Strong-coupling of a semiconductor quantum dot in a microcavity

A coherent exchange of a single energy quantum between an “atom” (in our case a gated InAs quantum dot) and an optical cavity has been reported by our group in Nature. We achieve an atom-cavity Cooperativity of 150, and probe the transitions between singly and doubly excited photon-atom system using photon-statistics spectroscopy.

Strong-coupling of a semiconductor quantum dot in a microcavity

Correlations between optical properties and Voronoi-cell area of quantum dots

In our recent paper, we have shown that several optical properties of a quantum dot are correlated with the area of the so-called Voronoi cell surrounding it. In the image below, the Voronoi-diagram is shown in red for a few tens of quantum dots. The quantum dot positions are indicated as black dots.

Correlations between optical properties and Voronoi-cell area of quantum dots

Excitons in InGaAs quantum dots without electron wetting layer states

We have recently shown that InGaAs quantum dots can be grown without conduction band states of the so-called wetting layer. This work is the result of a collaboration with the Ruhr Universität Bochum and the Forschungszentrum Jülich. In the picture below, you see a transmission electron microscopy image of a quantum dot grown with the new technique.

Excitons in InGaAs quantum dots without electron wetting layer states

Daniel Riedel wins PhD prize!

Daniel Riedel has won the Swiss Nanotechnology PhD Award 2018, an award sponsored by the Hightech Zentrum Aargau. He received the award for his publication in Physical Review X on improving the quality of the photons emitted by NV centres in diamond.

Christian Schönenberger, Daniel Riedel and Martin Bopp (Director of the Hightech Zentrum Aargau) during the award ceremony of the Swiss Nanotechnology PhD Award (Image: Edward Byrne).

Daniel Riedel wins PhD prize!

Tomek secures Marie-Skłodowska-Curie Fellowship

Our Post-doctoral research, Tomek, has been awarded a Marie-Skłdowska-Curie Individual Fellowship!

Tomek’s project, entitled “High-Frequency Spin Entanglement Generation in Diamond” (Hi-FrED) aims at establishing the Nitrogen-Vacancy center in diamond not only as spin-coherent, but also as an optically-coherent efficient emitter of single photons.

More details about the project, starting in September 2019, can be found here.

Tomek secures Marie-Skłodowska-Curie Fellowship

Hi-FRED Project

Host: Prof. Dr. Richard J. Warburton, in collaboration with the Quantum Sensing group (prof. Patrick Maletinsky) and the Nano-phononics group (prof. Ilaria Zardo)

Fellow: Dr. Tomasz Jakubczyk

Core participants: Viktoria Yurgens, Sigurd Flågan, Brendan Shields

The ultimate goal of the project is to achieve high-frequency generation of spin-spin entanglement in spatially separated nitrogen vacancy (NV) centers in diamond. While current photon collection efficiencies (few percent) and entanglement rates (approx. one entanglement event per minute) may be sufficient for proof-of-principle experiments, they need to be greatly improved for the implementation in practical quantum networks.

The project exploits deterministic cavity-assisted enhancement of the coherent photon emission rate of NV centers embedded in a micrometer-thin diamond sample. The increase of the decay rate results in enhanced radiative efficiency and makes the emission robust against dephasing, enhancing the photon indistinguishability and boosting the photon extraction efficiency. This work helps in establishing the NV center as not only spin- but also optically- coherent.

The main experimental challenge is to eliminate the optical linewidth broadening resulting from necessary processing of the diamond crystal, even though the processing remains at a minimal level in the selected open cavity scheme. We’re examining the suitability of novel NV fabrication methods to tackle this issue. In parallel, we are examining sources of optical losses in our open cavity design and trying to eliminate them to push the performance to a new level.

In the long term, the project aims at enabling new ways of studying phenomena resulting from the enhanced light-matter coupling of the NV center and other quantum emitters. Success of this project may provide a route to the realisation of scalable quantum computers based on optical networks of electronic and nuclear spins.

 

 

News:

-We examined the effect of a novel approach of creating NV centers in thin diamond samples, where the nitrogen is implanted after all etching processes have been completed. Results of our studies are described in Ref. [1].

-We achieved pulsed laser-induced and solid-immersion lens (SIL)-assisted generation of low-noise NV centers in diamond. The NV center ensemble features a distribution of zero-phonon-lines with a mean of 58 MHz and standard deviation of 19 MHz. This includes the effect of long-term spectral diffusion caused by a necessary green repump laser for charge stabilization. Such high-quality NVs are excellent candidates for practical implementations employing two-photon quantum interference with separate NV centers.  We are currently working on implementing these NVs in our open microcavities.

Publications:

[1] Kasperczyk, M., Zuber, J. A., Kölbl, J., Yurgens, V., Flågan, S., Jakubczyk, T., … Warburton, R. & Maletinsky, P. (2020). Statistically Modeling Optical Linewidths of Nitrogen Vacancy Centers in Post-Implanted Nanostructures. arXiv preprint arXiv:2005.03666.

 

 

Hi-FRED Project

Nanoscopy on single quantum dots

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.

Nanoscopy on single quantum dots

Matthias and Immo’s paper on the n-i-n-i-p structure appears in press

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!

Matthias and Immo’s paper on the n-i-n-i-p structure appears in press

NV centres in a tunable microcavity

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

NV centres in a tunable microcavity

Immo secures Marie-Skłodowska-Curie Fellowship

With great pleasure we announce that Dr. Immo Söllner, postdoctoral researcher in the Nano-Photonics Group, has been awarded a Marie-Skłodowska-Curie Individual Fellowship for the project “Emitter-mediated Photon-Phonon InteraCtion (EPPIC)”. The project will focus on engineering the interaction of single seminconductor quantum dots with the phononic environment. Immo will work closely with PhDs and postdoctoral researchers at the University of Basel to carry out this project and will profit greatly from a close collaboration with European partners in Germany and Denmark. We thank the European Union for the generous support and hope that our work will help exploit the potential of quantum science!

Immo secures Marie-Skłodowska-Curie Fellowship

Fabrication paper published

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.

Fabrication paper published

Paper published on quantum dot hole spin

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.

Paper published on quantum dot hole spin

Paper published on nuclear spin coherence in a quantum dot

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.

Paper published on nuclear spin coherence in a quantum dot

Paper published on quantum dots in an optomechanical crystal

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.

 

Paper published on quantum dots in an optomechanical crystal

Paper published on quantum fibre pigtail

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.

Paper published on quantum fibre pigtail

Andreas Kuhlmann leaves for IBM

Andreas Kuhlmann starts today at IBM, Rüshlikon in the team of Andreas Fuhrer. Andreas was RJW’s first PhD student in Basel, subsequently a post-doc in the group. We shall miss him but wish him well with his new job, incidentally a QSIT-funded project.

Andreas Kuhlmann leaves for IBM