We present experimental and theoretical studies of a hemispherical, high-solid angle external optical micro-cavity strongly coupled to nanoscale optical centers for cavity-quantum electrodynamics (QED) strong coupling and efficient single-photon sources. Implementations of single-photon sources based on various optical centers have been reported in the last three decades. The need for efficient single-photon sources, however, is still a major challenge in the context of quantum information processing. In order to efficiently produce single photons, single optical centers are coupled to a resonant high-finesse optical micro-cavity. A cavity can channel the spontaneously emitted photons into a well-defined spatial mode and in a desired direction to improve the overall efficiency, and can alter the spectral width of the emission. It can also provide an environment where dissipative mechanisms are overcome so that a pure quantum-state emission takes place.
We present experimental and theoretical studies of a hemispherical, high-solid angle external optical micro-cavity strongly coupled to nanoscale optical centers for cavity-quantum electrodynamics (QED) strong coupling and efficient single-photon sources. Implementations of single-photon sources based on various optical centers have been reported in the last three decades. The need for efficient single-photon sources, however, is still a major challenge in the context of quantum information processing. In order to efficiently produce single photons, single optical centers are coupled to a resonant high-finesse optical micro-cavity. A cavity can channel the spontaneously emitted photons into a well-defined spatial mode and in a desired direction to improve the overall efficiency, and can alter the spectral width of the emission. It can also provide an environment where dissipative mechanisms are overcome so that a pure quantum-state emission takes place.
Solid-state sources capable of emitting single photons on demand are of great interest in quantum information applications. Ideally, such a source should emit exactly one photon into the collection optics per trigger, the emitted photons should be indistinguishable, and the source should be electrically driven. Several design strategies addressing these requirements have been proposed. In the cavity-based source, light emission is controlled using resonant cavity quantum electrodynamics effects, whereas in the waveguide-based source, broadband electric field screening effects are employed to direct the light emission into the optical mode of interest. For all the strategies, accurate modeling and careful optical engineering is required to achieve high performance. In this study, we discuss the models and numerical techniques used to analyze such structures. The physical effects governing the light emission profile and the possibilities of tailoring it as well as the mechanisms governing the coherence are elucidated. The major design strategies pursued to optimize the single-photon source performance and the remaining challenges are reviewed.
In the pursuit of developing a quantum information network, photons appear to be the most convenient carriers to interconnect distant ports. The need to get on-demand single photons that is one and only one photon, with a high reliability, is the main driving force for the development of bright solid-state sources. One important parameter is the brightness defined as the probability that one collected pulse contains a single photon. For some applications like quantum information processing or long distance quantum communications, the emitted single photons must be also indistinguishable, so that can make use of their quantum interference to implement effective photon-photon interactions. To reach the ideal source of single and indistinguishable photons, different systems are explored : defects in diamond (NV centers), trapped atoms or ions, single molecules or semiconductor quantum dots.By coupling a semiconductor quantum dot to an optical cavity, the spontaneous emission of the emitter can be modified to obtain bright single-photon sources. An innovative technique was developed by Pr. Pascale Senellart and her team at the Laboratory of Photonics and Nanostructures (LPN) from CNRS that allows making such sources in a very reproducible way.This work explores the performance of single quantum dot coupled to micropillars. Various techniques are used in order to increase the single photon purity and indistinguishability while keeping a high source brightness. First, the cavity was modified using an adiabatic architecture such that a strong acceleration of the spontaneous emission was implemented. Then, a technique to apply an electric bias on the micropillars has been developed. The combination of the electric bias with a resonant optical excitation of the quantum dot allows to generation purely indistinguishable photons with a high brightness.The results developed in this thesis open a vast field of novel applications in quantum technologies, from quantum cryptography, metrology to quantum imaging.
Solid-state sources capable of emitting single photons on demand are of great interest in quantum information applications. Ideally, such a source should emit exactly one photon into the collection optics per trigger, the emitted photons should be indistinguishable and the source should be electrically driven. Several design strategies addressing these requirements have been proposed. In the cavity-based source, light emission is controlled using resonant cavity quantum electrodynamics effects, whereas in the waveguide-based source, broadband electric field screening effects are employed to direct the light emission into the optical mode of interest. For all the strategies, accurate modeling and careful optical engineering is required to achieve high performance.
A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.
Historically non-classical sources of single photons have been used to test quantum mechanics and to develop the field of quantum optics. Nowadays the single photon source has become a critical device for quantum communications and optical quantum information processing [Knill, et al., 2001]. A single photon source emits one photon at a time, and its quality can be evaluated by three parameters: quantum efficiency, multi-photon probability, and photon distinguishability. Among the different types single photon sources, InGaAs/GaAs quantum dots (QDs) are a promising solid state candidate showing high quantum efficiency, no bleaching effect, long-term stability, capable of high repetition rate, and compatible with standard semiconductor processing techniques. Furthermore, QDs can be embedded in micro-cavities or photonic crystal nano-cavities by in situ growth, and thus cavity quantum electrodynamics (CQED) can be exploited to improve the performance of QD-based single photon sources in all aspects mentioned above [Santori, et al., 2002].
Photons are excellent carriers of quantum information. The generation, manipulation and read-out of photonic quantum states have enabled pioneering investigations in quantum physics, paving the way to groundbreaking technologies. Integrated quantum photonics, that is the processing of photons on-chip, is at the forefront of this revolution. In particular, silicon integrated circuits have emerged as the dominant material platform for large-scale quantum photonics. However, the difficulty to inject single photons on-demand in a silicon chip is a major obstacle to the level of complexity of current systems. In this context, the isolation of individual fluorescent defects in silicon, called color centers, opens an avenue toward the integration of atomic sources of single photons. While several color centers have demonstrated an ability to generate antibunched photons, practical applications require tailoring of their emission through quantum electrodynamics effects in optical cavities.This PhD thesis is devoted to cavity quantum electrodynamics with color centers in silicon optical cavities, in view of building an efficient source of single photons. The efforts are first focused on ensembles of G centers in microring cavities. The optical properties of cavity-coupled G centers are studied using microphotoluminescence spectroscopy, and an enhancement of their zero-phonon emission rate by Purcell effect is demonstrated. However, a quantitative analysis reveals a poor quantum efficiency, motivating the choice of another emitter for further experiments. Subsequently, the efforts are focused on the W center. Purcell enhancement of the overall recombination rate is observed for an ensemble of W centers in a circular Bragg grating cavity. Using a quantitative analysis based on finite-difference time-domain calculation, a high quantum efficiency is estimated. The results being encouraging for the purpose of single photon emission, an optical setup is developed for single-emitter spectroscopy and a strategy is devised to fabricate individually-adressable emitters, leading to the isolation of single W centers in unpatterned silicon-on-insulator. Using a nanofabrication process allowing for deterministic positionning, the emitters are embedded into circular Bragg grating cavities, resulting in an increase of the photoluminescence intensity by orders of magnitude by Purcell effect. This work leads to the demonstration of a source of triggered single photons.A next milestone will be the demonstration of a single photon source for integrated photonic circuits, based on a color center in a waveguide-coupled optical cavity. Beyond the interest for integrated quantum photonics, the observation of bright photoluminescence from single color centers opens perspectives for further experiments, allowing to probe fundamental questions relative to their photophysics.