A coherent spin-photon interface with waveguide induced cycling transitions

Solid-state quantum dots are promising candidates for efficient light-matter interfaces connecting internal spin degrees of freedom to the states of emitted photons. However, selection rules prevent the combination of efficient spin control and optical cyclicity in this platform. By utilizing a photonic crystal waveguide we here experimentally demonstrate optical cyclicity up to $\approx15$ through photonic state engineering while achieving high fidelity spin initialization and coherent optical spin control. These capabilities pave the way towards scalable multi-photon entanglement generation and on-chip spin-photon gates.Comment: 5 pages, 4 figure

Spin-photon interface and spin-controlled photon switching in a nanobeam waveguide

Access to the electron spin is at the heart of many protocols for integrated and distributed quantum-information processing [1-4]. For instance, interfacing the spin-state of an electron and a photon can be utilized to perform quantum gates between photons [2,5] or to entangle remote spin states [6-9]. Ultimately, a quantum network of entangled spins constitutes a new paradigm in quantum optics [1]. Towards this goal, an integrated spin-photon interface would be a major leap forward. Here we demonstrate an efficient and optically programmable interface between the spin of an electron in a quantum dot and photons in a nanophotonic waveguide. The spin can be deterministically prepared with a fidelity of 96\%. Subsequently the system is used to implement a "single-spin photonic switch", where the spin state of the electron directs the flow of photons through the waveguide. The spin-photon interface may enable on-chip photon-photon gates [2], single-photon transistors [10], and efficient photonic cluster state generation [11]

Photography-based taxonomy is inadequate, unnecessary, and potentially harmful for biological sciences

The question whether taxonomic descriptions naming new animal species without type specimen(s) deposited in collections should be accepted for publication by scientific journals and allowed by the Code has already been discussed in Zootaxa (Dubois & Nemésio 2007; Donegan 2008, 2009; Nemésio 2009a–b; Dubois 2009; Gentile & Snell 2009; Minelli 2009; Cianferoni & Bartolozzi 2016; Amorim et al. 2016). This question was again raised in a letter supported by 35 signatories published in the journal Nature (Pape et al. 2016) on 15 September 2016. On 25 September 2016, the following rebuttal (strictly limited to 300 words as per the editorial rules of Nature) was submitted to Nature, which on 18 October 2016 refused to publish it. As we think this problem is a very important one for zoological taxonomy, this text is published here exactly as submitted to Nature, followed by the list of the 493 taxonomists and collection-based researchers who signed it in the short time span from 20 September to 6 October 2016

Excitons in quantum dots and design of their environment

Self-assembled semiconductor quantum dots confine single carriers on the nanometer-scale. For the confined carriers, quantum mechanics only allows states with discrete energies. Due to the Pauli exclusion principle, two carriers of identical spin cannot occupy the same energy level. When the quantum dot hosts more carriers (electrons or electron-holes), they fill the states according to Hund's rules. The recombination of a single exciton (a bound electron-hole pair) confined to the quantum dot gives rise to the emission of a single photon. For these reasons, quantum dots are often regarded as artificial atoms or even two-level systems. However, the environment of a quantum dot has a strong effect on it. The properties of a quantum dot can significantly deviate from that of an atom when it couples to continuum states in the surrounding semiconductor material; charge noise can strongly broaden the absorption of the quantum dot beyond its natural linewidth. On the other hand, designing the environment of a quantum dot enables to control its properties. Tunnel-coupling the quantum dot to a Fermi-reservoir or integrating it into cavities and waveguides are important examples. The first part of this thesis investigates a situation in which the environment of the quantum dot is especially problematic: when the quantum dot is integrated into a nanostructured device, close-by surfaces cause significant charge noise. To reduce the charge noise, a new type of ultra-thin diode structure is developed as a host for the quantum dots. The design of the diode is challenging as it must fulfill several requirements to enable spin-physics and quantum optics on single quantum dots in nanostructures. For quantum dots embedded in the final diode structure, we simultaneously achieve full electrical control of their charge state, ultra-low charge noise, and excellent spin properties. Even when the quantum dots have a large distance to surfaces, coupling to interfaces within the semiconductor heterostructure can be a problematic source of noise and decoherence. For InGaAs quantum dots, the so-called wetting layer is an interface that forms during the growth of the quantum dots and is located in their direct spatial proximity. The continuum states of the two-dimensional wetting layer are energetically close to the $p$- and $d$-shells of the quantum dots. Problematic coupling between quantum dot and wetting layer states takes place for charged excitons. The second part of this work shows that a slight modification to the growth process of the quantum dots removes wetting layer states for electrons. The wetting-layer free quantum dots can contain more electrons than conventional InGaAs quantum dots and the linewidths of highly charged excitons significantly improve. Importantly, these quantum dots retain other excellent properties of conventional InGaAs quantum dots: control of charge and spin state, and narrow linewidths in resonance fluorescence. Also for different types of self-assembled semiconductor quantum dots, the growth has a significant influence on the optical properties of confined excitons. In the third part of this thesis, it is investigated how nucleation processes during the growth are connected to the optical properties of GaAs quantum dots in AlGaAs. Remarkably, this connection can be studied post-growth by spatially resolved optical spectroscopy. The main experimental observation is the presence of strong correlations between the optical properties of a quantum dot and its proximity to neighboring quantum dots. In particular, the emission energy and the diamagnetic shift of the quantum dot emission are strongly correlated with the area of the so-called Voronoi cell surrounding the quantum dot. The observations can be explained with the capture zone model from nucleation theory, which shows that the optical quantum dot properties reveal information about the material diffusion during the semiconductor growth. As explained before, the surrounding semiconductor environment can have a strong effect on the properties of quantum dots. However, even for a well-isolated quantum dot, there are higher shells of the quantum dot itself which can lead to effects beyond a two-level system. In the final part of this thesis, a radiative Auger process is investigated. The radiative Auger effect is directly connected to higher shells of the quantum dot and appears in its emission spectrum. It arises when resonantly exciting the singly charged exciton (trion). When one electron recombines radiatively with the hole, the other one can be promoted into a higher shell. The radiative Auger emission is red-shifted by the energy that is transferred to the second electron. The corresponding emission lines show a strong magnetic field dispersion which is characteristic for higher shells. The radiative Auger effect is observed on both types of quantum dots investigated before. Radiative Auger offers powerful applications: the single-particle spectrum of the quantum dot can be easily deduced from the corresponding emission energies; carrier dynamics inside the quantum dot can be studied with a high temporal resolution by performing quantum optics measurements on the radiative Auger photons

Charge tunable GaAs quantum dots in a photonic n-i-p diode

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties

Wafer-scale epitaxial modulation of quantum dot density

Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/$\mu$$m^{2}$ and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer