By investigating the interaction of single atoms with a cavity, pioneering experiments in the 1980's demonstrated that the spontaneous emission from an atom is intimately tied to the electromagnetic vacuum field distribution. By adjusting the field distribution with a cavity a discrete mode distribution was created, and enhanced and inhibited spontaneous emission from the atom was observed. We have demonstrated a semiconductor analog of the single-atom cavity experiments using a single InAs quantum dot (QD) as an artificial atom. With this solid-state implementation it is possible to make compact, regulated single-photon sources with high efficiency and high repetition rate for applications such as quantum cryptography.
By investigating the interaction of single atoms with a cavity, pioneering experiments in the 1980's demonstrated that the spontaneous emission from an atom is intimately tied to the electromagnetic vacuum field distribution. By adjusting the field distribution with a cavity a discrete mode distribution was created, and enhanced and inhibited spontaneous emission from the atom was observed. We have demonstrated a semiconductor analog of the single-atom cavity experiments using a single InAs quantum dot (QD) as an artificial atom. With this solid-state implementation it is possible to make compact, regulated single-photon sources with high efficiency and high repetition rate for applications such as quantum cryptography.
Spontaneous emission of a source, such as an atom, is often thought of as an inherent property of the source, but is a result of the interaction between the radiating source dipole and the vacuum electromagnetic field. Therefore, the radiation emitted from an atom can be altered by suitably modifying the surrounding vacuum field [1] with a cavity. This has been verified by experiments initiated in the 1980s using atoms which are passed through a cavity and show enhanced[2] or inhibited[3] spontaneous emission. This field, called cavity quantum electrodynamics (QED), is currently addressing important issues in semiconductor nanostructures, in particular, high-efficiency light-emitting diodes for communications and single photon sources for quantum cryptography. [4,5] For example, quantum cryptography has emerged as a significant field of study over the last fifteen years, because it offers the promise of private communication whose security is assured by the laws of physics. Most implementations of quantum cryptography so far have used a protocol introduced by Bennet and Brassard, which uses four different states of a quantum system (generally known as BB84) [6 ]. For example, the message can be encoded on the polarization state of single photons, with a random choice between two non-othogonal polarization bases when the photons are sent (by Alice) and recieved (by Bob). Since an eavesdropper (Eve) does not know what bases have been chosen, any measurement she makes will impose a back-action on the states of the transmitted photons, which can be easily detected. Using error correction and privacy amplification, Alice and Bob can distill the transmitted message into a secure key about which Eve knows arbitrarily little.
Though QD arrays have initially been mostly studied in an attempt to improve semiconductor lasers, much attention has been recently devoted to the revolutionary prospects opened by the unique properties of single QDs. These artificial atoms allow to mimic in solid-state systems some of the quantum optics experiments performed on atoms since the early 80’s. Recent demonstrations of coherent control experiments on QDs illustrate this fruitful analogy [10]. We will focus here our discussion on the impact of QDs on two other fields, solid-state cavity quantum electrodynamics (CQED) and the generation of quantum states of light. A single QD can be used to emit triggered single photon pulses and photon pairs, whereas its narrow emission spectrum permits to implement microcavities very efficiently to tailor its emission properties. By combining both kind of effects, it is possible to control simultaneously the number of emitted photons as well as their quantum state and to build a solid-state singlemode source of single photons (singlemode S4P) as proposed [11] and demonstrated recently by our group [12]. This novel device represents the ultimate step in a field which has been extremely active in the last 15 years, spontaneous emission (SE) control in optoelectronic devices using confined electronic and/or photonic systems [13]. Before reviewing the development of this single mode S4P, based on a single QD in a pillar microcavity, we will briefly discuss its potential applications in the field of quantum information processing and quantum communications (QIPC).
The interaction of light with matter is at the heart of quantum optics, which itself enables insight into the fundamental aspects of quantum mechanics. First experimental access to this research field has been realized by coupling atoms to light. Here, the transition between discrete energy states of the atom is associated with the absorption and emission of a photon, a single quantum of the electromagnetic field. As a central aspect of quantum optics, the light-emitter interaction can be significantly enhanced by placing the emitter in optical cavity that is on resonance with the emitter. In recent years, this has led to a rapidly evolving research field known as cavity quantum electrodynamics (CQED). In CQED two different regimes are distinguished: the strong and the weak coupling regimes. In the strong coupling regime, the emitted photon is reflected from the cavity mirrors and eventually reabsorbed by the emitter. In contrast, the weak coupling regime describes the irreversible emission, where the photon leaks out of the cavity before it can be reabsorbed. Both the weak and strong coupling regimes enabled fundamental experiments for a better understanding of quantum optical phenomena. \n \n \n \nCQED grants access to the quantum world and hence offers potentially revolutionizing applications, particularly in the field of quantum information processing. A central aspect for the successful implementation of quantum applications is the system's scalability. Unfortunately, placing atoms deterministically inside a cavity remains technologically elaborate and hence minimizes the prospect of scaling a atom-CQED system. \n \nA possibility to address this issue is to implement CQED in the solid state, where sophisticated fabrication strategies allow miniaturization and scalability of the system. Particularly the development of self-assembled quantum dots (QD) in semiconductors represent a promising route. QDs can be considered as artificial atoms that mimic the atomic two-level system. These structures interact strongly with light and therefore have the potential for replacing atoms in CQED. As a central advantage, QDs are naturally trapped, which greatly simplifies the deterministic incorporation into the cavity. \n \nIn recent years, many efforts have been made to couple self-assembled QDs to microcavities. Generally, the successful implementation of CQED requires a cavity with a high quality factor Q and a low mode volume. In a majority of the approaches, the high Q/small mode volume cavities were monolithically defined around the QD, embedding the QD at a fixed position inside the cavity. Both the weak and strong coupling regimes have been reached with these systems. However, for future applications they suffer from several disadvantages. The fixed position of the QD inside the cavity minimizes the prospects for spectral tunability and spatial positioning the QD inside the cavity. Furthermore, prospects for further increasing of the cavity Q-factor and minimization of the mode volume remain limited in these systems. \n \nIn this thesis the mentioned disadvantages are addressed by developing a fully tunable miniaturized Fabry-Pérot microcavity with low mode volume. The design enables both spatial positioning of the emitter inside the cavity and spectral tunability. Successful coupling of a single QD to the microcavity is demonstrated including the strong coupling regime. Further a new approach to decrease the cavity mode volume is presented, where we demonstrate weak coupling is achieved
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 h g = ∣〈 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 γ c . When g << γ c , 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.
Semiconductor quantum dots have emerged as promising candidates for studying quantum optical phenomena. Observation of photon antibunching and spontaneous emission limited coherence decay rates have demonstrated that quantum dots behave as artificial atoms. Cavity-quantum electrodynamics (QED) phenomena can be investigated using a single quantum dot embedded inside a photonic nanostructure, where both carriers and photons are confined within sub-micron length scales in all three dimensions. Since quantum dot location inside the cavity is fixed by the growth, this system is free of the stringent trapping requirements that limit its atomic counterpart. Possibility of fabricating photonic nanostructures with ultra-small optical-mode volumes and long photon lifetimes enhances the prospects for applications in quantum information processing. 1
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.