In our most recent manuscript, published in Applied Physics Letter, we present an improved method for the creation of narrow-linewidth nitrogen-vacancy centers (NVs) in microstructured diamond. NVs are known for their exceptional spin coherence and convenience in optical spin initialization and readout, and are increasingly used both as a quantum sensor and as a building block for quantum networks. The standard method relying on nitrogen implantation to create NVs tends to create NV populations with broadened optical linewidth. The phenomenon is much aggravated when photonic structures allowing a better photon collection efficiency are created around the emitters. We demonstrate that implantation of carbon ions yields a comparable density of NVs as implantation of nitrogen ions, and that implantation after instead of before the diamond fabrication process results in NV populations with narrow optical linewidths and low charge-noise levels even in thin diamond microstructures. We measure a median NV linewidth of 150 MHz for structures thinner than 5 μm, with no trend of increasing linewidths down to the thinnest measured structure of 1.9 μm and confirm our results in multiple samples implanted with different ion energies and fluences.

Natasha Tomm has received two awards for her PhD thesis “A quantum dot in a microcavity as a bright source of coherent single photons” this year!

The first prize was the Prix Schläfli in Physics 2022, awarded by the Swiss Academy of Sciences (SCNAT). The Prix Schläfli is one of the oldest prizes in Switzerland and awards every year the best dissertations in natural sciences. Read more here.

The second one was given by the Philosophisch-Naturwissenschaftliche Fakultät of the University of Basel. Every year the Faculty awards two outstanding dissertations that have demonstrated great importance and quality of the scientific contributions.

Congratulations!!!

In our recent publication in Physical Review Letters, we experimentally and theoretically study exciton-exciton couplings in a two-dimensional semiconductor, homobilayer MoS₂. More information can be found in the SNI news or in this short explanation video.

Simon Geyer was recognized with the Swiss Nanotechnology PhD award sponsored by the IBM Research Zürich for the publication “A hole spin qubit in a fin field-effect transistor above 4 kelvin”. Congratulations!!

Recently published in Nature Nanotechnology, we demonstrate quantum interference between single photons from separate quantum dots. Such a demonstration has been attempted multiple times in the past decade and appeared challenging. The problem is the noise: the noise affects the photon creation process of different quantum dots in different ways. Thus, single photons created by separate quantum dots are sometimes distinguishable. Low-noise GaAs quantum dots are employed for our work, each operated in an independent cryostat. Single photons are created from two distant quantum dots and simultaneously sent to two inputs of a beamsplitter. Despite their (different) origins, the photons exhibit bunching in the beamsplitter output with 93% visibility: they are almost identical. Using an optical CNOT operation, we achieve high-fidelity entanglement between photons from two quantum dots. Our results establish low-noise GaAs quantum dots as interconnectable sources of identical photons.

In out recent publication in Journal of Applied Physics, we report on an open microcavity containing a diamond micromembrane. Despite operation in the so-called diamond confined regime, where the electric field profile inside the cavity is maximized across the diamond-air interface, we demonstrate quality factors exceeding 120 000. We next develop a qualitative model describing the losses in our cavity and determine that the dominant source of losses to be surface waviness – surface textures with dimensions comparable to the cavity beam waist. The large quality factor combined with a small mode volume of 3.9λ₀³, yield a predicted Purcell factor of 170, a significant improvement over the state-of-the-art.

In our recent publication in npj Quantum Information we have demonstrated a chiral one-dimensional atom using a single semiconductor quantum dot in a tunable microcavity. In a chiral atom, photons propagating in one direction interact with the atom, while photons propagating in the other direction do not. Here, we achieve strong non-reciprocal absorption of single-photons, a single-photon diode. Proof that chirality arises from a single emitter is found in the nonlinear behaviour at low powers – light propagating in the backward direction of the diode is highly bunched.

One of the greatest challenges in quantum computing is scalability. Classical computing overcome this problem by integrating bilions of nm-scale fin field-effect transistors (FinFETs) on a silicon chip. Here, we operate a silicon FinFET as a hole spin qubit above 4 K. At this elevated temperature cooling power increases by order of magnitudes compared to typical qubit operation temperatures, such that on-chip integration with control electronics becomes feasible. We achieve fast electrical 3-axis control with speeds up to 150 MHz, single-qubit fidelities at the fault-tolerance threshold, and a Rabi quality factor greater than 87. The devices feature both industry compatibility and quality, yet are fabricated in a flexible and agile way accelerating future development. The work was published in Nature Electronics.

Prof. Richard Warburton received the prestigious Nevill Mott Medal and Prize from the Institute of Physics (UK)! The prize was awarded in recognition of his outstanding research in solid-state physics and quantum optics, especially the invention and application of Coulomb blockade devices to create coherent spin-photon interfaces and quantum light sources. Congratulations!

