05 March 2010 New PhD Graduate

Dr. Xiaojuan Shi

Thesis Committee

Dr. Xiaojuan Shi obtained her PhD with a thesis on quantum engineering of light. Dr. Xiaojuan Shi holds an MSc in Optics from Shanxi University, China. Since 2005, she has been working at ICFO on the field of Quantum Optics, developing systems for engineering the frequency properties of paired photons.

Dr. Shi’s research project is called \"Quantum Engineering of Light: Frequency Entanglement of Paired Photons\". Her thesis was supervised by ICFO Prof. Juan P. Torres.


One of the main objectives of quantum optics is to prepare quantum light sources with the characteristics required for various quantum optical applications. Pure single photons and entangled paired photons are two of the most widely used types of quantum light. Single photons in a pure, well-defined quantum state may prove to be the building blocks for future quantum technologies. These technologies promise enormous improvements over their classical counterparts in terms of speed or security. For example, in quantum information processing, single photons may be used in quantum logic gates, playing the role of electrons in current electric circuits.

Although light is all around us in all possible colors, polarizations and intensities, it is extremely difficult to get a single photon in a pure quantum state. On the other hand, it is relatively easy to produce pairs of photons, and pure single photons can be obtained from a two-photon source in a heralded way. Thus, generating paired photons with desired photonic properties becomes one of the crucial tasks in the research area of quantum optics and related applications.

The process of spontaneous parametric down-conversion (SPDC) is one of the most convenient ways to produce pairs of photons. In SPDC, a strong pump laser beam is shone on a nonlinear crystal. Occasionally, a pump photon disappears and two new photons with lower energy are created simultaneously. The production of photon pairs in the nonlinear crystal is probabilistic and the fact that the two photons are produced simultaneously is an advantage. The two photons can be taken apart and one of them can be used as a trigger to indicate (herald) the presence of the other, thereby telling us when a photon is ready to be used. These pairs of photons have special types of correlation in their color (frequency), in their spatial shape, and/or in their polarization. For many applications, such as quantum cryptography, these types of correlation may be useful, but at the moment of generating pure single photons they are detrimental since they diminish the purity of the single photon.

In this thesis, we are interested in the frequency properties of SPDC pairs of photons, and our main goal is to demonstrate new methods to control these properties. The frequency properties of the photon pairs are described by its joint spectrum, which contains the information of the bandwidth, the waveform and the type of frequency correlation possessed by the two photons.

In the first part of our work, two new toolkits are added to the current toolbox to manipulate the joint spectrum of photon pairs. One tool is called \"pulse-front-tilt\" technique. This technique is based on the appropriate introduction of angular dispersion into the interacting waves. The feasibility of this method is demonstrated by observing a sevenfold increase of the bandwidth of photon pairs. The corresponding experiment has been accomplished by adding angular dispersion into a collinear SPDC geometry, where the interacting waves propagate at the same direction. Also, we discuss the implication of this method for the generation of narrow temporal Fourier transform limited biphotons, and paired photons with high degree of entanglement. The second tool is called \"spatial-to-spectral\" mapping technique. This technique takes effect in a noncollinear geometry, in which case the pump beam and the down-converted paired photons propagate at different directions. With this situation, the spatial features of the pump beam will be imprinted into the frequency distribution of the new created photon pairs. We experimentally prove this method by shaping the waveform, and tuning the types of frequency correlation of the down-converted two photons.

In addition, we propose a new scheme to fully control the joint spectrum of the photon pairs by combining the above two discussed techniques. In this proposal, angular dispersion is introduced into a noncollinear SPDC geometry. This opens a new way to obtain pure single photons with tunable bandwidth.

Another topic discussed in this thesis is the exploration of more compact quantum light sources with versatile characteristics. We describe an SPDC two-photon source by using Bragg reflection waveguide (BRW) in AlxGa11-xAs material as down-converting material. Here, the nonlinear process happens between a pump beam in a Bragg reflection waveguide mode, and down-converted photon pairs in total internal reflection (TIR) mode. The waveguide dispersion properties can be largely modified by implementing different ridge sizes, and this allows the described monolithic source to offer significant control over the process bandwidth. We numerically demonstrate the possibility of tuning the bandwidth of the photon pairs between 1 nm and 450 nm with the same wafer structure. The results presented in this part have been obtained as a result of the collaboration with Prof. Amr S. Helmy and his PhD student Payam Abolghasem from University of Toronto. In particular, the programs used for the presented simulations have been developed by the University of Toronto team.

To summarize, in this thesis, we add new elements to the current ways of engineering quantum light sources. Two ready to be used methods to engineer the spectrum of the photon pairs are presented. Particularly, new routes are provided to get pure heralded single photons and narrow temporal biphoton. In addition, a compact monolithic two-photon source with tunable bandwidth control is investigated, which could be very promising for further performance of various quantum applications towards reality.