Antonio Badolato

Quantum nature of a strongly coupled single quantum dot–cavity system

Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot–cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.

Self-tuned quantum dot gain in photonic crystal lasers

We demonstrate that very few (2-4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.

Strongly correlated photons on a chip

Researchers observe a continuous change in photon correlations from strong antibunching to bunching by tuning either the probe laser or the cavity mode frequency. These results, which demonstrate unprecedented strong single-photon nonlinearities in quantum dot cavity system, are explained by the photon blockade and tunnelling in the anharmonic Jaynes–Cummings model.

Tuning photonic crystal nanocavity modes by wet chemical digital etching

We have developed a wet chemical digital etching technique for tuning the resonant wavelengths of photonic crystal (PC) nanocavities over a wide range of 80nm in precise 2–3nm steps while preserving high cavity quality factors. In one tuning step, a few monolayers of material are removed from the cavity surface by etching a self-formed native oxide in 1mol citric acid. Due to the self-limiting oxide thickness, total tuning range is based only on the number of etch steps, resulting in a highly controlled, digital tuning ability. We have characterized the tuning behavior of GaAs PC defect cavities of both square and triangular lattice symmetry and proven the effectiveness of this method by tuning a mode into resonance with the charged exciton, and then later the biexciton, transition of a single InAs∕GaAs self-assembled quantum dot.

Effect of uniaxial stress on excitons in a self-assembled quantum dot

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of an applied uniaxial stress. The spectrum of the excitonic Rayleigh scattering was measured in reflectivity using high-resolution laser spectroscopy while the sample was submitted to a tunable uniaxial stress along its [110] crystal axis. We show that using this stretching technique, the quantum dot potential is elastically deformable such that the exciton fine structure splitting can be substantially reduced.

The nonlinear Fano effect

The Fano effect is ubiquitous in the spectroscopy of, for instance, atoms, bulk solids and semiconductor heterostructures. It arises when quantum interference takes place between two competing optical pathways, one connecting the energy ground state and an excited discrete state, the other connecting the ground state with a continuum of energy states. The nature of the interference changes rapidly as a function of energy, giving rise to characteristically asymmetric lineshapes. The Fano effect is particularly important in the interpretation of electronic transport and optical spectra in semiconductors. Whereas Fano’s original theory applies to the linear regime at low power, at higher power a laser field strongly admixes the states and the physics becomes rich, leading, for example, to a remarkable interplay of coherent nonlinear transitions. Despite the general importance of Fano physics, this nonlinear regime has received very little attention experimentally, presumably because the classic autoionization processes, the original test-bed of Fano’s ideas, occur in an inconvenient spectral region, the deep ultraviolet. Here we report experiments that access the nonlinear Fano regime by using semiconductor quantum dots, which allow both the continuum states to be engineered and the energies to be rescaled to the near infrared. We measure the absorption cross-section of a single quantum dot and discover clear Fano resonances that we can tune with the device design or even in situ with a voltage bias. In parallel, we develop a nonlinear theory applicable to solid-state systems with fast relaxation of carriers. In the nonlinear regime, the visibility of the Fano quantum interferences increases dramatically, affording a sensitive probe of continuum coupling. This could be a unique method to detect weak couplings of a two-level quantum system (qubits), which should ideally be decoupled from all other states.

Explanation of Photon Correlations in the Far-Off-Resonance Optical Emission from a Quantum-Dot-Cavity System

Martin Winger, Thomas Volz, Guillaume Tarel, Stefano Portolan, Antonio Badolato, Kevin J. Hennessy, Evelyn L. Hu, Alexios Beveratos, Jonathan Finley, Vincenzo Savona, and Ataç Imamoğlu Institute of Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne EPFL, CH-1015 Lausanne, Switzerland Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA California NanoSystems Institute, University of California, Santa Barbara, California 93106, USA CNRS Laboratoire Photonique et Nanostructures, Route de Nozay, F-91460 Marcoussis, France Walter Schottky Institut, Am Coulombwall 3, D-85748 Garching, Germany (Dated: November 23, 2009)

Observation of Faraday rotation from a single confined spin

The ability to read out the state of a single confined spin lies at the heart of solid-state quantum-information processing1. Although spin measurements using Faraday rotation of light polarization have been implemented in semiconductor spin ensembles2,3,4, single-spin read-out has only been achieved using transport measurements5,6. Here, we demonstrate an all-optical dispersive measurement of the time-averaged spin state of a single electron in a quantum dot. We obtain information on the spin state through conditional Faraday rotation of a spectrally detuned laser, induced by the polarization- and spin-selective trion (charged quantum dot) transitions. To assess the sensitivity of the technique, we use an independent resonant laser for spin-state preparation7. We infer that there are ∼10 spin-flip Raman scattering events (that is, back-action) within our measurement timescale. Straightforward improvements such as incorporating solid-immersion lenses8,9 and higher efficiency detectors should allow for back-action-evading spin measurements, without the need for a cavity.

Tuning photonic nanocavities by atomic force microscope nano-oxidation

The authors demonstrate a technique to achieve high-precision tuning of photonic crystal nanocavities by atomic force microscope nano-oxidation of the cavity surface. Relative tuning between two nanocavity modes is achieved though careful choice of the oxide pattern, allowing them to restore the spectral degeneracy conditions necessary to create polarization-entangled quantum states. Tuning steps less than the linewidth (1A) of the high quality factor modes are obtained, allowing for virtually continuous tuning ability.The authors demonstrate a technique to achieve high-precision tuning of photonic crystal nanocavities by atomic force microscope nano-oxidation of the cavity surface. Relative tuning between two nanocavity modes is achieved though careful choice of the oxide pattern, allowing them to restore the spectral degeneracy conditions necessary to create polarization-entangled quantum states. Tuning steps less than the linewidth (1A) of the high quality factor modes are obtained, allowing for virtually continuous tuning ability.

Manipulating exciton fine structure in quantum dots with a lateral electric field

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero.