Our paper on imaging single quantum dots has appeared in Nature Photonics. The paper describes how rapid adiabatic passage on a two-level system can be exploited as an on-off switch. This non-linearity enables imaging the quantum dot with a resolution much less than that of a confocal microscope: 30 nm (λ/31) was achieved in this experiment.

Left: image of quantum dots in a semiconductor recorded with a confocal microscope operating close to the diffraction limit. Right: image of the same quantum dots using the switching protocol based on rapid adiabatic passage.

Our paper Narrow optical linewidths and spin pumping on charge-tunable close-to-surface self-assembled quantum dots in an ultrathin diode has been published in Physical Review B. The paper is part of a successful collaboration with our partners at the Ruhr Universität Bochum and the University of Copenhagen. In this work, InGaAs self-assembled quantum dots are embedded into an ultrathin p-i-n-i-n diode structure allowing for the deterministic charging of the quantum dots in the Coulomb blockade regime. The bias voltages, and thus tunnelling currents, can be kept very small, a particular advantage of the p-i-n-i-n diode. These excellent electrical properties allowed us to demonstrate narrow optical linewidths as well as optical spin pumping on single quantum dots in this ultrathin diode structure. The structure is fully compatible with the fabrication of photonic crystals. At least the editor seems to like it: the work was highlighted as an “Editor’s Suggestion”, as was another paper (Indistinguishable and efficient single photons from a quantum dot in a planar nanobeam waveguide) just recently published as part of this collaboration!

The flux of coherent photons emitted by single NV centres in diamond is very low on account of three problems: (i) only a small percentage are emitted into the zero-phonon-line (ZPL); (ii) the extraction efficiency out of the high-index diamond host is small; and (iii) the radiative lifetime is large. All three problems can be solved by embedding a single NV centre into a high-Q-factor, low mode-volume microcavity tuned to the ZPL. In our paper just published in Physical Review X, we show that this concept can be implemented using a tunable, highly-miniaturised microcavity. The ZPL fraction increases from ∼3% to close to 50%.

Schematic of a diamond membrane embedded in a highly miniaturised Fabry-Perot cavity

Our paper Fabrication of mirror templates in silica with micron-sized radii of curvature was published in Applied Physics Letters. Routinely, we use a laser ablation process to create mirror templates in silica, usually on flat substrates but also on the end facets of optical fibres. For cavity QED applications, the mode volume should be small. We therefore strive to make the radius of curvature as small as possible. However, while the standard process can produce radii down to 5 microns or so, there is a strong correlation between radius and depth – as the radius decreases, the depth increases – such that the smallest radii templates do not support stable cavity modes. In this paper we describe two techniques which allow shallow, small-radius mirror templates to be fabricated. The essential advantage of the laser ablation technique, the creation of super-smooth surfaces, is retained and we demonstrate stable, small-radius microcavity modes with mirror-limited finesse values up to 25,000.

Our paper entitled Decoupling a hole spin qubit from the nuclear spins was published in Nature Materials. An electron spin in a quantum dot interacts with the nuclear spins (via the hyperfine interaction) and this limits the spin dephasing times to quite modest values. What about a hole spin? Theory predicts that a heavy-hole spin decouples from the nuclear spins in an in-plane magnetic field. This is an idealized limit. How does it work out in practice? We have attempted to answer this question with an experiment: we apply an in-plane magnetic field, we measure the hole Zeeman frequency spectroscopically with an ultrahigh resolution technique (“coherent population trapping”, equivalently “dark-state spectroscopy”) and then look for changes in the hole Zeeman frequency as we polarize the nuclear spins. We tried to carry out this experiment several years ago but charge noise defeated us. Now though we have super-quiet hole devices thanks to the efforts of our partners Arne Ludwig and Andreas Wieck in Bochum and the dark-state spectroscopy works really nicely. The result? The hole spin decouples from the nuclear spins, even when we look with 10 neV resolution. The paper was featured in UniNews.

Our paper entitled Role of the electron spin in determining the coherence of the nuclear spins in a quantum dot was published in Nature Nanotechnology. The work is a close collaboration between the Poggio and Loss groups in Basel and our partners Arne Ludwig and Andreas Wieck in Bochum. Our motivation was to determine the limits set by the hyperfine interaction on the decoherence rate of an electron spin inside a quantum dot. To do this, we decided to measure the coherence of the nuclear spins as the nuclear spins have long-lived coherence in the absence of an electron. We found that the nuclear spin coherence time (as measured with a Hahn echo protocol) falls from several milli-seconds to around 20 μs on adding a single electron to the quantum dot. The nuclear spin recovers to its original value on adding two electrons pointing to a spin-based and not a charge-based decoherence mechanism (in the case of two electrons, a spin singlet is formed). We propose that a single electron mediates a nuclear spin-nuclear spin coupling, an RKKY-like effect. This hypothesis is backed up with a quantitative calculation. The paper was featured in UniNews.

Immo Söllner’s paper entitled Deterministic Single-Phonon Source Triggered by a Single Photon was published in Physical Review Letters. In the manuscript, a collaboration with Leonardo Midolo and Peter Lodahl from the Niels Bohr Institute, we propose an optomechanical crystal that allows for the simultaneous control of a quantum dot’s photonic and phononic environments. We go on to show how this platform enables the generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. Future work will focus on the experimental realization of these ideas, in collaboration with groups at the University of Bochum and at the Niels Bohr Institute.

Our paper entitled A fiber-coupled quantum-dot on a photonic tip was published in Applied Physics Letters. In the manuscript we present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum dot bonded to the end of an optical fibre. The quantum dot is embedded in a photonic “trumpet” to ensure good mode-matching to the mode in the optical fibre. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% and suggest realistic improvements for the implementation of a useful device in the context of quantum information. We also discuss potential applications in scanning probe microscopy. The approach is generic and transferable to other materials including diamond and silicon. The research represents a collaboration between three groups in Basel (Poggio, Maletinsky, Warburton), two groups in Grenoble (photonic trumpet fabrication) and a group at Technical University of Denmark (photonic modeling). A video showing the fabrication process can be found here.

Our paper entitled An artificial Rb atom in a semiconductor with lifetime-limited linewidth was published in Physical Review B. This work, a collaboration with Philipp Treutlein (Basel), Armando Rastelli (Linz), Fei Ding and Oliver Schmidt (Dresden), describes single photon creation using a “droplet” quantum dot. Surprisingly, our first attempts at creating spectrally narrow single photons with this system were successful: the quantum dot linewidth is just 1.42 GHz corresponding to the transform limit for one quantum dot we probed in detail. The major noise is not upper level dephasing but a relatively benign blinking on timescale much larger than radiative recombination. The results strike us as important for single photon generation at a key wavelength for quantum technology: we achieve a perfect match to the wavelength of the Rb D2 transition. In Basel, the results are particularly relevant for our on-going efforts to create a hybrid semiconductor-cold atom quantum memory.