Introduction

Quantum optics provides a complete description of the resonant interaction of light and a material system. The light scattered as a result of this interaction – the resonance fluorescence – provides a means to monitor and affect the material system's dynamics and also allows for the generation of nonclassical states of the electromagnetic field. Initial theoretical studies characterizing resonance fluorescence addressed fundamental questions regarding the necessity of quantizing the electromagnetic field. This is a particularly illustrative period in optical physics where multiple theories existed that might describe resonance fluorescence – semi-classical optical Bloch equations [1, 2] or quantum electrodynamics [4, 3] – and experimental evidence was necessary to identify the appropriate theoretical framework. Indeed, experiments in 1974 and 1977 investigating the resonance fluorescence lineshape [5, 6] and the photon statistics of the scattered light [7] vindicated the full quantum electrodynamical description of light-matter interaction over other models. In the subsequent 30 years, resonance fluorescence has been used as a high-resolution spectroscopic probe of a variety of physical systems, as a means to investigate quantum coherence in atomic gases and, with the current interest in quantum information science (QIS), as a tool for quantum-state preparation, measurement and control in trapped ions [8].

During this same time period, advances in material science have ushered in an era where it is possible to grow and fabricate solid-state devices at length scales commensurate with the quantum confinement of electrons and holes. It is only recently that resonance fluorescence has been observed from the prototypical artificial nanostructure exhibiting three-dimensional quantum confinement – a single semiconductor quantum dot (QD) [9].

Introduction

During the past two decades, the development of micro- and nano-fabrication technologies has positively impacted multiple areas of science and engineering. In the photonics community, these technologies had numerous early adopters, which led to photonic devices that exhibit features at the nano-scale and operate at the most fundamental level of light–matter interaction [28, 39, 18, 29]. One of the leading platforms for these types of devices is based on gallium arsenide (GaAs) planar photonic crystals (PC) with embedded indium arsenide (InAs) quantum dots (QDs). The PC architecture is advantageous because it enables monolithic fabrication of photonic networks for efficient routing of light signals of the chip [26]. At the same time, PC devices have low loss and ultra-small optical mode volumes, which enable strong light–matter interactions. The InAs quantum dots are well suited for quantum photonic applications because they have excellent quantum efficiencies, large dipole moments, and a variety of quantum states that can be optically controlled [24, 3].

Currently, the development of these photonic technologies is geared mainly towards applications in quantum and classical information processing. The first proposals for quantum information processing using QDs in optical microresonators were developed more than a decade ago in the broader context of quantum information processing using quantum systems (such as atoms, ion, molecules) that can be optically controlled [23, 17]. Compared to other systems, the solid-state quantum photonic platform is attractive for quantum information applications because of its potential for large-scale integration [27].

We show in this review that the spin state of a single magnetic atom embedded in an individual semiconductor quantum dot can be optically probed. A high degree of spin polarization can be achieved for an individual Mn atom using quasi-resonant or fully resonant optical excitation of the quantum dot at zero magnetic field. Under quasi-resonant excitation, optically created spin-polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. Monitoring the time dependence of the intensity of the fluorescence during a resonant optical pumping process allows us to directly probe the dynamics of the initialization of the Mn spin. The dynamics and the magnetic field dependence of the optical-pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. The Mn spin state prepared by optical pumping is fully conserved for a few microseconds. These experiments open the way to full optical control of the spin state of an individual magnetic atom in a solid state environment.

Introduction

The ability to control spins in semiconductor nanostructures is an important issue for spintronics and quantum information processing. Single-spin detection and control is a key but very challenging step for any spin-based solid-state quantum computing device. In the past few years, efficient optical techniques have been developed to control the spin of individual carriers [34] or ensemble of nuclei [22] in semiconductor quantum dots (QDs).

Introduction

Colloidal semiconductor nanocrystals were the first model systems to evidence radiusdependent energy shifts of excitonic states caused by three-dimensional quantum confinement (for reviews see e.g. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and references therein). Besides the large tunability of optical emission wavelength, such nanocrystal quantum dots exhibit high quantum efficiency q and can nowadays replace organic dyes in various applications, e.g. act as biomarkers or active laser medium. In the broad field of nanoplasmonics, combinations of metallic nanostructures with semiconductor nanocrystals have potential applications in optoelectronics. The enhancement of spontaneous emission rate near metallic surfaces makes nanocrystals attractive candidates as probes of electromagnetic field distribution and nano-antenna effects. Charge separation at the metal–semiconductor interface can be applied in photocatalytic processes as was recently shown for production of hydrogen in multicomponent metal–semiconductor nanocrystal structures [11, 12]. The modification of optical and electronic properties of colloidal nanocrystals close to a metallic surface is a longstanding issue of research and will be reviewed in this chapter, in particular with a focus on the latest developments. We will start with an overview of fundamental properties of colloidal quantum dots along with a presentation of the most recent results in the field of functionalized colloidal nanostructures. We consider semiconductor and metallic nanostructures separately and describe their properties as individual building blocks for future complex nanosystems. In the following sections we deal with coupling schemes of quantum dots to metal surfaces, discuss practical applications in all-optical plasmonic devices and outline perspectives in quantum optics with surface plasmons.

In 1946, E. M. Purcell predicted that the radiative lifetime of an emitter is not an intrinsic property but can be modified by structuring the surrounding electromagnetic field [36]. By inserting a semiconductor quantum dot (QD) in an optical cavity, one can accelerate or inhibit its spontaneous emission. In the present article, we show that the QD spontaneous emission can be deterministically controlled to fabricate bright sources of quantum light.

QDs in cavities: basics, motivation, first demonstrations

Light-matter coupling

We note f the ground state of the QD and e its excited state. For a cavity mode close to resonance with the QD optical transition, we consider only the states with 0 or 1 photon in the cavity mode. The states ∣e, 0〉 and ∣ f, 1〉 are coupled through light–matter interaction, with a constant g, where hg = ∣〈e, 0∣ Ed∣ f, 1∣, with d the dipole of the optical transition e → f and E the electric field at the QD position.

Each of the states ∣e, 0 > and ∣ f, 1 > are also coupled to continua of states: continuum of the free-space optical mode, phonons of the semiconductor matrix, etc. [4]. Here, we consider only the coupling to the continuum of the free-space optical mode, related to the cavity losses, with a constant γc. When g << γc, the photon emitted by the recombination of an exciton efficiently escapes outside the cavity. The QD optical transition radiative recombination rate can be accelerated (Purcell effect) or inhibited.

Introduction

Quantum information technology promises to offer incredible advantages over current digital systems, allowing intractable problems in science and engineering to be tackled almost instantaneously through quantum computing, and unconditionally secure communication over long distances using quantum key distribution. Many schemes have been developed to implement quantum computing, including using linear optics [28]. The linear optical approach has proved popular due to the limited decoherence of photons with the environment, and accessibility of the components required for simple experiments. At the heart of an optical quantum computer, or extended range quantum key distribution using quantum relays or repeaters [14, 8, 24], lie entangled photons. The characteristics of the sources that create entangled photons, and their properties, are therefore central to realizing the full potential of such applications.

Quantum dots are one technology with which entangled light sources can be built [6]. Although first realised only relatively recently [49], they in principle offer key fundamental and practical advantages over other entangled photon sources. In the fundamental sense, quantum dots can be triggered, so that no more than one entangled photon pair is emitted at a time. This is in stark contrast to Poissonian entangled light sources [47, 27, 13], including the most widely used parametric down-conversion, where zero or multiple photon-pairs are usually emitted due to their probabilistic nature. Furthermore quantum dots have the potential to operate with high efficiency, with current experiments reporting up to 72% collection efficiency for the first and second photon [9, 12].