L. Lanco

Near-optimal single-photon sources in the solid state

A single photon with near-unity indistinguishability is generated from quantum dots in electrically controlled cavity structures. The cavity allows for efficient photon collection while application of an electrical bias cancels charge noise effects.

Controlled light-matter coupling for a single quantum dot embedded in a pillar microcavity using far-field optical lithography.

Using far-field optical lithography, a single quantum dot is positioned within a pillar microcavity with a 50 nm accuracy. The lithography is performed in situ at 10 K while measuring the quantum dot emission. Deterministic spectral and spatial matching of the cavity-dot system is achieved in a single step process and evidenced by the observation of strong Purcell effect. Deterministic coupling of two quantum dots to the same optical mode is achieved, a milestone for quantum computing.

Bright solid-state sources of indistinguishable single photons

Bright sources of indistinguishable single photons are strongly needed for the scalability of quantum information processing. Semiconductor quantum dots are promising systems to build such sources. Several works demonstrated emission of indistinguishable photons while others proposed various approaches to efficiently collect them. Here we combine both properties and report on the fabrication of ultrabright sources of indistinguishable single photons, thanks to deterministic positioning of single quantum dots in well-designed pillar cavities. Brightness as high as 0.79±0.08 collected photon per pulse is demonstrated. The indistinguishability of the photons is investigated as a function of the source brightness and the excitation conditions. We show that a two-laser excitation scheme allows reducing the fluctuations of the quantum dot electrostatic environment under high pumping conditions. With this method, we obtain 82±10% indistinguishability for a brightness as large as 0.65±0.06 collected photon per pulse.

Continuous-wave second-harmonic generation in modal phase matched semiconductor waveguides

We report the observation of continuous-wave second-harmonic generation in a modalphase-matched GaAs/AlGaAs waveguide. The heterostructure has been designed to generate a second-harmonic signal via a type-II process with a fundamental signal around 1.55 μm. Continuous wave conversion enables a quantitative estimation of the conversion efficiency. In our case, η=30%±5% W−1 cm−2 is found experimentally.

Deterministic and electrically tunable bright single-photon source

The scalability of a quantum network based on semiconductor quantum dots lies in the possibility of having an electrical control of the quantum dot state as well as controlling its spontaneous emission. The technological challenge is then to define electrical contacts on photonic microstructures optimally coupled to a single quantum emitter. Here we present a novel photonic structure and a technology allowing the deterministic implementation of electrical control for a quantum dot in a microcavity. The device consists of a micropillar connected to a planar cavity through one-dimensional wires; confined optical modes are evidenced with quality factors as high as 33,000. We develop an advanced in-situ lithography technique and demonstrate the deterministic spatial and spectral coupling of a single quantum dot to the connected pillar cavity. Combining this cavity design and technology with a diode structure, we demonstrate a deterministic and electrically tunable single-photon source with an extraction efficiency of around 53±9%.

Optical nonlinearity with few-photon pulses using a quantum dot-pillar cavity device

Giant optical nonlinearity is observed under both continuous wave and pulsed excitation in a deterministically coupled quantum dot-micropillar system, in a pronounced strong-coupling regime. Using absolute reflectivity measurements we determine the critical intracavity photon number as well as the input and output coupling efficiencies of the device. Thanks to a near-unity input-coupling efficiency, we demonstrate a record nonlinearity threshold of only 8 incident photons per pulse. The output-coupling efficiency is found to strongly influence this nonlinearity threshold. We show how the fundamental limit of single-photon nonlinearity can be attained in realistic devices, which would provide an effective interaction between two coincident single-photons.

Quantum dot-cavity strong-coupling regime measured through coherent reflection spectroscopy in a very high-Q micropillar

We report on the coherent reflection spectroscopy of a high-quality factor micropillar, in the strong-coupling regime with a single InGaAs annealed quantum dot. The absolute reflectivity measurement is used to study the characteristics of the device at low and high excitation powers. The strong coupling is obtained with a g=16 μeV coupling strength in a 7.3 μm diameter micropillar, with a cavity spectral width κ=20.5 μeV (Q=65 000). The factor of merit of the strong-coupling regime, 4g/κ=3, is the current state-of-the-art for a quantum dot-micropillar system.

Macroscopic rotation of photon polarization induced by a single spin

Entangling a single spin to the polarization of a single incoming photon, generated by an external source, would open new paradigms in quantum optics such as delayed-photon entanglement, deterministic logic gates or fault-tolerant quantum computing. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10−3 degrees due to poor spin–photon coupling. Here we report the enhancement by three orders of magnitude of the spin–photon interaction, using a cavity quantum electrodynamics device. A single hole spin in a semiconductor quantum dot is deterministically coupled to a micropillar cavity. The cavity-enhanced coupling between the incoming photons and the solid-state spin results in a polarization rotation by ±6° when the spin is optically initialized in the up or down state. These results open the way towards a spin-based quantum network.

Measuring propagation loss in a multimode semiconductor waveguide

The measurement of propagation loss based on the Fabry–Perot transmission fringes is a powerful tool for the characterization of single-mode optical waveguides. This method is well established for lithium niobate waveguides, but its implementation with semiconductor devices is more delicate. A method to extend this technique to the case of multimode semiconductor waveguides is presented. Our procedure involves Fabry–Perot measurements on a large spectral range, in order to find an interval where multimode effects do not alter the loss measurement. Two experimental examples are given, showing also the domain of validity of this approach.

Origin of the optical emission within the cavity mode of coupled quantum dot-cavity systems.

The origin of the emission within the optical mode of a coupled quantum dot-micropillar system is investigated. Time-resolved photoluminescence is performed on a large number of deterministically coupled devices in a wide range of temperature and detuning. The emission within the cavity mode is found to exhibit the same dynamics as the spectrally closest quantum dot state. Our observations indicate that fast dephasing of the quantum dot state is responsible for the emission within the cavity mode. An explanation for recent photon correlation measurements reported on similar systems is proposed.