Showing 1–50 of 278 results for author: Wieck, A

Real-time dynamics of triplet-resonant tunneling driven by nonequilibrium phonons

Authors: Kazuyuki Kuroyama, Sasha R. Valentin, Arne Ludwig, Andreas D. Wieck, Yasuhiro Tokura, Seigo Tarucha, Sadashige Matsuo

Abstract: Driven nonequilibrium systems can host emergent functionalities beyond equilibrium, but real-time access to excited-state dynamics remains limited. Here we report real-time measurements of phonon-driven charge and spin dynamics in excited states of a double quantum dot. Under phonon irradiation, resonant inter-dot tunneling emerges at triplet resonance. Time-resolved charge sensing reveals that th… ▽ More Driven nonequilibrium systems can host emergent functionalities beyond equilibrium, but real-time access to excited-state dynamics remains limited. Here we report real-time measurements of phonon-driven charge and spin dynamics in excited states of a double quantum dot. Under phonon irradiation, resonant inter-dot tunneling emerges at triplet resonance. Time-resolved charge sensing reveals that the resonant inter-dot tunneling is strongly modified by spin blockade. For weaker inter-dot coupling, the nonequilibrium phonon environment generates a unidirectional transport cycle along the phonon density gradient. △ Less

Submitted 1 July, 2026; originally announced July 2026.

Beam-Offset Thermoreflectance with Bayesian Optimization to Measure the Anisotropic Thermal Properties of Semiconductor Superlattices

Authors: A. Chatterjee, N. Spitzer, T. Kruck, P. Song, A. Ludwig, A. D. Wieck, J. Ordonez-Miranda, M. Pawlak

Abstract: The directional nature of heat conduction in semiconductor superlattices--marked by significant differences between in-plane and cross-plane pathways--poses substantial challenges for precise thermal property assessment. Conventional frequency-domain thermoreflectance (FDTR) techniques, while proficient at evaluating cross-plane thermal conductivity, suffer from restricted capability in resolving… ▽ More The directional nature of heat conduction in semiconductor superlattices--marked by significant differences between in-plane and cross-plane pathways--poses substantial challenges for precise thermal property assessment. Conventional frequency-domain thermoreflectance (FDTR) techniques, while proficient at evaluating cross-plane thermal conductivity, suffer from restricted capability in resolving in-plane transport due to inherent phase-delay constraints and inadequate lateral resolution. In this investigation, we establish a non-contact beam-offset FDTR (BO-FDTR) approach that concurrently determines both directional thermal conductivities within layered semiconductor architectures. Our methodology implements spatial separation between excitation and detection beams while utilizing coupled normalized amplitude and phase responses as analytical inputs, thereby improving discrimination between anisotropic thermal parameters. We combine this experimental configuration with a Bayesian optimization scheme incorporating Gaussian Process Regression (BO-GPR) to reduce estimation inaccuracies, attaining measurement uncertainties under 1% to 2% at 95% confidence intervals. This technique demonstrates particular efficacy for intricate multilayer nanostructures, furnishing a structured protocol for superlattice thermal evaluation. Experimental characterization of an AlAs/GaAs superlattice (period thickness 52 nm) delivers thermal conductivity values of 14.7 W m-1 K-1 (cross-plane) and 37.4 W m-1 K-1 (in-plane). Our findings indicate that integrating frequency sweeps with varied beam offset locations yields superior measurement precision, exceeding conventional single-variable methods and confirming thermal assessment validity across both geometric arrangements. △ Less

Submitted 21 February, 2026; originally announced February 2026.

Exchange interaction in gate-defined quantum dots beyond the Hubbard model

Authors: Alexander Willmes, Patrick Bethke, M. Mohamed El Kordy Shehata, George Simion, M. A. Wolfe, Tim Botzem, Robert P. G. McNeil, Julian Ritzmann, Arne Ludwig, Andreas D. Wieck, Dieter Schuh, Dominique Bougeard, Hendrik Bluhm

Abstract: A quantitative description of the exchange interaction in quantum dots is relevant for modeling gate operations of spin qubits. By measuring the amplitude and frequency of exchange-driven qubit state oscillations, we measure the detuning dependence of the exchange coupling in a GaAs double quantum dot over three orders of magnitude. Both 1D and 3D full configuration interaction simulations can rep… ▽ More A quantitative description of the exchange interaction in quantum dots is relevant for modeling gate operations of spin qubits. By measuring the amplitude and frequency of exchange-driven qubit state oscillations, we measure the detuning dependence of the exchange coupling in a GaAs double quantum dot over three orders of magnitude. Both 1D and 3D full configuration interaction simulations can replicate the observed behavior. Extending a Hubbard model by including excited states increases the range of detuning where it provides a good fit, thus elucidating the underlying physics. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: 11 pages, 8 figures

High fidelity flopping-mode single spin operation with tuning inter-dot orbital levels

Authors: Yuta Matsumoto, Xiao-Fei Liu, Arne Ludwig, Andreas D. Wieck, Keisuke Koike, Takefumi Miyoshi, Takafumi Fujita, Akira Oiwa

Abstract: Fast spin manipulation and long spin coherence time in quantum dots are essential features for high fidelity semiconductor spin qubits. However, generally it has not been well established how to optimize these two properties simultaneously, because these two properties are usually not independent from each other. Therefore, the scheme for high fidelity operation by simultaneous tuning Rabi frequen… ▽ More Fast spin manipulation and long spin coherence time in quantum dots are essential features for high fidelity semiconductor spin qubits. However, generally it has not been well established how to optimize these two properties simultaneously, because these two properties are usually not independent from each other. Therefore, the scheme for high fidelity operation by simultaneous tuning Rabi frequency and coherence time, which does not rely on the material-dependent strong spin-orbit interaction and the local magnetic field gradient limiting their scalability, are strongly demanded. Here, we demonstrate an approach to achieve high-fidelity spin control by tuning inter-dot spin-orbit coupling in a GaAs triple quantum dot (TQD), where the third dot provides precise control over orbital energy levels. In an electrically stable charge state with optimized tunnel coupling, we achieve Rabi frequencies exceeding 100 MHz while maintaining coherence through proper tuning of the inter-dot orbital levels of the TQD. By implementing a machine learning-based feedback control that efficiently estimates qubit frequency using past measurement data, we characterize and mitigate the impact of low frequency noise on qubit coherence with minimal measurement overhead. Finally, we demonstrate a $π$/2 gate fidelity of 99.7\% with a gate time of 4 ns through randomized benchmarking, even in a GaAs quantum dot device where electron spin coherence is typically limited by strong hyperfine interaction with nuclear spins. Our approach provides a scalable strategy for high-fidelity spin control in semiconductor quantum dot arrays by utilizing device-specific parameters rather than relying on material properties or external field gradients. △ Less

Submitted 29 August, 2025; originally announced August 2025.

Spectral shadows of a single GaAs quantum dot

Authors: Kai Hühn, Lena Klar, Fei Ding, Arne Ludwig, Andreas D. Wieck, Jens Hübner, Michael Oestreich

Abstract: Semiconductor quantum dots are a promising platform for generating single and entangled photons.Still, their use is limited even in the most advanced structures by changes in the charge state of the quantum dot and its environment. Here, we present detailed time-resolved resonance fluorescence measurements on a single charge-tunable GaAs quantum dot, shedding new light on the spectral shadows invo… ▽ More Semiconductor quantum dots are a promising platform for generating single and entangled photons.Still, their use is limited even in the most advanced structures by changes in the charge state of the quantum dot and its environment. Here, we present detailed time-resolved resonance fluorescence measurements on a single charge-tunable GaAs quantum dot, shedding new light on the spectral shadows invoked by the complex impurity environment. Detuning-dependent measurements reveal the existence of multiple Stark-shifted resonances, which are associated with rare spectral jumps smaller than the homogeneous linewidth and, therefore, typically concealed in the measurement noise. We observe similar environmentally induced Stark shifts for both the neutral exciton and negatively charged trion transitions, while the positively and doubly negatively charged trions exhibit significant differences. Our investigation quantifies the underlying impurity charge dynamics over a range from well below milliseconds to seconds, revealing that the hole occupation of the positively charged trion transition is constrained by rapid hole loss and slow hole recapture dynamics. Utilizing a second non-resonant laser, we increase the hole occupancy by over an order of magnitude and identify both a prolonged hole residence time and an enhanced hole tunneling rate into the quantum dot. These findings are supported by complementary spin noise spectroscopy measurements, which offer a significantly higher bandwidth compared to the time-resolved resonance fluorescence measurements. △ Less

Submitted 22 April, 2026; v1 submitted 27 July, 2025; originally announced July 2025.

