Quantum interference of identical photons from remote GaAs quantum dots

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.

Quantum interference of identical photons from remote GaAs quantum dots

A diamond-confined open microcavity featuring a high quality-factor and a small mode-volume

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.

A diamond-confined open microcavity featuring a high quality-factor and a small mode-volume

A chiral one-dimensional atom using a quantum dot in an open microcavity

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.

A chiral one-dimensional atom using a quantum dot in an open microcavity

A hole spin qubit in a fin field-effect transistor above 4 kelvin

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.

A hole spin qubit in a fin field-effect transistor above 4 kelvin

Richard receives the Nevill Mott Medal and Prize from IOP!

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

Richard receives the Nevill Mott Medal and Prize from IOP!

Optically driving the radiative Auger transition

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.

Optically driving the radiative Auger transition

Nadine Leisgang wins the Nano Image Award 2021!

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.

Nadine Leisgang wins the Nano Image Award 2021!

Laser writing of low-charge-noise nitrogen-vacancy centers in diamond

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.

Laser writing of low-charge-noise nitrogen-vacancy centers in diamond

Tuning the polarization splitting of a microcavity with stress

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.

Tuning the polarization splitting of a microcavity with stress

Natasha Tomm wins the QCQT Excellence Award!

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/

Natasha Tomm wins the QCQT Excellence Award!

Surface passivation of GaAs mitigates surface-related losses in a microcavity

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.

Surface passivation of GaAs mitigates surface-related losses in a microcavity

QSIT features Nadine Leisgang in #NCCRWomen 2021 campaign

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

 

QSIT features Nadine Leisgang in #NCCRWomen 2021 campaign

Silicon quantum dots with a self-aligned gates

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.

Silicon quantum dots with a self-aligned gates

A record-breaking single photon source

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.

A record-breaking single photon source

Low-Noise GaAs Quantum Dots for Quantum Photonics

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.

Low-Noise GaAs Quantum Dots for Quantum Photonics

Large-Range Frequency Tuning of a Narrow-Linewidth Quantum Emitter

We have achieved large-range frequency tuning of a single-photon emitter, a GaAs quantum dot in a bulk sample. The total tuning range is three orders of magnitude large than the quantum dot’s linewidth, which remains narrow throughout the entire tuning process. Our results are an important step towards building a hybrid system connecting a single-photon emitter to a rubidium quantum memory. You find our work published in Applied Physics Letters

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Large-Range Frequency Tuning of a Narrow-Linewidth Quantum Emitter

Interlayer excitons in bilayer MoS2

In our recent paper, we have shown that interlayer excitons in bilayer MoS2 exhibit both a high oscillator strength and highly tunable energies in an applied electric field. Owing to this very large tunability, we were able to optically probe the interaction between intra- and interlayer excitons as they were energetically tuned into resonance. These results have been published in Nature Nanotechnology. More details can also be read at UniNews.

Interlayer excitons in bilayer MoS2

Daniel Najer wins the Swiss Nanotechnology PhD award!

Daniel Najer was one of the 5 young scientists to win the Swiss Nanotechnology PhD award. The prize is awarded by the company Bühler for Daniel’s publication on an efficient light-matter interface, coupling a semiconductor QD strongly to an optical microcavity. Congratulations!

Read more here: https://nanoscience.ch/en/2020/07/15/five-awards-for-young-scientists/ 

Daniel Najer wins the Swiss Nanotechnology PhD award!