Read more here: https://www.iop.org/about/awards/2021-nevill-mott-medal-and-prize

Radiative Auger is a process that leads to red-shifted satellite peaks in the emission of atoms and solid-state quantum emitters. It is caused by Coulomb interactions between charged carriers. In our recent paper in Nature Communications, we show for the first time that it is possible to turn the whole process around by optical driving of the radiative Auger transition. Possible applications could be fast optical switching or THz spectroscopy.

Nadine Leisgang was recognized with the Swiss Nanotechnology PhD award sponsored by the Hightech Zentrum Aargau for the publication “Giant Stark splitting of an exciton in bilayer MoS₂”. Congratulations!!

Read more here: https://nanoscience.ch/en/2021/08/09/excellent-prizes-for-outstanding-doctoral-students/

Nadine Leisgang received the Nano Image Award 2021 for the optical micrograph image of a gated van der Waals heterostructure. The device consists of two monolayers of transition metal dichalcogenides encapsulated between insulating hexagonal boron nitride flakes. Direct gold contacts allow tuning the carrier concentration in the optically active layers in the middle of the structure. Few-layer graphene sheets on the top and bottom of the stack serve as local gates to apply an electric field across the device.

This image reflects the complexity and – at the same time – the beauty of the fabrication of van der Waals heterostructures.

In our recent publication in ACS Photonics, we report on the pulsed-laser-induced generation of high-quality nitrogen-vacancy (NV) centers in diamond facilitated by a solid-immersion lens (SIL). The SIL enables laser writing at energies as low as 5.8 nJ per pulse and allows vacancies to be formed close to a diamond surface without inducing surface graphitization. We present samples in which NV center arrays were laser-written across the full diamond thickness, all presenting narrow optical linewidth distributions with means down to 62.1 MHz. The linewidths include the effect of long-term spectral diffusion induced by a 532 nm repump laser for charge-state stabilization, underlining the extremely low charge-noise environment of the created color centers.

In our recent open-access publication at Physical Review Applied we report a technique to control the frequency splitting of two orthogonal polarization modes in an open semiconductor microcavity. We employ the photoelastic effect by stressing uniaxially the sample and controlling the birefringence of the semiconductor crystal. The semiconductor mirror is mounted on a strain piezo, and the amount of stress is gauged by observing the emission spectrum shift of quantum dots embedded in the sample. We achieve up to 11 GHz tuning at the center of the stopband.

Natasha Tomm was one of the 3 PhD students from the QCQT PhD school to be awarded with the QCQT Excellence award. The prize is awarded by the Basel Center for Quantum Computing and Quantum Coherence in recognition of her outstanding work “A bright and fast source of coherent single photons“. Congratulations!

Read more here: https://www.quantum.unibas.ch/qcqt-phd-school/awards-support/

We found that GaAs surface passivation is key to minimize losses in a gated semiconductor microcavity and thus increase its quality factor by almost two orders of magnitude. The procedure not only eliminates a Franz-Keldysh-like surface loss inherent to gated semiconductor microcavities, but also mitigates the effect of surface scattering due to roughness. We elucidate these findings in our open-access publication at Physical Review Applied.

Nadine Leisgang was featured by NCCR (National Centres of Competence in Research) in their new campaign #NCCRWomen. In the video, Nadine explains a bit about her work as a physicist at the University of Basel and her motivation to do science. Congratulations!

Read more here: https://nccr-qsit.ethz.ch/equal-opportunity/NCCRWomen_campaign.html

Video here: https://www.youtube.com/watch?v=3eaSz3fry84

In our recent publication at Applied Physics Letters we present our work on silicon-finFET quantum dots with perfectly self-aligned 2nd gate layer and gate lengths down to 15 nm. The fabrication is industry compatible and scalable and gives very high-quality devices. We observe Pauli spin blockade and extract the hole g-factor and strong spin-orbit coupling with spin-orbit length of ~50 nm, thus paving the way for scalable silicon spin qubits with fast, all-electrical control. Device fabrication and measurements were done in collaboration between IBM Zürich and University of Basel team.

In our recent publication in Nature Nanotechnology we present the results of our record-breaking single photon source. The source, based on InAs quantum dots coupled in a carefully designed open-access microcavity, is able to emit up to 1 billion single photons per second with an end-to-end efficiency of 57%. Furthermore, photons present high single-photon purity (98%) and preserve high coherence (HOM visibility = 97%) in timescales of up to 1.5us. This result allows for significant improvement in quantum processing with photons. More information can be found at UniNews.

We have realized electrical tuning of the energy and the charge-state of GaAs quantum dots in AlGaAs. In contrast to previous work on the same system, the quantum dots do not suffer from a fluctuating charge-state. At the same time, we achieve linewidths that are just a few percent broader than the lifetime-limit. Our results are an important step towards connecting a quantum dot as a single-photon emitter to a rubidium memory in which quantum information can be stored. You can find our results in the open-access journal Nature Communications.