Quantum optics with single-wall carbon nanotubes

Abstract

In this dissertation, we report on experimental and theoretical investigations of the optical properties of semiconducting single-wall carbon nanotubes (SWNTs). We focus on aspects and phenomena involving typically quantum effects, whose descrip¬tions require going beyond the realm of classical physics and Maxwell equations. Our most significant experimental result is the observation of photon antibunching in the photoluminescence (PL) emitted by SWNTs. Considering the particularities of our sample, in which surfactant-embedded SWNTs are deposited on a functionalized substrate, we show that the suppression of multi-photon emission events is due to localization of the excitons in nanometer-scale traps along the nanotubes. Fast and efficient exciton-exciton annihilation, a consequence of the reduced dimensionality of carbon nanotubes, is playing a determinant role in forbidding photon-pair emission. The successful reproduction of the broad and asymmetric PL lineshapes by a phy¬sical model relying on strong exciton confinement supports this picture. We calculate the PL spectrum of a quantum dot (QD) embedded in a SWNT and demonstrate that exciton coupling to the low-energy acoustic phonons of the nanotube leads to ultrafast, non-markovian pure dephasing of the optically excited state. In the spec¬tral domain, the oscillator strength is transferred from the zero-phonon line into phonon wings baring a strong asymmetry at cryogenic temperatures. We prove hereby that our PL data are direct evidences of the experimental realiza¬tion of the spin-boson model in the (sub-)ohmic regime. This is a consequence of the one-dimensionality of the phonon bath reflected in the spectral density governing the dissipation. We emphasize the qualitative differences compared to traditional QDs embedded in a three-dimensional matrix, and briefly discuss the consequences for the use of SWNT-QDs in quantum information processing. An exciting possibility opened by strong exciton-phonon coupling in carbon na-notubes is their use as mechanical resonators for laser-assisted cooling. We propose a device based on a suspended SWNT where exciton confinement is controlled by sharp tips acting as gates. The potential applied on the tips can additionally be used to induce exciton coupling to the flexural mode of the SWNT and tune its strength. Inelastic scattering of a weak red-detuned laser then permits to reduce the occupation number of the fundamental flexural mode down to the quantum ground state. In an attempt to give a unified picture for all our experimental observations, we also suggest a physical origin for the unintentional formation of SWNT-QDs in our sample. We consider the presence of a charged impurity in the surrounding of the nanotube and demonstrate that the resulting electric field effectively traps the SWNT excitons. The peculiar characteristics of the confining potential would explain most of the experimental features. Finally, we show how non-vanishing spin-orbit coupling recently measured in transport experiments allows for all-optical spin manipulation in carbon nanotubes. d We perform numerical simulations based on Bloch-equations to demonstrate that high-fidelity spin-state preparation is achievable. Coherent optical spin manipulation and possible schemes for the use of SWNT spins in quantum information processing are also discussed. Combining surprising novel experimental results with diverse theoretical and nu-merical studies, this work emphasizes on the fascinating potential of carbon nano-tubes in the study of quantum physics in materials of reduced dimensionality.