Enhancement of Indistinguishable Photon Emission from a GaAs Quantum Dot via Charge Noise Suppression

Authors: Priyabrata Mudi, Avijit Barua, Kartik Gaur, Steffen Wilksen, Alexander Steinhoff, Setthanat Wijitpatima, Sarthak Tripathi, Julian Ritzmann, Andreas D. Wieck, Sven Rodt, Christopher Gies, Arne Ludwig, Stephan Reitzenstein

Abstract: The generation of indistinguishable single photons is a fundamental requirement for future quantum technologies, particularly in quantum repeater networks and for distributed quantum computing based on entanglement distribution. However, spectral jitter, often induced by charge noise in epitaxial quantum dots, leads to exciton dephasing, thereby limiting their practical usage in quantum applicatio… ▽ More The generation of indistinguishable single photons is a fundamental requirement for future quantum technologies, particularly in quantum repeater networks and for distributed quantum computing based on entanglement distribution. However, spectral jitter, often induced by charge noise in epitaxial quantum dots, leads to exciton dephasing, thereby limiting their practical usage in quantum applications. We present a straightforward approach to mitigate charge noise-induced decoherence in droplet-etched GaAs quantum dots embedded in an n-i-p diode structure and integrated deterministically into an electrically contacted circular Bragg grating resonator for emission enhancement. The quantum device allows for the stabilization of the charge environment by applying an external electrical field while producing a photon extraction efficiency of approximately (37 +- 2)%. Hong-Ou-Mandel two-photon interference measurements reveal a strong voltage dependence of the exciton dephasing time and interference visibility on the applied bias in excellent agreement with our theoretical predictions. Notably, the reduction in visibility from a maximum, charge-stabilized corrected value of 97 percent at the optimum bias point follows an inverse square dependence (proportional to 1/I^2) with increasing diode current (I) in forward direction. Under a quasi-resonant excitation scheme, we achieve a maximum exciton dephasing time (T2*) of approximately (6.8 +-0.5) ns, reaching nearly the Fourier limit (T2 = 2T1) without the need for complex echo schemes like Ramsey or Carr-Purcell-Meiboom-Gill sequences. These findings are consistent with theoretical predictions from rate equation modeling and quantum optical analysis as well as voltage-dependent linewidth measurements, demonstrating optimized electrical control of exciton dephasing. △ Less

Submitted 16 July, 2025; originally announced July 2025.

Exciton-resonant detection of high-frequency surface acoustic waves from subwavelength metal gratings

Authors: Olga Ken, Dmytro Horiachyi, Ilya Akimov, Vladimir Korenev, Vitalyi Gusev, Leonid Litvin, Michael Kahl, Arne Ludwig, Nikolai Spitzer, Andreas D. Wieck, Manfred Bayer

Abstract: We report on all-optical generation and detection of high-frequency (up to about 30 GHz) surface acoustic waves (SAWs) in GaAs/AlGaAs heterostructures with short-period Au gratings on top. A highly sensitive method for SAW detection is demonstrated using a polarization-resolved pump-probe technique that exploits the narrow exciton resonance in high-quality GaAs. The elastic strain of the SAW modul… ▽ More We report on all-optical generation and detection of high-frequency (up to about 30 GHz) surface acoustic waves (SAWs) in GaAs/AlGaAs heterostructures with short-period Au gratings on top. A highly sensitive method for SAW detection is demonstrated using a polarization-resolved pump-probe technique that exploits the narrow exciton resonance in high-quality GaAs. The elastic strain of the SAW modulates the exciton energy in the time domain. As a result, even a small deformation produces a noticeable change in the dielectric function at the detection wavelength leading to an order of magnitude increase in the detection sensitivity as compared to off-resonant conditions. A theoretical model is developed that considers two detection schemes: one accounting for probe light diffraction and one corresponding to a non-diffractive situation. △ Less

Submitted 14 January, 2026; v1 submitted 15 July, 2025; originally announced July 2025.

Comments: Accepted for publication in Physical Review B

Near transform-limited single photons from rapid-thermal annealed quantum dots

Authors: Hendrik Mannel, Fabio Rimek, Marcel Zoellner, Nico Schwarz, Andreas D. Wieck, Nikolai Bart, Arne Ludwig, Martin Geller

Abstract: Single-photon emitters are essential components for quantum communication systems, enabling applications such as secure quantum key distribution and the long-term vision of a quantum internet. Among various candidates, self-assembled InAs/GaAs quantum dots (QDs) remain highly promising due to their ability to emit coherent and indistinguishable photons, as well as their compatibility with photonic… ▽ More Single-photon emitters are essential components for quantum communication systems, enabling applications such as secure quantum key distribution and the long-term vision of a quantum internet. Among various candidates, self-assembled InAs/GaAs quantum dots (QDs) remain highly promising due to their ability to emit coherent and indistinguishable photons, as well as their compatibility with photonic integration. In this work, we investigate the impact of post-growth rapid thermal annealing (RTA) on the quantum optical properties of single self-assembled QDs embedded in a p-i-n diode structure. The annealing process induces a controlled blueshift of the emission wavelength by promoting Ga in-diffusion and intermixing. Using resonance fluorescence measurements at cryogenic temperatures (4.2 K), we investigate the single-photon statistics, the emission linewidths, and coherence time $T_2$ of the emitted photons. Our results show that, despite the high annealing temperature of $760^\circ$C, the process does not degrade the optical quality of the quantum dots strongly. Instead, we observe single-photon emission with near transform-limited linewidths, where the dephasing time $T_2$ is only a factor 1.5 above the Fourier-limit $T_2=2T_1$. These findings demonstrate that rapid thermal annealing (RTA) serves as an effective tuning method that preserves the key single-photon emission properties and may help reduce undesirable effects such as non-radiative Auger recombination in quantum photonic applications. △ Less

Submitted 22 January, 2026; v1 submitted 15 July, 2025; originally announced July 2025.

Quantum stochastic resonance in a single-photon emitter

Authors: H. Mannel, J. Zöllner, E. Kleinherbers, M. Zöllner, N. Schwarz, F. Rimek, A. D. Wieck, A. Ludwig, A. Lorke, J. König, M. Geller

Abstract: Stochastic resonance is a phenomenon in which fluctuations enhance an otherwise weak signal. It has been found in many different systems in paleoclimatology, biology, medicine, and physics. The classical stochastic resonance due to thermal noise has recently been experimentally extended to the quantum regime, where the fundamental randomness of individual quantum events provides the noise source.… ▽ More Stochastic resonance is a phenomenon in which fluctuations enhance an otherwise weak signal. It has been found in many different systems in paleoclimatology, biology, medicine, and physics. The classical stochastic resonance due to thermal noise has recently been experimentally extended to the quantum regime, where the fundamental randomness of individual quantum events provides the noise source. Here, we demonstrate quantum stochastic resonance in the single-electron tunneling dynamics of a periodically driven single-photon emitter, consisting of a self-assembled quantum dot that is tunnel-coupled to an electron reservoir. Such highly-controllable quantum emitters are promising candidates for future applications in quantum information technologies. We monitor the charge dynamics by resonant optical excitation and identify quantum stochastic resonance with the help of full counting statistics of tunneling events in terms of the Fano factor and extend the statistical evaluation to factorial cumulants to gain a deeper understanding of this far-reaching phenomenon. △ Less

Submitted 23 October, 2025; v1 submitted 20 May, 2025; originally announced May 2025.

