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.

We describe a general method for producing ultrahigh-density arrays of aligned metal and semiconductor nanowires and nanowire circuits. The technique is based on translating thin film growth thickness control into planar wire arrays. Nanowires were fabricated with diameters and pitches (center-to-center distances) as small as 8 nanometers and 16 nanometers, respectively. The nanowires have high aspect ratios (up to 10(6)), and the process can be carried out multiple times to produce simple circuits of crossed nanowires with a nanowire junction density in excess of 10(11) per square centimeter. The nanowires can also be used in nanomechanical devices; a high-frequency nanomechanical resonator is demonstrated.

We demonstrate a deterministic approach to the implementation of solid-state cavity quantum electrodynamics (QED) systems based on a precise spatial and spectral overlap between a single self-assembled quantum dot and a photonic crystal membrane nanocavity. By fine-tuning nanocavity modes with a high quality factor into resonance with any given quantum dot exciton, we observed clear signatures of cavity QED (such as the Purcell effect) in all fabricated structures. This approach removes the major hindrances that had limited the application of solid-state cavity QED and enables the realization of experiments previously proposed in the context of quantum information processing.

We have demonstrated laser cooling of a single electron spin trapped in a semiconductor quantum dot. Optical coupling of electronic spin states was achieved using resonant excitation of the charged quantum dot (trion) transitions along with the heavy-light hole mixing, which leads to weak yet finite rates for spin-flip Raman scattering. With this mechanism, the electron spin can be cooled from 4.2 to 0.020 kelvin, as confirmed by the strength of the induced Pauli blockade of the trion absorption. Within the framework of quantum information processing, this corresponds to a spin-state preparation with a fidelity exceeding 99.8%.

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.

An outstanding goal in quantum optics is the realization of fast optical non-linearities at the single-photon level. Such non-linearities would allow for the realization of optical devices with new functionalities such as a single-photon switch/transistor or a controlled-phase gate, which could form the basis of future quantum optical technologies. While non-linear optics effects at the single-emitter level have been demonstrated in different systems, including atoms coupled to Fabry-Perot or toroidal micro-cavities, super-conducting qubits in strip-line resonators or quantum dots (QDs) in nano-cavities, none of these experiments so far has demonstrated single-photon switching on ultrafast timescales. Here, we demonstrate that in a strongly coupled QD-cavity system the presence of a single photon on one of the fundamental polariton transitions can turn on light scattering on a transition from the first to the second Jaynes-Cummings manifold with a switching time of 20 ps. As an additional device application, we use this non-linearity to implement a single-photon pulse-correlator. Our QD-cavity system could form the building-block of future high-bandwidth photonic networks operating in the quantum regime

