A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED). The strong coupling regime between the atom and cavity-confined photon corresponds to the reversible exchange of energy between the two modes, and underpins a wide range of CQED phenomena with applications in quantum information science, including for example as quantum logic gates and as sources of entangled states. An important advance was achieved recently when strong coupling between excitons and cavity photons was reported for the first time for localized quantum dots (QDs) in micron-size solid state cavities. This has significance in terms of scalability and integration with other optical devices, and could lead to the emergence of "quantum optics on a chip" technology. However the results presented so far for quantum dots are in the linear regime, corresponding to coupling to the vacuum field (vacuum Rabi splitting); they are not a true QED effect and can equally well be described by classical physics as the coupling between two oscillators. In this paper, we present evidence for a purely quantum phenomenon for the QD/cavity photon system, namely the increase in splitting of the levels when the mean number of photons in the cavity is increased. This corresponds to non-linearities on the single-photon scale: the presence of a single excitation in the cavity changes the level structure, affecting the emission energies for a second photon. Such results are a first step in demonstrating the promise of quantum dots for CQED applications.
A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED). The strong coupling regime between the atom and cavity-confined photon corresponds to the reversible exchange of energy between the two modes, and underpins a wide range of CQED phenomena with applications in quantum information science, including for example as quantum logic gates and as sources of entangled states. An important advance was achieved recently when strong coupling between excitons and cavity photons was reported for the first time for localized quantum dots (QDs) in micron-size solid state cavities. This has significance in terms of scalability and integration with other optical devices, and could lead to the emergence of "quantum optics on a chip" technology. However the results presented so far for quantum dots are in the linear regime, corresponding to coupling to the vacuum field (vacuum Rabi splitting); they are not a true QED effect and can equally well be described by classical physics as the coupling between two oscillators. In this paper, we present evidence for a purely quantum phenomenon for the QD/cavity photon system, namely the increase in splitting of the levels when the mean number of photons in the cavity is increased. This corresponds to non-linearities on the single-photon scale: the presence of a single excitation in the cavity changes the level structure, affecting the emission energies for a second photon. Such results are a first step in demonstrating the promise of quantum dots for CQED applications.
In this paper, exciting progress of quantum optics in solid state is reviewed. The focus is on semiconductor microcavities with self-assembled quantum dots embedded in the active layer. Due to enormous progress in semiconductor nanotechnology, such photonic structures have become a model system for the study of quantum optics on a scalable and integrable technology platform with high potential for future applications in quantum information technology. Quantum optical phenomena have become accessible due to 3-D confinement of light and matter on the length scale of their wavelength in state-of-the-art semiconductor micro- and nanostructures. This confinement leads to a quantization of the associated photonic and electronic energy levels and requires a quantum mechanical description of the system in the framework of cavity quantum electrodynamics (cQED). This approach considers the dipole interaction between two-level quantum emitters and discrete photonic states of a microcavity. Within the well-known Jaynes-Cummings model, the dipole interaction is described in terms of a coherent exchange of energy between the emitter and the resonator mode. This coherent interaction in the so-called strong coupling regime of cQED is reflected in a normal mode splitting, the vacuum Rabi splitting, of the involved modes and represents a central feature of quantum optics in solid state. Another important example of quantum optics in semiconductor nanostructures is the generation of nonclassical light in specific quantum devices. Of particular interest is the realization of Fock states which represent states containing a well-defined number of photons. Single-photon sources, for instance, allow for the generation of single photons on demand, which is highly desirable for quantum communication systems. In this context, this review paper will present recent experimental studies of quantum optics in solid state. This paper is meant for readers who would like to become familiar with this topic and for experts being interested in the progress in this field. It will cover a broad range of studies ranging from examples of fundamental light-matter interaction in the quantum limit to devices capable of emitting single photons and entangled photon pairs on demand.
This thesis is devoted to theoretically study the systems of single quantum dots (QDs) embedded in various photonic nanostructures, in close relation with experimental data. The word single is essential: due to their nanometer size, QDs have mainly been studied as ensembles. It is only recently that the experimental progress, in particular in the domain of QDs growth, has allowed to address single QDs. This helped to observe that the confined states in a QD resemble those of an atom, leading in turn to the macro-atom picture for QDs. It opened the path to the study of cavity quantum electrodynamics (CQED) in the solid state, as the analogous of a system of an atom in an electromagnetic cavity. However, already at the level of the QD (i.e. without a solid state cavity), the macro-atom picture has been proven to be oversimplified. Various effects related to the solid state environment of the QD have been experimentally and theoretically evidenced, like the influence of acoustic and optical phonons. These are channels of decoherence for the otherwise well isolated and confined QD levels. When adding the cavity, these effects are possibly enhanced, which can lead to very large deviations from the CQED ideal picture. Therefore the clear understanding of these effects is of essential importance, especially in view of the application of solid state systems to quantum information science. In Chapter 2, we address the radiation-matter coupling between two QDs in a nanocavity. This field of research combines the high quality of light resonators to the nonlinearities of the semiconductor material. It is promising in view of manipulating and transferring single long-lived excitations, an essential goal for quantum information science. Then, we study the deviations of a QD from the idealized macroatom picture. First, in Chapter 3, we include the influence of acoustic phonons on the light emitted by a QD-cavity system. We show that the QD modified susceptibility leads to the qualitative explanation of the cavity feeding effects, which is one of the most striking deviations of the QD-cavity system from the ideal CQED picture. In addition to phonons are extended two-dimensional states populated by the non resonant pumping laser in the QD's wetting layer (WL). In Chapters 4 and 5 we build a Monte-Carlo model of the excitation-decay mechanism in a QD-cavity system. We use it to describe the influence of WL states on the dynamics of the QD, leading in turn to a non-CQED behavior. This approach is particularly successful in the case of large QD-cavity detuning, where it predicts both time-dependent and spectral characteristics of the emitted light. Finally, in Chapter 6, we discuss our conclusions, emphasizing in particular that the knowledge of the actual solid state environment of the QD leads to a better control over the parameters allowing to reach the CQED regime. We also discuss the possible extensions of this work, in particular in relation to the analysis of the stimulated emission regime.