Journal ref: Commun Phys 8, 404 (2025)

Towards quantitative understanding of quantum dot ensemble capacitance-voltage spectroscopy

Authors: Nico F. Brosda, Phil J. Badura, İsmail Bölükbaşı, İbrahim Engin, Patrick Lindner, Sascha R. Valentin, Andreas D. Wieck, Björn Sothmann, Arne Ludwig

Abstract: Inhomogeneous ensembles of quantum dots (QDs) coupled to a charge reservoir are widely studied by using, e.g., electrical methods like capacitance-voltage spectroscopy. We present experimental measurements of the QD capacitance as a function of varying parameters such as ac frequency and bath temperature. The experiment reveals distinct shifts in the position of the capacitance peaks. While temper… ▽ More Inhomogeneous ensembles of quantum dots (QDs) coupled to a charge reservoir are widely studied by using, e.g., electrical methods like capacitance-voltage spectroscopy. We present experimental measurements of the QD capacitance as a function of varying parameters such as ac frequency and bath temperature. The experiment reveals distinct shifts in the position of the capacitance peaks. While temperature-induced shifts have been explained by previous models, the observation of frequency-dependent shifts has not been explained so far. Given that existing models fall short in explaining these phenomena, we propose a refined theoretical model based on a master equation approach which incorporates energy-dependent tunneling effects. This approach successfully reproduces the experimental data. We highlight the critical role of energy-dependent tunneling in two distinct regimes: at low temperatures, ensemble effects arising from energy-level dispersion in differently sized QDs dominate the spectral response; at high temperatures and frequencies, we observe a peak shift of a different nature, which is best described by optimizing the conjoint probability of successive in- and out-tunneling events. Our findings contribute to a deeper understanding of tunnel processes and the physical properties of QD ensembles coupled to a common reservoir, with implications for their development in applications such as single-photon sources and spin qubits. △ Less

Submitted 11 April, 2025; originally announced April 2025.

Comments: 13 pages, 9 figures. Nico F. Brosda and Phil J. Badura contributed equally. Submitted to Physical Review B

Optical and magnetic response by design in GaAs quantum dots

Authors: Christian Schimpf, Ailton J. Garcia Jr., Zhe X. Koong, Giang N. Nguyen, Lukas L. Niekamp, Martin Hayhurst Appel, Ahmed Hassanen, James Waller, Yusuf Karli, Saimon Philipe Covre da Silva, Julian Ritzmann, Hans-Georg Babin, Andreas D. Wieck, Anton Pishchagin, Nico Margaria, Ti-Huong Au, Sebastien Bossier, Martina Morassi, Aristide Lemaitre, Pascale Senellart, Niccolo Somaschi, Arne Ludwig, Richard Warburton, Mete Atatüre, Armando Rastelli , et al. (2 additional authors not shown)

Abstract: Quantum networking technologies use spin qubits and their interface to single photons as core components of a network node. This necessitates the ability to co-design the magnetic- and optical-dipole response of a quantum system. These properties are notoriously difficult to design in many solid-state systems, where spin-orbit coupling and the crystalline environment for each qubit create inhomoge… ▽ More Quantum networking technologies use spin qubits and their interface to single photons as core components of a network node. This necessitates the ability to co-design the magnetic- and optical-dipole response of a quantum system. These properties are notoriously difficult to design in many solid-state systems, where spin-orbit coupling and the crystalline environment for each qubit create inhomogeneity of electronic g-factors and optically active states. Here, we show that GaAs quantum dots (QDs) obtained via the quasi-strain-free local droplet etching epitaxy growth method provide spin and optical properties predictable from assuming the highest possible QD symmetry. Our measurements of electron and hole g-tensors and of transition dipole moment orientations for charged excitons agree with our predictions from a multiband k.p simulation constrained only by a single atomic-force-microscopy reconstruction of QD morphology. This agreement is verified across multiple wavelength-specific growth runs at different facilities within the range of 730 nm to 790 nm for the exciton emission. Remarkably, our measurements and simulations track the in-plane electron g-factors through a zero-crossing from -0.1 to 0.3 and linear optical dipole moment orientations fully determined by an external magnetic field. The robustness of our results demonstrates the capability to design - prior to growth - the properties of a spin qubit and its tunable optical interface best adapted to a target magnetic and photonic environment with direct application for high-quality spin-photon entanglement. △ Less

Submitted 3 April, 2025; originally announced April 2025.

Eigenstate control of plasmon wavepackets with electron-channel blockade

Authors: Shintaro Takada, Giorgos Georgiou, Junliang Wang, Yuma Okazaki, Shuji Nakamura, David Pomaranski, Arne Ludwig, Andreas D. Wieck, Michihisa Yamamoto, Christopher Bäuerle, Nobu-Hisa Kaneko

Abstract: Coherent manipulation of plasmon wavepackets in solid-state systems is crucial for advancing nanoscale electronic devices, offering a unique platform for quantum information processing based on propagating quantum bits. Controlling the eigenstate of plasmon wavepackets is essential, as it determines its propagation speed and hence the number of quantum operations that can be performed during its f… ▽ More Coherent manipulation of plasmon wavepackets in solid-state systems is crucial for advancing nanoscale electronic devices, offering a unique platform for quantum information processing based on propagating quantum bits. Controlling the eigenstate of plasmon wavepackets is essential, as it determines its propagation speed and hence the number of quantum operations that can be performed during its flight-time through a quantum system. When plasmon wavepackets are generated by short voltage pulses and transmitted through nanoscale devices, they distribute among multiple electron conduction channels via Coulomb interactions, a phenomenon known as charge fractionalisation. This spreading complicates plasmon manipulation in quantum circuits and makes precise control of the eigenstates of plasmon wavepackets challenging. Using a cavity, we demonstrate the ability to isolate and select electron conduction channels contributing to plasmon excitation, thus enabling precise control of plasmon eigenstate. Specifically, we observe an electron-channel blockade effect, where charge fractionalisation into cavity-confined channels is suppressed due to the plasmon's narrow energy distribution, enabling more stable and predictable plasmonic circuits. This technique provides a versatile tool for designing plasmonic circuits, offering the ability to tailor plasmon speed through local parameters, minimise unwanted plasmon excitation in adjacent circuits, and enable the precise selection of electron-channel plasmon eigenstates in quantum interferometers. △ Less

Submitted 22 November, 2025; v1 submitted 9 March, 2025; originally announced March 2025.

Comments: 24 pages, 5 figures, 7 supplementary figures

Journal ref: Nature Communications 16, 9942 (2025)

Semiconductor Circuits for Quantum Computing with Electronic Wave Packets

Authors: David Pomaranski, Ryo Ito, Ngoc Han Tu, Arne Ludwig, Andreas D. Wieck, Shintaro Takada, Nobu-Hisa Kaneko, Seddik Ouacel, Christopher Bauerle, Michihisa Yamamoto

Abstract: Standard approaches to quantum computing require significant overhead to correct for errors. The hardware size for conventional quantum processors in solids often increases linearly with the number of physical qubits, such as for transmon qubits in superconducting circuits or electron spin qubits in quantum dot arrays. While photonic circuits based on flying qubits do not suffer from decoherence o… ▽ More Standard approaches to quantum computing require significant overhead to correct for errors. The hardware size for conventional quantum processors in solids often increases linearly with the number of physical qubits, such as for transmon qubits in superconducting circuits or electron spin qubits in quantum dot arrays. While photonic circuits based on flying qubits do not suffer from decoherence or lack of potential scalability, they have encountered significant challenges to overcome photon loss in long delay circuits. Here, we propose an alternative approach that utilizes flying electronic wave packets propagating in solid-state quantum semiconductor circuits. Using a novel time-bin architecture for the electronic wave packets, hardware requirements are drastically reduced because qubits can be created on-demand and manipulated with a common hardware element, unlike the localized approach of wiring each qubit individually. The electronic Coulomb interaction enables reliable coupling and readout of qubits. Improving upon previous devices, we realize electronic interference at the level of a single quantized mode that can be used for manipulation of electronic wavepackets. This important landmark lays the foundation for fault-tolerant quantum computing with a compact and scalable architecture based on electron interferometry in semiconductors. △ Less

Submitted 21 October, 2024; originally announced October 2024.

