A. Ulhaq

Cascaded single-photon emission from the Mollow triplet sidebands of a quantum dot

Researchers demonstrate that an individual Mollow sideband channel of the resonance fluorescence from an InGaAs quantum dot can act as an efficient single-photon source. The central frequency of the bright and narrow sideband emission can be changed by laser detuning over a range spanning 15 times the emission linewidth.

In situ laser microprocessing of single self-assembled quantum dots and optical microcavities

One of the biggest challenges of nanotechnology is the fabrication of nano-objects with perfectly controlled properties. Here we employ a focused laser beam both to characterize and to {\it in-situ} modify single semiconductor structures by heating them from cryogenic to high temperatures. The heat treatment allows us to blue-shift, in a broad range and with resolution-limited accuracy, the quantized energy levels of light and charge carriers confined in optical microcavities and self-assembled quantum dots (QDs). We demonstrate the approach by tuning an optical mode into resonance with the emission of a single QD and by bringing different QDs in mutual resonance. This processing method may open the way to a full control of nanostructures at the quantum level.The authors employ a focused laser beam both as a probe and as a local heat source to tune in situ, over a broad range and with resolution-limited accuracy, the quantized energy states of single optical microcavities and self-assembled quantum dots (QDs). The approach is demonstrated by bringing an optical mode of a microdisk into resonance with the emission of a single QD and by tuning spatially separated QDs in mutual resonance. This processing method may be used, e.g., to fabricate arrays of perfectly resonant QDs.

Phonon-assisted incoherent excitation of a quantum dot and its emission properties

We present a detailed study of a phonon-assisted incoherent excitation mechanism of single quantum dots. A spectrally detuned continuous-wave laser couples to a quantum dot transition by mediation of acoustic phonons, whereby excitation efficiencies up to 20

Detuning-dependent Mollow triplet of a coherently-driven single quantum dot

%

Vanishing electron g factor and long-lived nuclear spin polarization in weakly strained nanohole-filled GaAs/AlGaAs quantum dots

with respect to strictly resonant excitation can be achieved at

Mollow quintuplets from coherently excited quantum dots

T=9

1 – Resonance fluorescence emission from single semiconductor quantum dots coupled to high-quality microcavities

K. Laser-frequency-dependent analysis of the quantum dot intensity distinctly maps the underlying acoustic phonon bath and shows good agreement with our polaron master equation theory. An analytical solution for the steady-state exciton density (which is proportional to the photoluminescence) is introduced which predicts a broadband incoherent coupling process mediated by electron-phonon scattering. Moreover, we investigate the coherence properties of the emitted light with respect to strictly resonant versus phonon-assisted excitation, revealing the importance of narrow band triggered emitter-state initialization for possible applications of a quantum dot exciton system as a qubit.

Linewidth broadening and emission saturation of a resonantly excited quantum dot monitored via an off-resonant cavity mode

We present both experimental and theoretical investigations of a laser-driven quantum dot (QD) in the dressed-state regime of resonance fluorescence. We explore the role of phonon scattering and pure dephasing on the detuning-dependence of the Mollow triplet and show that the triplet sidebands may spectrally broaden or narrow with increasing detuning. Based on a polaron master equation approach, which includes electron-phonon interaction nonperturbatively, we derive a fully analytical expression for the spectrum. With respect to detuning dependence, we identify a crossover between the regimes of spectral sideband narrowing or broadening. We also predict regimes of phonon-induced squeezing and anti-squeezing of the spectral resonances. A comparison of the theoretical predictions to detailed experimental studies on the laser detuning-dependence of Mollow triplet resonance emission from single In(Ga)As QDs reveals excellent agreement.

Fabrication and characterization of microdisk resonators with In(Ga)As/GaAs quantum dots