The strong-coupling regime of cavity-quantum-electrodynamics (cQED) represents the light-matter interaction at the fully quantum level. Adding a single photon shifts the resonance frequencies, a profound nonlinearity. cQED is a test-bed of quantum optics and the basis of photon-photon and atom-atom entangling gates. At microwave frequencies, success in cQED has had a transformative effect. At optical frequencies, the gates are potentially much faster; the photons can propagate over long distances; and the photons can be detected easily, ideal features for quantum networks. Following pioneering work on single atoms, solid-state implementations using semiconductor quantum dots are emerging, an important prospect for quantum technology. We present here a gated, ultralow-loss microcavity-device which forms a highly coherent photon−quantum-dot interface with a cooperativity of C=150. The gates allow both the quantum dot charge state and resonance frequency to be controlled electrically; crucially, they eliminate the noise-source which has complicated quantum dot cQED in the past − scattering from the bare microcavity mode even at the quantum dot-microcavity resonance. Even in the microcavity, the quantum dot has a linewidth close to the radiative limit. In addition to a very pronounced avoided-crossing in the spectral domain, we observe a clear coherent exchange of a single energy-quantum between the ``atom" and cavity in the time domain (vacuum Rabi-oscillations). Decoherence arises predominantly via the atom and photon loss-channels. The coherence is exploited to probe the transitions between the singly- and doubly-excited photon-atom system via photon-statistics spectroscopy. We propose this system as a platform for quantum technology, for instance a photon-photon entangling gate.
Theoretical study on entangled photon generation from a quantum dot (QD) embedded in microcavity is briefly reviewed within a strong coupling regime. In this system, entangled photon pairs can be generated through the cascade photon emission via dressed states in the cavity quantum electrodynamics (CQED). Excitation levels of a QD are modeled by a V-type three-level system and four-level system including a biexciton. In contrast to the entangled-photon generation until now, all four Bell states can be generated from an identical cavity system by simply selecting applied-field polarizations and frequencies as characteristic features of the CQED. The CQED effects play a crucial role in providing a high degree of entanglement: (i) spectral fi ltering can be used to extract entangled photons due to the vacuum Rabi splitting and (ii) non-entangled co-polarized photons are strongly suppressed due to the photon blockade effect.
Semiconductor quantum dots (QDs) are commonly considered as artificial atoms due to the 3D confinement of the electrons. These nanostructures are very promising for the realization of integrated devices such as single photon sources for quantum cryptography applications [1,2]. However, contrarily to genuine atoms, QDs are condensed matter systems and they consequently suffer from the coupling to their environment [3] which degrades the quality of the emitted photons in view of quantum cryptography applications. One way to circumvent this coupling is to excite strictly resonantly the QD at low temperature [4-10]. This difficult task has been achieved by designing an original excitation scheme to spatially decouple the excitation and detection paths. We first report that experimental studies of the resonant emission (RE) of single QDs can be strongly limited, even impossible, since most of the QDs show a very strong quenching of the RE of the fundamental transition. This issue is a broadly met experimental situation which is highly problematic for cavity quantum electrodynamics experiments in single QDs. We propose an efficient way to overcome this issue based on the use of an ultra-weak non-resonant laser that optically gates the QD RE [8] which controls the residual doping inherent to the QDs fabrication. It induces a complete recovery of the properties of an artificial atom, and the single QDs behave as quasi-ideal two-level systems. Up to now [7,9], the measurements on the QDs RE reported in the literature mainly focus on the incoherent fluorescence signal for achieving efficient single photon emission. In this regime, the spectrum linewidth is always limited to the radiative limit. We present experimental studies on the coherent laser light scattering by a single QD [7]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. By reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in what we name an " ultra-coherent " character [7]. In conclusion, the resulting ultra-coherent single photon source promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. References [1] P. Michler et al., Science 290 (2000). [6] S. Ates et al., PRL 103 (2009). [2] C. Santori et al., Nature 419 (2002). [7] H. S. Nguyen et al., APL 99 (2011). [3] A. Berthelot et al., Nat. Phys. 2 (2006). [8] H. S. Nguyen et al., PRL 108 (2012). [4] A. Muller et al., PRL 99 (2007). [9] C. Matthiesen et al., PRL 1082 (2012). [5] R. Melet et al., PRB 78 (2008). [10] A. Reinhard et al., Nat. Phot. 6 (2012).
Semiconductor quantum dots (QDs) have emerged as promising candidates for studying quantum optical phenomena. In particular, cavity-quantum electrodynamics effects can be investigated using a single QD embedded inside a photonic nanostructure, where both the carriers and photons are confined within sub-micron length scales in all three dimensions. Since QD location inside the cavity is fixed by the growth, this system is free of the stringent trapping requirements that limit its atomic counterpart. The possibility of fabricating photonic nanostructures with ultra-small optical-mode volumes and long photon lifetimes enhances the prospects for applications in quantum information processing.