High-fidelity spin readout via the double latching mechanism

Authors: Haruki Kiyama, Danny van Hien Hien, Arne Ludwig, Andreas D. Wieck, Akira Oiwa

Abstract: Projective measurement of single electron spins, or spin readout, is among the most fundamental technologies for spin-based quantum information processing. Implementing spin readout with both high-fidelity and scalability is indispensable for developing fault-tolerant quantum computers in large-scale spin-qubit arrays. To achieve high fidelity, a latching mechanism is useful. However, the fidelity… ▽ More Projective measurement of single electron spins, or spin readout, is among the most fundamental technologies for spin-based quantum information processing. Implementing spin readout with both high-fidelity and scalability is indispensable for developing fault-tolerant quantum computers in large-scale spin-qubit arrays. To achieve high fidelity, a latching mechanism is useful. However, the fidelity can be decreased by spin relaxation and charge state leakage, and the scalability is currently challenging. Here, we propose and demonstrate a double-latching high-fidelity spin readout scheme, which suppresses errors via an additional latching process. We experimentally show that the double-latching mechanism provides significantly higher fidelity than the conventional latching mechanism and estimate a potential spin readout fidelity of 99.94% using highly spin-dependent tunnel rates. Due to isolation from error-inducing processes, the double-latching mechanism combined with scalable charge readout is expected to be useful for large-scale spin-qubit arrays while maintaining high fidelity. △ Less

Submitted 4 October, 2024; originally announced October 2024.

Comments: 17 pages, 5 figures

Journal ref: npj Quantum Information 10, 95 (2024)

Evidence of Coulomb liquid phase in few-electron droplets

Authors: Jashwanth Shaju, Elina Pavlovska, Ralfs Suba, Junliang Wang, Seddik Ouacel, Thomas Vasselon, Matteo Aluffi, Lucas Mazzella, Clément Geffroy, Arne Ludwig, Andreas D. Wieck, Matias Urdampilleta, Christopher Bäuerle, Vyacheslavs Kashcheyevs, Hermann Sellier

Abstract: Emergence of universal collective behaviour from interactions within a sufficiently large group of elementary constituents is a fundamental scientific paradigm. In physics, correlations in fluctuating microscopic observables can provide key information about collective states of matter such as deconfined quark-gluon plasma in heavy-ion collisions or expanding quantum degenerate gases. Mesoscopic c… ▽ More Emergence of universal collective behaviour from interactions within a sufficiently large group of elementary constituents is a fundamental scientific paradigm. In physics, correlations in fluctuating microscopic observables can provide key information about collective states of matter such as deconfined quark-gluon plasma in heavy-ion collisions or expanding quantum degenerate gases. Mesoscopic colliders, through shot-noise measurements, have provided smoking-gun evidence on the nature of exotic electronic excitations such as fractional charges, levitons and anyon statistics. Yet, bridging the gap between two-particle collisions and the emergence of collectivity as the number of interacting particles increases remains a challenging task at the microscopic level. Here we demonstrate all-body correlations in the partitioning of electron droplets containing up to N = 5 electrons, driven by a moving potential well through a Y-junction in a semiconductor device. Analyzing the partitioning data using high-order multivariate cumulants and finite-size scaling towards the thermodynamic limit reveals distinctive fingerprints of a strongly-correlated Coulomb liquid. These fingerprints agree well with a universal limit where the partitioning of a droplet is predicted by a single collective variable. Our electron-droplet collider provides critical insight into the interplay of confinement and interaction effects in small electron systems and highlights a new way to study engineered states of matter. △ Less

Submitted 12 January, 2025; v1 submitted 26 August, 2024; originally announced August 2024.

Journal ref: Nature 642, 928 (2025)

Electronic interferometry with ultrashort plasmonic pulses

Authors: Seddik Ouacel, Lucas Mazzella, Thomas Kloss, Matteo Aluffi, Thomas Vasselon, Hermann Edlbauer, Junliang Wang, Clement Geffroy, Jashwanth Shaju, Arne Ludwig, Andreas D. Wieck, Michihisa Yamamoto, David Pomaranski, Shintaro Takada, Nobu-Hisa Kaneko, Giorgos Georgiou, Xavier Waintal, Matias Urdampilleta, Hermann Sellier, Christopher Bäuerle

Abstract: Electronic flying qubits offer an interesting alternative to photonic qubits: electrons propagate slower, hence easier to control in real time, and Coulomb interaction enables direct entanglement between different qubits. Although their coherence time is limited, flying electrons in the form of picosecond plasmonic pulses could be competitive in terms of the number of achievable coherent operation… ▽ More Electronic flying qubits offer an interesting alternative to photonic qubits: electrons propagate slower, hence easier to control in real time, and Coulomb interaction enables direct entanglement between different qubits. Although their coherence time is limited, flying electrons in the form of picosecond plasmonic pulses could be competitive in terms of the number of achievable coherent operations. The key challenge in achieving this critical milestone is the development of a new technology capable of injecting 'on-demand' single-electron wavepackets into quantum devices, with temporal durations comparable to or shorter than the device dimensions. Here, we take a significant step towards achieving this regime in a quantum nanoelectronic system by injecting ultrashort single-electron plasmonic pulses into a 14-micrometer-long Mach-Zehnder interferometer. Our results establish that quantum coherence is robust under the on-demand injection of ultrashort plasmonic pulses, as evidenced by the observation of coherent oscillations in the single-electron regime. Building on this, our results demonstrate for the first time the existence of a new "non-adiabatic" regime that is prominent at high frequencies. This breakthrough highlights the potential of flying qubits as a promising alternative to localised qubit architectures, offering advantages such as a reduced hardware footprint, enhanced connectivity, and scalability for quantum information processing. △ Less

Submitted 5 January, 2025; v1 submitted 23 August, 2024; originally announced August 2024.

Electrical control of a Kondo spin screening cloud

Authors: Ngoc Han Tu, Donghoon Kim, Minsoo L. Kim, Jeongmin Shim, Ryo Ito, David Pomaranski, Ivan V. Borzenets, Arne Ludwig, Andreas D. Wieck, Heung-Sun Sim, Michihisa Yamamoto

Abstract: Quantitative analysis of quantum many-body systems, consisting of numerous itinerant electrons that interact with localized spins or electrons, is a long-standing issue. The Kondo cloud, a quantum many-body object of conduction electrons that screens a single localized spin, is the building block of such strongly correlated electronic systems. While quantitative analysis of the Kondo cloud associa… ▽ More Quantitative analysis of quantum many-body systems, consisting of numerous itinerant electrons that interact with localized spins or electrons, is a long-standing issue. The Kondo cloud, a quantum many-body object of conduction electrons that screens a single localized spin, is the building block of such strongly correlated electronic systems. While quantitative analysis of the Kondo cloud associated with a single magnetic impurity is well established for uniform conduction electrons, the fundamental properties of a deformed Kondo cloud influenced by conduction electrons with a modulated density of states remain unsolved. Here we report engineering of the Kondo cloud deformation by confining a part of the cloud into a quantum box called the Kondo box that mimics realistic material systems. We demonstrate quantitative control of the Kondo cloud by developing a way of tuning quantum interference in the box and monitoring the Kondo entanglement. The temperature dependence of the entanglement reveals counterintuitively that the cloud shape is altered mainly outside the box although the quantum interference in the box is tuned. Our work provides a way to simulate various strongly correlated systems by integrating the Kondo cloud, which is not possible in the current theoretical framework. △ Less

Submitted 25 March, 2025; v1 submitted 18 April, 2024; originally announced April 2024.