Optical non-linearities at the single-photon level are key ingredients for future photonic quantum technologies [1]. Prime candidates for the realization of strong photon-photon interactions necessary for implementing quantum information processing tasks [2] as well as for studying strongly correlated photons [3,4] in an integrated photonic device setting are quantum dots embedded in photonic crystal nanocavities. Here, we report strong quantum correlations between photons on picosecond timescales. We observe (a) photon antibunching upon resonant excitation of the lowest-energy polariton state, proving that the first cavity photon blocks the subsequent injection events, and (b) photon bunching when the laser field is in two-photon resonance with the polariton eigenstates of the second Jaynes-Cummings manifold, demonstrating that two photons at this color are more likely to be injected into the cavity jointly, than they would otherwise. Together, these results demonstrate unprecedented strong single-photon non-linearities, paving the way for realizing a single-photon transistor [5] or a quantum optical Josephson interferometer [6].Cavity quantum electrodynamics (cQED) studies the quantum limit of light-matter interaction where a single two-level quantum emitter is coupled to a single cavity mode [2]. In the strong coupling regime of cavity-QED where the coherent interaction strength between the emitter and the cavity mode exceeds the dissipative rates, the elementary excitations (polaritons) have an anharmonic spectrum (Fig. 1a). As a consequence, this system embodies the ultimate non-linear optical device enabling the observation of photon-photon interactions at the single-photon level [7]. Various implementations of cavity-QED systems have been reported with atoms in high finesse cavities [8], with quantum dots (QDs) in different types of monolithic cavities [9][10][11][12] and in the microwave domain [13][14][15]. Recent experiments in the optical domain using a single atom coupled to FabryPerot [16] or toroidal cavities [17] have demonstrated the * These authors contributed equally to this work. FIG. 1:Resonant scattering spectroscopy of a QD strongly coupled to a PC cavity. a) Non-linear JaynesCummings level scheme up to the second manifold. b) Sketch of the experimental setup with crossed-polarized laser suppression. c) On-resonance cw scattering spectrum for a probe power of 1 nW. The black trace was recorded without the additional re-pump laser. With the re-pump switched on, the resonant signal from the polaritons is restored (red trace).photon blockade effect by observing photon antibunching in correlation measurements. Photon bunching upon two-photon excitation of the second Jaynes-Cummings manifold has been observed for a single atom cavity-QED system [18]. In the solid-state, early results in quantum dot (QD) cavity-QED systems indicating optical nonlinearities have been reported [19,20].Here, we show that a single QD deterministically coupled to a photonic crystal (PC) defect cavity exhibits pronoun...

Resonant laser scattering along with photon correlation measurements have established the atomlike character of quantum dots. Here, we present measurements which challenge this identification for a wide range of experimental parameters: the absorption lineshapes that we measure at magnetic fields exceeding 1 Tesla indicate that the nuclear spins polarize by an amount that ensures locking of the quantum dot resonances to the incident laser frequency. In contrast to earlier experiments, this nuclear spin polarization is bi-directional, allowing the electron+nuclear spin system to track the changes in laser frequency dynamically on both sides of the quantum dot resonance. Our measurements reveal that the confluence of the laser excitation and nuclear spin polarization suppresses the fluctuations in the resonant absorption signal. A master equation analysis shows narrowing of the nuclear Overhauser field variance, pointing to potential applications in quantum information processing. PACS numbers:A number of ground-breaking experiments have demonstrated fundamental atom-like properties of quantum dots (QD), such as photon antibunching [1] and radiative lifetime limited Lorentzian absorption lineshape [2] of optical transitions. Successive experiments using transport [3] as well as optical spectroscopy [4] however, revealed that the nature of hyperfine interactions in QDs is qualitatively different than that of atoms: coupling of a single electron spin to the mesoscopic ensemble of ∼ 10 5 QD nuclear spins results in non-Markovian electron spin decoherence [5] and presents a major drawback for applications in quantum information science. Nevertheless, it is still customary to refer to QDs as artificial atoms; i.e. two level emitters with an unconventional dephasing mechanism. Here, we present resonant absorption experiments demonstrating that for a wide range of system parameters, such as the gate voltage, the length of the tunnel barrier that separates the QDs from the back contact and the external magnetic field, it is impossible to isolate the optical excitations of QD electronic states from a strong influence of nuclear spin physics. We determine that the striking locking effect of any QD transition to an incident near-resonant laser, which we refer to as dragging, is associated with dynamic nuclear spin polarization (DNSP); in stark contrast to previous experiments [6,7,8,9,10,11,12] the relevant nuclear spin polarization is bi-directional and its orientation is determined simply by the sign of the excitation laser detuning. We find that fluctuations in the QD transition energy, either naturally occurring [2] or introduced by externally modulating the Stark field, are suppressed when the laser and the QD resonances are locked. We also find that when the exchange interaction between the confined QD electron and the nearby electron Fermi-sea that leads to spin-flip co-tunneling [13] is sufficiently strong, it can suppress the confluence of laser and QD transition energies by inducing fast nuclear spin depolarization...