Quantum dots in photonic crystals are interesting because of their potential in quantum information processing 1,2 and as a testbed for cavity quantum electrodynamics. Recent advances in controlling 3,4 and coherent probing 5,6 of such systems open the possibility of realizing quantum networks originally proposed for atomic systems [7][8][9] . Here, we demonstrate that non-classical states of light can be coherently generated using a quantum dot strongly coupled to a photonic crystal resonator 10,11 . We show that the capture of a single photon into the cavity affects the probability that a second photon is admitted. This probability drops when the probe is positioned at one of the two energy eigenstates corresponding to the vacuum Rabi splitting, a phenomenon known as photon blockade, the signature of which is photon antibunching 12,13 . In addition, we show that when the probe is positioned between the two eigenstates, the probability of admitting subsequent photons increases, resulting in photon bunching. We call this process photon-induced tunnelling. This system represents an ultimate limit for solid-state nonlinear optics at the single-photon level. Along with demonstrating the generation of non-classical photon states, we propose an implementation of a single-photon transistor 14 in this system.The optical system consists of a self-assembled InAs quantum dot with decay rate γ /2π ≈ 0.1 GHz coupled to a three-hole defect cavity 15 in a two-dimensional GaAs photonic crystal, as described in ref. 5. The quantum-dot/cavity coupling rate g /2π = 16 GHz equals the cavity field decay rate κ/2π = 16 GHz (corresponding to a cavity quality factor Q = 10,000), which puts the system in the strong coupling regime 10,11 . We first characterize the system in photoluminescence by pumping the structure above the GaAs bandgap. The photoluminescence scans in Fig. 1b show the anticrossing characteristic of strong coupling between the quantum dot and the cavity. Here, the quantum dot is tuned into resonance using local temperature tuning 16 around an average temperature of 20 K maintained in a continuous He flow cryostat. To generate non-classical light, we coherently probe the system with linearly polarized laser beams (Fig. 1a) and observe the cross-polarized output, as described in our previous work 5 . The cross-polarized set-up enables us to separate the cavity-coupled signal from the direct probe reflection, which is essential for achieving large signal-to-noise ratios needed in autocorrelation measurements. Figure 1 Schematic diagram of the experimental set-up. a, Laser pulses (40 ps FWHM) are reflected from a photonic crystal cavity that is linearly polarized at 45 • relative to the input polarization set by the polarizing beam splitter (PBS). The output light, observed in cross-polarization and carrying the cavity-coupled signal, is analysed using an HBT set-up that measures second-order correlation. The inset shows the suspended structure with the photonic crystal cavity and the metal pad for local temperature tuni...

Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity-quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot-cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity-quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.

Optical nonlinearities enable photon-photon interaction and lie at the heart of several proposals for quantum information processing, quantum nondemolition measurements of photons, and optical signal processing. To date, the largest nonlinearities have been realized with single atoms and atomic ensembles. We show that a single quantum dot coupled to a photonic crystal nanocavity can facilitate controlled phase and amplitude modulation between two modes of light at the single-photon level. At larger control powers, we observed phase shifts up to π/4 and amplitude modulation up to 50%. This was accomplished by varying the photon number in the control beam at a wavelength that was the same as that of the signal, or at a wavelength that was detuned by several quantum dot linewidths from the signal. Our results present a step toward quantum logic devices and quantum nondemolition measurements on a chip.

Abstract:We describe a general recipe for designing high-quality factor (Q) photonic crystal cavities with small mode volumes. We first derive a simple expression for out-of-plane losses in terms of the k-space distribution of the cavity mode. Using this, we select a field that will result in a high Q. We then derive an analytical relation between the cavity field and the dielectric constant along a high symmetry direction, and use it to confine our desired mode. By employing this inverse problem approach, we are able to design photonic crystal cavities with Q > 4 · 10 6 and mode volumes V ∼ (λ /n) 3 . Our approach completely eliminates parameter space searches in photonic crystal cavity design, and allows rapid optimization of a large range of photonic crystal cavities. Finally, we study the limit of the out-of-plane cavity Q and mode volume ratio. 9. Y. Akahane, T. Asano, B.-S. Song, and S. Noda, "Fine-tuned high-Q photonic-crystal nanocavity," Opt. Express 13(4), 1202-1214 (2005 (Prentice-Hall, 1987).

Coupling of photonic crystal (PC) linear three-hole defect cavities (L3) to PC waveguides is theoretically and experimentally investigated. The systems are designed to increase the overlap between the evanescent cavity field and the waveguide mode, and to operate in the linear dispersion region of the waveguide. Our simulations indicate increased coupling when the cavity is tilted by 60 o with respect to the waveguide axis, which we have also confirmed by experiments. We obtained up to 90% coupling efficiency into the waveguide.

Electroluminescence from quantum dots fabricated with nanosphere lithography Appl. Phys. Lett. 101, 103105 (2012) Nucleation features and energy levels of type-II InAsSbP quantum dots grown on InAs(100) substrate Appl. Phys. Lett. 101, 093103 (2012) Highly luminescing multi-shell semiconductor nanocrystals InP/ZnSe/ZnS Appl. Phys. Lett. 101, 073107 (2012) Eliminating the fine structure splitting of excitons in self-assembled InAs/GaAs quantum dots via combined stresses Appl.

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We demonstrate fast (up to 20 GHz), low power (5 µW ) modulation of photonic crystal (PC) cavities in GaAs containing InAs quantum dots. Rapid modulation through blue-shifting of the cavity resonance is achieved via free carrier injection by an above-band picosecond laser pulse.Slow tuning by several linewidths due to laser-induced heating is also demonstrated.