Lateral 2D superlattices in GaAs heterostructures with independent control of carrier density and modulation potential

Authors: D. Q. Wang, D. Reuter, A. D. Wieck, A. R. Hamilton, O. Klochan

Abstract: We present a new double-layer design for 2D surface superlattice systems in GaAs-AlGaAs heterostructures. Unlike previous studies, our device (1) uses an in-situ gate, which allows very short period superlattice in high mobility, shallow heterostructures; (2) enables independent control of the carrier density and the superlattice modulation potential amplitude over a wide range. We characterise th… ▽ More We present a new double-layer design for 2D surface superlattice systems in GaAs-AlGaAs heterostructures. Unlike previous studies, our device (1) uses an in-situ gate, which allows very short period superlattice in high mobility, shallow heterostructures; (2) enables independent control of the carrier density and the superlattice modulation potential amplitude over a wide range. We characterise this device design using low-temperature magneto-transport measurements and show that the fabrication process caused minimal damage to the system. We demonstrate the tuning of potential modulation from weak (much smaller than Fermi energy) to strong (larger than the Fermi energy) regimes. △ Less

Submitted 11 March, 2024; originally announced March 2024.

Comments: 4 pages, 3 figures

Dynamics of quantum cellular automata electron transition in triple quantum dots

Authors: Takumi Aizawa, Motoya Shinozaki, Yoshihiro Fujiwara, Takeshi Kumasaka, Wataru Izumida, Arne Ludwig, Andreas D. Wieck, Tomohiro Otsuka

Abstract: The quantum cellular automata (QCA) effect is a transition in which multiple electron move coordinately by Coulomb interactions and observed in multiple quantum dots. This effect will be useful for realizing and improving quantum cellular automata and information transfer using multiple electron transfer. In this paper, we investigate the real-time dynamics of the QCA charge transitions in a tripl… ▽ More The quantum cellular automata (QCA) effect is a transition in which multiple electron move coordinately by Coulomb interactions and observed in multiple quantum dots. This effect will be useful for realizing and improving quantum cellular automata and information transfer using multiple electron transfer. In this paper, we investigate the real-time dynamics of the QCA charge transitions in a triple quantum dot by using fast charge-state readout realized by rf reflectometry. We observe real-time charge transitions and analyze the tunneling rate comparing with the first-order tunneling processes. We also measure the gate voltage dependence of the QCA transition and show that it can be controlled by the voltage. △ Less

Submitted 10 March, 2024; originally announced March 2024.

Comments: 11pages, 4 figures

Journal ref: Physical Review B 110, 235421 (2024)

Accelerated adiabatic passage of a single electron spin qubit in quantum dots

Authors: Xiao-Fei Liu, Yuta Matsumoto, Takafumi Fujita, Arne Ludwig, Andreas D. Wieck, Akira Oiwa

Abstract: Adiabatic processes can keep the quantum system in its instantaneous eigenstate, which is robust to noises and dissipation. However, it is limited by sufficiently slow evolution. Here, we experimentally demonstrate the transitionless quantum driving (TLQD) of the shortcuts to adiabaticity in gate-defined semiconductor quantum dots (QDs) to greatly accelerate the conventional adiabatic passage for… ▽ More Adiabatic processes can keep the quantum system in its instantaneous eigenstate, which is robust to noises and dissipation. However, it is limited by sufficiently slow evolution. Here, we experimentally demonstrate the transitionless quantum driving (TLQD) of the shortcuts to adiabaticity in gate-defined semiconductor quantum dots (QDs) to greatly accelerate the conventional adiabatic passage for the first time. For a given efficiency of quantum state transfer, the acceleration can be more than twofold. The dynamic properties also prove that the TLQD can guarantee fast and high-fidelity quantum state transfer. In order to compensate for the diabatic errors caused by dephasing noises, the modified TLQD is proposed and demonstrated in experiment by enlarging the width of the counter-diabatic drivings. The benchmarking shows that the state transfer fidelity of 97.8% can be achieved. This work will greatly promote researches and applications about quantum simulations and adiabatic quantum computation based on the gate-defined QDs. △ Less

Submitted 28 January, 2024; v1 submitted 20 December, 2023; originally announced December 2023.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. Lett. 132, 027002 (2024)

On-Demand Single-Electron Source via Single-Cycle Acoustic Pulses

Authors: Shunsuke Ota, Junliang Wang, Hermann Edlbauer, Yuma Okazaki, Shuji Nakamura, Takehiko Oe, Arne Ludwig, Andreas D. Wieck, Hermann Sellier, Christopher Bäuerle, Nobu-Hisa Kaneko, Tetsuo Kodera, Shintaro Takada

Abstract: Surface acoustic waves (SAWs) are a reliable solution to transport single electrons with precision in piezoelectric semiconductor devices. Recently, highly efficient single-electron transport with a strongly compressed single-cycle acoustic pulse has been demonstrated. This approach, however, requires surface gates constituting the quantum dots, their wiring, and multiple gate movements to load an… ▽ More Surface acoustic waves (SAWs) are a reliable solution to transport single electrons with precision in piezoelectric semiconductor devices. Recently, highly efficient single-electron transport with a strongly compressed single-cycle acoustic pulse has been demonstrated. This approach, however, requires surface gates constituting the quantum dots, their wiring, and multiple gate movements to load and unload the electrons, which is very time-consuming. Here, on the contrary, we employ such a single-cycle acoustic pulse in a much simpler way - without any quantum dot at the entrance or exit of a transport channel - to perform single-electron transport between distant electron reservoirs. We observe the transport of a solitary electron in a single-cycle acoustic pulse via the appearance of the quantized acousto-electric current. The simplicity of our approach allows for on-demand electron emission with arbitrary delays on a ns time scale. We anticipate that enhanced synthesis of the SAWs will facilitate electron-quantum-optics experiments with multiple electron flying qubits. △ Less

Submitted 30 November, 2023; originally announced December 2023.

Journal ref: Phys. Rev. Applied 21, 024034 (2024)

Electrical resistance associated with the scattering of optically oriented electrons in n-GaAs

Authors: M. D. Ragoza, N. V. Kozyrev, S. V. Nekrasov, B. R. Namozov, Yu. G. Kusrayev, N. Bart, A. Ludwig, A. D. Wieck

Abstract: In a bulk GaAs crystal, an unusual magnetoresistance effect, which takes place when a spin-polarized current flows through the sample, was detected. Under conditions of optical pumping of electron spins, an external magnetic field directed along the electric current and perpendicular to the oriented spins decreases the resistance of the material. The phenomenon is due to the spin-dependent scatter… ▽ More In a bulk GaAs crystal, an unusual magnetoresistance effect, which takes place when a spin-polarized current flows through the sample, was detected. Under conditions of optical pumping of electron spins, an external magnetic field directed along the electric current and perpendicular to the oriented spins decreases the resistance of the material. The phenomenon is due to the spin-dependent scattering of electrons by neutral donors. It was found that the sign of the magnetoresistance does not depend on the sign of the exciting light circular polarization, the effect is even with respect to the sign of the spin polarization of the carriers, which indicates a correlation between the spins of optically oriented free electrons and electrons localized on donors. △ Less

Submitted 30 November, 2023; originally announced November 2023.

Comments: 9 pages, 4 figures

A quantum dot coupled to a suspended-beam mechanical resonator: from the unresolved- to the resolved-sideband regime

Authors: Clemens Spinnler, Giang N. Nguyen, Ying Wang, Marcel Erbe, Alisa Javadi, Liang Zhai, Sven Scholz, Andreas D. Wieck, Arne Ludwig, Peter Lodahl, Leonardo Midolo, Richard J. Warburton

Abstract: We present experiments in which self-assembled InAs quantum dots are coupled to a thin, suspended-beam GaAs resonator. The quantum dots are driven resonantly and the resonance fluorescence is detected. The narrow quantum-dot linewidths, just a factor of three larger than the transform limit, result in a high sensitivity to the mechanical motion. We show that one quantum dot couples to eight mechan… ▽ More We present experiments in which self-assembled InAs quantum dots are coupled to a thin, suspended-beam GaAs resonator. The quantum dots are driven resonantly and the resonance fluorescence is detected. The narrow quantum-dot linewidths, just a factor of three larger than the transform limit, result in a high sensitivity to the mechanical motion. We show that one quantum dot couples to eight mechanical modes spanning a frequency range from $30$ to $600~\mathrm{MHz}$: one quantum dot provides an extensive characterisation of the mechanical resonator. The coupling spans the unresolved-sideband to the resolved-sideband regimes. Finally, we present the first detection of thermally-driven phonon sidebands (at $4.2~\mathrm{K}$) in the resonance-fluoresence spectrum. △ Less

Submitted 9 November, 2023; originally announced November 2023.

