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Frontiers in Optics

Cavity Quantum Electrodynamics with Epitaxial Quantum Dots

A. Badolato

Published Jan 1, 2012• 0 citations
Publisher

Abstract

Much in physics is explained through an understanding of harmonic oscillation, but nonlinearity makes physics more exciting and greatly contributes for its many applications. Optical nonlinearities at the single-photon level are key ingredients for future photonic quantum technologies. Here we report unprecedented strong single-photon nonlinearities and single-photon switching on ultrafast timescales in epitaxial quantum dots strongly coupled to microcavities.

DOI
10.1364/ls.2012.lm4j.2

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Much in physics is explained through an understanding of harmonic oscillation, but nonlinearity makes physics more exciting and greatly contributes for its many applications. Optical nonlinearities at the single-photon level are key ingredients for future photonic quantum technologies. Here we report unprecedented strong single-photon nonlinearities and single-photon switching on ultrafast timescales in epitaxial quantum dots strongly coupled to microcavities.
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.
Next-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments.
A single semiconductor quantum dot that is strongly coupled to a photonic crystal nanocavity yields optical nonlinearities at the single photon level. The promise of quantum information processing is driving developments in several different physical implementations. Photons are attractive as carriers of quantum information be-cause they can travel long distances without losing their infor-mation through interactions with their environment. However, their passive nature can be a problem in applications such as the construction of logic gates, where it is necessary for two or more photons to interact. This interaction requires a nonlinear optical medium. Particularly strong nonlinearities are achieved when photons drive an emitter near its resonance frequency, and this nonlinearity is even greater if the photon is confined near the emitter in an optical cavity.
A quantum dot strongly coupled to a photonic crystal resonator is used to investigate cavity quantum electro- dynamics phenomena in solid state physics. Nonlinear optical phenomena such as photon blockade and photon induced tunneling are observed in this system. The nonlinearity of this system is sensitive to intra-cavity photon numbers close to unity, and it has been used to demonstrate conditional phase shifts of 28° at a single photon level and a second order auto-correlation of g2(0) = 0.9 in the photon blockade regime.
We coherently probe a quantum dot that is strongly coupled to a photonic crystal nano-cavity by scattering of a resonant laser beam. The coupled system's response is highly nonlinear as the quantum dot saturates with nearly one photon per cavity lifetime. This system enables large amplitude and phase shifts of a signal beam via a control beam, both at single photon levels. We demonstrate photon-photon interactions with short pulses in a system that is promising for ultra-low power switches and two-qubit quantum gates.