measurements.