A single-photon emitter coupled to a phononic-crystal resonator in the resolved-sideband regime

Authors: Clemens Spinnler, Giang N. Nguyen, Ying Wang, Liang Zhai, Alisa Javadi, Marcel Erbe, Sven Scholz, Andreas D. Wieck, Arne Ludwig, Peter Lodahl, Leonardo Midolo, Richard J. Warburton

Abstract: A promising route towards the heralded creation and annihilation of single-phonons is to couple a single-photon emitter to a mechanical resonator. The challenge lies in reaching the resolved-sideband regime with a large coupling rate and a high mechanical quality factor. We achieve all of this by coupling self-assembled InAs quantum dots to a small-mode-volume phononic-crystal resonator with mecha… ▽ More A promising route towards the heralded creation and annihilation of single-phonons is to couple a single-photon emitter to a mechanical resonator. The challenge lies in reaching the resolved-sideband regime with a large coupling rate and a high mechanical quality factor. We achieve all of this by coupling self-assembled InAs quantum dots to a small-mode-volume phononic-crystal resonator with mechanical frequency $Ω_\mathrm{m}/2π= 1.466~\mathrm{GHz}$ and quality factor $Q_\mathrm{m} = 2.1\times10^3$. Thanks to the high coupling rate of $g_\mathrm{ep}/2π= 2.9~\mathrm{MHz}$, and by exploiting a matching condition between the effective Rabi and mechanical frequencies, we are able to observe the interaction between the two systems. Our results represent a major step towards quantum control of the mechanical resonator via a single-photon emitter. △ Less

Submitted 9 November, 2023; originally announced November 2023.

The interplay between electron tunneling and Auger emission in a single quantum emitter weakly coupled to an electron reservoir

Authors: Marcel Zöllner, Hendrik Mannel, Fabio Rimek, Britta Maib, Nico Schwarz, Andreas D. Wieck, Arne Ludwig, Axel Lorke, Martin Geller

Abstract: In quantum dots (QDs) the Auger recombination is a non-radiative scattering process in which the optical transition energy of a charged exciton (trion) is transferred to an additional electron leaving the dot. Electron tunneling from a reservoir is the competing process that replenishes the QD with an electron again. Here, we study the dependence of the tunneling and Auger recombintaion rate on th… ▽ More In quantum dots (QDs) the Auger recombination is a non-radiative scattering process in which the optical transition energy of a charged exciton (trion) is transferred to an additional electron leaving the dot. Electron tunneling from a reservoir is the competing process that replenishes the QD with an electron again. Here, we study the dependence of the tunneling and Auger recombintaion rate on the applied electric field using high-resolution time-resolved resonance fluorescence (RF) measurements. With the given p-i-n diode structure and a tunnel barrier between the electron reservoir and the QD of $45\,$nm, we measured a tunneling rate into the QD in the order of ms$^{-1}$. This rate shows a strong decrease by almost an order of magnitude for decreasing electric field, while the Auger emission rate decreases by a factor of five in the same voltage range. Furthermore, we study in detail the influence of the Auger recombination and the tunneling rate from the charge reservoir into the QD on the intensity and linewidth of the trion transition. Besides the well-known quenching of the trion transition, we observe in our time-resolved RF measurements a strong influence of the tunneling rate on the observed linewidth. The steady-state RF measurement yields a broadened trion transition of about $1.5\,$GHz for an Auger emission rate of the same order as the electron tunneling rate. In a non-equilibrium measurement, the Auger recombination can be suppressed, and a more than four times smaller linewidth of $340\,$MHz ($1.4\,$$μ$eV) is measured. △ Less

Submitted 20 October, 2023; originally announced October 2023.

Enhanced Electron Spin Coherence in a GaAs Quantum Emitter

Authors: Giang N. Nguyen, Clemens Spinnler, Mark R. Hogg, Liang Zhai, Alisa Javadi, Carolin A. Schrader, Marcel Erbe, Marcus Wyss, Julian Ritzmann, Hans-Georg Babin, Andreas D. Wieck, Arne Ludwig, Richard J. Warburton

Abstract: A spin-photon interface should operate with both coherent photons and a coherent spin to enable cluster-state generation and entanglement distribution. In high-quality devices, self-assembled GaAs quantum dots are near-perfect emitters of on-demand coherent photons. However, the spin rapidly decoheres via the magnetic noise arising from the host nuclei. Here, we address this drawback by implementi… ▽ More A spin-photon interface should operate with both coherent photons and a coherent spin to enable cluster-state generation and entanglement distribution. In high-quality devices, self-assembled GaAs quantum dots are near-perfect emitters of on-demand coherent photons. However, the spin rapidly decoheres via the magnetic noise arising from the host nuclei. Here, we address this drawback by implementing an all-optical nuclear-spin cooling scheme on a GaAs quantum dot. The electron-spin coherence time increases 156-fold from $T_2^*$ = 3.9 ns to 0.608 $μ$s. The cooling scheme depends on a non-collinear term in the hyperfine interaction. The results show that such a term is present even though the strain is low and no external stress is applied. Our work highlights the potential of optically-active GaAs quantum dots as fast, highly coherent spin-photon interfaces. △ Less

Submitted 5 July, 2023; originally announced July 2023.

Controlled Coherent Coupling in a Quantum Dot Molecule Revealed by Ultrafast Four-Wave Mixing Spectroscopy

Authors: Daniel Wigger, Johannes Schall, Marielle Deconinck, Nikolai Bart, Paweł Mrowiński, Mateusz Krzykowski, Krzysztof Gawarecki, Martin von Helversen, Ronny Schmidt, Lucas Bremer, Frederik Bopp, Dirk Reuter, Andreas D. Wieck, Sven Rodt, Julien Renard, Gilles Nogues, Arne Ludwig, Paweł Machnikowski, Jonathan J. Finley, Stephan Reitzenstein, Jacek Kasprzak

Abstract: Semiconductor quantum dot molecules are considered as promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical s… ▽ More Semiconductor quantum dot molecules are considered as promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical spectroscopy, we here report on the first demonstration of a coherently controlled inter-dot tunnel-coupling focusing on the quantum coherence of the optically active trion transitions. We employ ultrafast four-wave mixing spectroscopy to resonantly generate a quantum coherence in one trion complex, transfer it to and probe it in another trion configuration. With the help of theoretical modelling on different levels of complexity we give an instructive explanation of the underlying coupling mechanism and dynamical processes. △ Less

Submitted 20 April, 2023; originally announced April 2023.

Journal ref: ACS Photonics 10 (5), 1504-1511 (2023)

Tailoring potentials by simulation-aided design of gate layouts for spin qubit applications

Authors: Inga Seidler, Malte Neul, Eugen Kammerloher, Matthias Künne, Andreas Schmidbauer, Laura Diebel, Arne Ludwig, Julian Ritzmann, Andreas D. Wieck, Dominique Bougeard, Hendrik Bluhm, Lars R. Schreiber

Abstract: Gate-layouts of spin qubit devices are commonly adapted from previous successful devices. As qubit numbers and the device complexity increase, modelling new device layouts and optimizing for yield and performance becomes necessary. Simulation tools from advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach for elect… ▽ More Gate-layouts of spin qubit devices are commonly adapted from previous successful devices. As qubit numbers and the device complexity increase, modelling new device layouts and optimizing for yield and performance becomes necessary. Simulation tools from advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach for electrostatically modelling new spin qubit device layouts, considering gate voltages, heterostructures, reservoirs and an applied source-drain bias. Exemplified by a specific potential, we study the influence of each parameter. We verify our model by indirectly probing the potential landscape of two design implementations through transport measurements. We use the simulations to identify critical design areas and optimize for robustness with regard to influence and resolution limits of the fabrication process. △ Less

Submitted 23 March, 2023; originally announced March 2023.