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole infilling offer a combination of strong charge confinement, optical efficiency, and spatial symmetry required for polarization entanglement and spin-photon interface. Here we study spin properties of such dots. We find nearly vanishing electron g-factor (ge < 0.05), providing a route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to 60% achieved. NMR spectroscopy reveals the structure of two types of quantum dots and yields the small magnitude of residual strain εb < 0.02% which nevertheless leads to long nuclear spin lifetimes exceeding 1000 s. The stability of the nuclear spin environment is advantageous for applications in quantum information processing. Central spin in semiconductor quantum dots is a prime candidate for applications in quantum information technologies.1,2 It is relatively isolated from the solid state effects and at the same time is accessible for coherent manipulation and can be interfaced optically. The coherence in this system is mainly limited by the hyperfine coupling with the nuclear spin bath.3,4 Single spin qubit manipulation in these structures, therefore, demands an auxiliary control over nuclear spin environment. Such control can be realized by maximizing polarization of 104− 105 nuclei in a single quantum dot,5–7 enabling the formation of well-defined nuclear spin states and in effect reducing the influence of the nuclear field fluctuations.8,9 Central spin manipulation in semiconductor quantum dot (QD) system using resonant ultrafast optical pulses10,11 has been demonstrated but scalability in such schemes is challenging. An alternative approach is to induce controlled spin rotation by manipulating the coupling to the external magnetic field.12 This can be achieved by electrical modulation of the g-factor. However, such scheme critically depends on the ability to change the sign of g, thus requiring quantum dots with close to zero electron or hole g-factor.13,14 Self-assembled InGaAs/GaAs QD has been the primary system of choice for spin studies over the last two decades, as quantum confinement in monolayer-fluctuation GaAs/AlGaAs dots is too weak. Only recently the potential of droplet epitaxial (DE) grown GaAs QDs has been identified.15–17 In particular nanohole-filled droplet epitaxial (NFDE) dots formed by in situ etching and nanohole infilling18 provide confinement and excellent optical efficiency, while on the other hand exhibiting high symmetry not achievable previously in self-assembled dots.19 Such unique com1 ar X iv :1 50 7. 06 55 3v 1 [ co nd -m at .m es -h al l] 2 3 Ju l 2 01 5 bination of properties make NFDE dots ideal candidates for polarization entanglement and spinphoton interface.20 This system has already exhibited an efficient interface between rubidium atoms and a quantum dot.21 However, the understanding of the spin properties in such quantum dots is still lacking. Here we use optical and nuclear magnetic resonance (NMR) spectroscopy to study the properties of the single charge spins and nuclear spin environment in NFDE grown GaAs/AlGaAs QDs. Magneto-photoluminescence measurements reveal close-to-zero electron g-factor, due to the electron wavefunction overlap with the AlGaAs barrier. We demonstrate efficient dynamic nuclear polarization (DNP) as large as 60 %. By measuring the excitation wavelength dependence we identify three mechanisms of DNP: (i) via optical excitation of the quantum well states, (ii) via resonant optical excitation of the dot ground or excited states, and (iii) via resonant excitation of the neighboring dot made possible by inter-dot charge tunneling. Radio frequency (rf) excitation is used to measure NMR spectra revealing the presence of small (< 0.02%) residual biaxial strain. Surprisingly, we observe two sub-ensembles of QDs one with compressive and another with tensile strain along the growth axis: this allows us to identify these two types of dots as formed in the nanoholes and at the rims of the nanoholes respectively. We show that small residual strain results in very stable nuclear spin bath with nuclear spin relaxation times > 1000 s, previously achievable only in selfassembled dots. The properties of the NFDE quantum dots revealed in this study make them a favorable system for electrical spin qubit manipulation with a potential for minimized decoherence effects from the nuclear spin bath. Single dot photoluminescence (PL) spectroscopy is performed with a confocal setup which collects PL at low temperature (T ≈ 4.2 K) from a ∼ 1 μm spot. Magnetic fields up to 10 T along sample growth axis (Faraday geometry) are employed in this study. The polarization degree of the nuclear spins is probed by measuring the hyperfine shifts in the Zeeman splitting of the quantum dot PL. Nuclear spin polarization and NMR spectroscopy studies are performed using the methods described in Reference.22 QD B QD A 1.56 1.58 1.60 1.62 1.64 1.66 1.68 1.70 Photon Energy (eV)

Highly indistinguishable photons from a quantum dot in a microcavity

Charge-neutral excitons in semiconductor quantum dots (QDs) have a small finite energy separation caused by the anisotropic exchange splitting. Coherent excitation of neutral excitons will generally excite both exciton components, unless the excitation is parallel to one of the dipole axes. We present a polaron master equation model to describe two-exciton pumping using a coherent continuous wave pump field in the presence of a realistic anisotropic exchange splitting. We predict a five-peak incoherent spectrum, namely a Mollow quintuplet under general excitation conditions. We experimentally confirm such spectral quintuplets for In(Ga)As QDs and obtain very good agreement with theory.