Comments: 10 pages, 6 figures

Journal ref: Phys. Rev. Applied 20, 044058 (2023)

Magnetic tuning of the tunnel coupling in an optically active quantum dot molecule

Authors: Frederik Bopp, Charlotte Cullip, Christopher Thalacker, Michelle Lienhart, Johannes Schall, Nikolai Bart, Friedrich Sbresny, Katarina Boos, Sven Rodt, Dirk Reuter, Arne Ludwig, Andreas D. Wieck, Stephan Reitzenstein, Filippo Troiani, Guido Goldoni, Elisa Molinari, Kai Müller, Jonathan J. Finley

Abstract: Self-assembled optically active quantum dot molecules (QDMs) allow the creation of protected qubits via singlet-triplet spin states. The qubit energy splitting of these states is defined by the tunnel coupling strength and is, therefore, determined by the potential landscape and thus fixed during growth. Applying an in-plane magnetic field increases the confinement of the hybridized wave functions… ▽ More Self-assembled optically active quantum dot molecules (QDMs) allow the creation of protected qubits via singlet-triplet spin states. The qubit energy splitting of these states is defined by the tunnel coupling strength and is, therefore, determined by the potential landscape and thus fixed during growth. Applying an in-plane magnetic field increases the confinement of the hybridized wave functions within the quantum dots, leading to a decrease of the tunnel coupling strength. We achieve a tuning of the coupling strength by $(53.4\pm1.7)$ %. The ability to fine-tune this coupling is essential for quantum network and computing applications that require quantum systems with near identical performance. △ Less

Submitted 22 March, 2023; originally announced March 2023.

Comments: Main: 6 pages, 3 figures, Supplemental: 4 pages, 2 figures

Independent electrical control of two quantum dots coupled through a photonic-crystal waveguide

Authors: Xiao-Liu Chu, Camille Papon, Nikolai Bart, Andreas D. Wieck, Arne Ludwig, Leonardo Midolo, Nir Rotenberg, Peter Lodahl

Abstract: Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled… ▽ More Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned 16$μ$m away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveals signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multi-emitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters. △ Less

Submitted 3 March, 2023; v1 submitted 1 March, 2023; originally announced March 2023.

Comments: 6 pages, 3 figures

Coherent driving of direct and indirect excitons in a quantum dot molecule

Authors: Frederik Bopp, Johannes Schall, Nikolai Bart, Florian Vogl, Charlotte Cullip, Friedrich Sbresny, Katarina Boos, Christopher Thalacker, Michelle Lienhart, Sven Rodt, Dirk Reuter, Arne Ludwig, Andreas Wieck, Stephan Reitzenstein, Kai Müller, Jonathan J. Finley

Abstract: Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic generation of one- and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybri… ▽ More Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic generation of one- and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybridized electron wave function. An off-resonant detection technique based on phonon-mediated state transfer allows for spectrally filtered detection under resonant excitation. Applying a gate voltage to the QDM-device enables a continuous transition between direct and indirect excitons and, thereby, control of the overlap of the electron and hole wave function. This does not only vary the Rabi frequency of the investigated transition by a factor of $\approx3$, but also allows to optimize graph state generation in terms of optical pulse power and reduction of radiative lifetimes. △ Less

Submitted 31 January, 2023; originally announced January 2023.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 107, 165426 (2023)

Visual explanations of machine learning model estimating charge states in quantum dots

Authors: Yui Muto, Takumi Nakaso, Motoya Shinozaki, Takumi Aizawa, Takahito Kitada, Takashi Nakajima, Matthieu R. Delbecq, Jun Yoneda, Kenta Takeda, Akito Noiri, Arne Ludwig, Andreas D. Wieck, Seigo Tarucha, Atsunori Kanemura, Motoki Shiga, Tomohiro Otsuka

Abstract: Charge state recognition in quantum dot devices is important in the preparation of quantum bits for quantum information processing. Toward auto-tuning of larger-scale quantum devices, automatic charge state recognition by machine learning has been demonstrated. For further development of this technology, an understanding of the operation of the machine learning model, which is usually a black box,… ▽ More Charge state recognition in quantum dot devices is important in the preparation of quantum bits for quantum information processing. Toward auto-tuning of larger-scale quantum devices, automatic charge state recognition by machine learning has been demonstrated. For further development of this technology, an understanding of the operation of the machine learning model, which is usually a black box, will be useful. In this study, we analyze the explainability of the machine learning model estimating charge states in quantum dots by gradient-weighted class activation mapping, which identified class-discriminative regions for the predictions. The model predicts the state based on the change transition lines, indicating that human-like recognition is realized. We also demonstrate improvements of the model by utilizing feedback from the mapping results. Due to the simplicity of our simulation and pre-processing methods, our approach offers scalability without significant additional simulation costs, demonstrating its suitability for future quantum dot system expansions. △ Less

Submitted 27 December, 2023; v1 submitted 26 October, 2022; originally announced October 2022.

Comments: 17 pages, 4 figures

Journal ref: APL Machine Learning 2, 026110 (2024)

Cavity-enhanced single-shot readout of a quantum dot spin within 3 nanoseconds

Authors: Nadia Olympia Antoniadis, Mark Richard Hogg, Willy Frederik Stehl, Alisa Javadi, Natasha Tomm, Rüdiger Schott, Sascha René Valentin, Andreas Dirk Wieck, Arne Ludwig, Richard John Warburton

Abstract: Rapid, high-fidelity single-shot readout of quantum states is a ubiquitous requirement in quantum information technologies, playing a crucial role in quantum computation, quantum error correction, and fundamental tests of non-locality. Readout of the spin state of an optically active emitter can be achieved by driving a spin-preserving optical transition and detecting the emitted photons. The spee… ▽ More Rapid, high-fidelity single-shot readout of quantum states is a ubiquitous requirement in quantum information technologies, playing a crucial role in quantum computation, quantum error correction, and fundamental tests of non-locality. Readout of the spin state of an optically active emitter can be achieved by driving a spin-preserving optical transition and detecting the emitted photons. The speed and fidelity of this approach is typically limited by a combination of low photon collection rates and measurement back-action. Here, we demonstrate single-shot optical readout of a semiconductor quantum dot spin state, achieving a readout time of only a few nanoseconds. In our approach, gated semiconductor quantum dots are embedded in an open microcavity. The Purcell enhancement generated by the microcavity increases the photon creation rate from one spin state but not from the other, as well as efficiently channelling the photons into a well-defined detection mode. We achieve single-shot readout of an electron spin state in 3 nanoseconds with a fidelity of (95.2$\pm$0.7)%, and observe quantum jumps using repeated single-shot measurements. Owing to the speed of our readout, errors resulting from measurement-induced back-action have minimal impact. Our work reduces the spin readout-time to values well below both the achievable spin relaxation and dephasing times in semiconductor quantum dots, opening up new possibilities for their use in quantum technologies. △ Less

Submitted 25 October, 2022; originally announced October 2022.

In-flight detection of few electrons using a singlet-triplet spin qubit

Authors: Vivien Thiney, Pierre-André Mortemousque, Konstantinos Rogdakis, Romain Thalineau, Arne Ludwig, Andreas D. Wieck, Matias Urdampilleta, Christopher Bäuerle, Tristan Meunier

Abstract: We investigate experimentally the capacitive coupling between a two-electron singlet-triplet spin qubit and flying electrons propagating in quantum Hall edge channels. After calibration of the spin qubit detector, we assess its charge sensibility and demonstrate experimentally the detection of less than five flying electrons with average measurement. This experiment demonstrates that the spin qubi… ▽ More We investigate experimentally the capacitive coupling between a two-electron singlet-triplet spin qubit and flying electrons propagating in quantum Hall edge channels. After calibration of the spin qubit detector, we assess its charge sensibility and demonstrate experimentally the detection of less than five flying electrons with average measurement. This experiment demonstrates that the spin qubit is an ultrasensitive and fast charge detector with the perspective of a future single shot-detection of a single flying electron. This work opens the route toward quantum electron optics experiments at the single electron level in semiconductor circuits. △ Less

Submitted 22 November, 2022; v1 submitted 17 October, 2022; originally announced October 2022.

Coulomb-mediated antibunching of an electron pair surfing on sound

Authors: Junliang Wang, Hermann Edlbauer, Aymeric Richard, Shunsuke Ota, Wanki Park, Jeongmin Shim, Arne Ludwig, Andreas Wieck, Heung-Sun Sim, Matias Urdampilleta, Tristan Meunier, Tetsuo Kodera, Nobu-Hisa Kaneko, Hermann Sellier, Xavier Waintal, Shintaro Takada, Christopher Bäuerle

Abstract: Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the contr… ▽ More Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the controlled interaction of flying electrons at the single particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realise a controlled-phase gate for in-flight quantum manipulations. △ Less

Submitted 7 October, 2022; originally announced October 2022.

Journal ref: Nature Nanotechnology 18, 721-726 (2023)

Complete readout of two-electron spin states in a double quantum dot

Authors: Martin Nurizzo, Baptiste Jadot, Pierre-André Mortemousque, Vivien Thiney, Emmanuel Chanrion, David Niegemann, Matthieu Dartiailh, Arne Ludwig, Andreas D. Wieck, Christopher Bäuerle, Matias Urdampilleta, Tristan Meunier

Abstract: We propose and demonstrate complete spin state readout of a two-electron system in a double quantum dot probed by an electrometer. The protocol is based on repetitive single shot measurements using Pauli spin blockade and our ability to tune on fast timescales the detuning and the interdot tunnel coupling between the GHz and sub-Hz regime. A sequence of three distinct manipulations and measurement… ▽ More We propose and demonstrate complete spin state readout of a two-electron system in a double quantum dot probed by an electrometer. The protocol is based on repetitive single shot measurements using Pauli spin blockade and our ability to tune on fast timescales the detuning and the interdot tunnel coupling between the GHz and sub-Hz regime. A sequence of three distinct manipulations and measurements allows establishing if the spins are in S, Tzero, Tplus or Tminus state. This work points at a procedure to reduce the overhead for spin readout, an important challenge for scaling up spin qubit platforms. △ Less

Submitted 1 September, 2022; originally announced September 2022.

Comments: 11 pages, 7 figures

Generation of a single-cycle acoustic pulse: a scalable solution for transport in single-electron circuits

Authors: Junliang Wang, Shunsuke Ota, Hermann Edlbauer, Baptiste Jadot, Pierre-André Mortemousque, Aymeric Richard, Yuma Okazaki, Shuji Nakamura, Arne Ludwig, Andreas D. Wieck, Matias Urdampilleta, Tristan Meunier, Tetsuo Kodera, Nobu-Hisa Kaneko, Shintaro Takada, Christopher Bäuerle

Abstract: The synthesis of single-cycle, compressed optical and microwave pulses sparked novel areas of fundamental research. In the field of acoustics, however, such a generation has not been introduced yet. For numerous applications, the large spatial extent of surface acoustic waves (SAW) causes unwanted perturbations and limits the accuracy of physical manipulations. Particularly, this restriction appli… ▽ More The synthesis of single-cycle, compressed optical and microwave pulses sparked novel areas of fundamental research. In the field of acoustics, however, such a generation has not been introduced yet. For numerous applications, the large spatial extent of surface acoustic waves (SAW) causes unwanted perturbations and limits the accuracy of physical manipulations. Particularly, this restriction applies to SAW-driven quantum experiments with single flying electrons, where extra modulation renders the exact position of the transported electron ambiguous and leads to undesired spin mixing. Here, we address this challenge by demonstrating single-shot chirp synthesis of a strongly compressed acoustic pulse. Employing this solitary SAW pulse to transport a single electron between distant quantum dots with an efficiency exceeding 99%, we show that chirp synthesis is competitive with regular transduction approaches. Performing a time-resolved investigation of the SAW-driven sending process, we outline the potential of the chirped SAW pulse to synchronize single-electron transport from many quantum-dot sources. By superimposing multiple pulses, we further point out the capability of chirp synthesis to generate arbitrary acoustic waveforms tailorable to a variety of (opto)nanomechanical applications. Our results shift the paradigm of compressed pulses to the field of acoustic phonons and pave the way for a SAW-driven platform of single-electron transport that is precise, synchronized, and scalable. △ Less

Submitted 31 July, 2022; originally announced August 2022.

Comments: To be published in Physical Review X

Journal ref: Phys. Rev. X 12, 031035 (2022)

Controlled quantum dot array segmentation via a highly tunable interdot tunnel coupling

Authors: Martin Nurizzo, Baptiste Jadot, Pierre-André Mortemousque, Vivien Thiney, Emmanuel Chanrion, Matthieu Dartiailh, Arne Ludwig, Andreas D. Wieck, Christopher Bäuerle, Matias Urdampilleta, Tristan Meunier

Abstract: Recent demonstrations using electron spins stored in quantum dots array as qubits are promising for developing a scalable quantum computing platform. An ongoing effort is therefore aiming at the precise control of the quantum dots parameters in larger and larger arrays which represents a complex challenge. Partitioning of the system with the help of the inter-dot tunnel barriers can lead to a simp… ▽ More Recent demonstrations using electron spins stored in quantum dots array as qubits are promising for developing a scalable quantum computing platform. An ongoing effort is therefore aiming at the precise control of the quantum dots parameters in larger and larger arrays which represents a complex challenge. Partitioning of the system with the help of the inter-dot tunnel barriers can lead to a simplification for tuning and offers a protection against unwanted charge displacement. In a triple quantum dot system, we demonstrate a nanosecond control of the inter-dot tunnel rate permitting to reach the two extreme regimes, large GHz tunnel coupling and sub-Hz isolation between adjacent dots. We use this novel development to isolate a sub part of the array while performing charge displacement and readout in the rest of the system. The degree of control over the tunnel coupling achieved in a unit cell should motivate future protocol development for tuning, manipulation and readout including this capability. △ Less

Submitted 19 July, 2022; originally announced July 2022.

Comments: 10 pages, 8 figures

Semiconductor membranes for electrostatic exciton trapping in optically addressable quantum transport devices

Authors: Thomas Descamps, Feng Liu, Sebastian Kindel, René Otten, Tobias Hangleiter, Chao Zhao, Mihail Ion Lepsa, Julian Ritzmann, Arne Ludwig, Andreas D. Wieck, Beata E. Kardynał, Hendrik Bluhm

Abstract: Combining the capabilities of gate defined quantum transport devices in GaAs-based heterostructures and of optically addressed self-assembled quantum dots could open broad perspectives for new devices and functionalities. For example, interfacing stationary solid-state qubits with photonic quantum states would open a new pathway towards the realization of a quantum network with extended quantum pr… ▽ More Combining the capabilities of gate defined quantum transport devices in GaAs-based heterostructures and of optically addressed self-assembled quantum dots could open broad perspectives for new devices and functionalities. For example, interfacing stationary solid-state qubits with photonic quantum states would open a new pathway towards the realization of a quantum network with extended quantum processing capacity in each node. While gated devices allow very flexible confinement of electrons or holes, the confinement of excitons without some element of self-assembly is much harder. To address this limitation, we introduce a technique to realize exciton traps in quantum wells via local electric fields by thinning a heterostructure down to a 220 nm thick membrane. We show that mobilities over $1 \times 10^{6}$ cm$^{2}$V$^{-1}$s$^{-1}$ can be retained and that quantum point contacts and Coulomb oscillations can be observed on this structure, which implies that the thinning does not compromise the heterostructure quality. Furthermore, the local lowering of the exciton energy via the quantum-confined Stark effect is confirmed, thus forming exciton traps. These results lay the technological foundations for devices like single photon sources, spin photon interfaces and eventually quantum network nodes in GaAs quantum wells, realized entirely with a top-down fabrication process. △ Less

Submitted 9 November, 2022; v1 submitted 15 July, 2022; originally announced July 2022.

Comments: v2: added missing acknowledgement. v3: fixed typos in acknolwedgement

Quantum dot molecule devices with optical control of charge status and electronic control of coupling

Authors: Frederik Bopp, Jonathan Rojas, Natalia Revenga, Hubert Riedl, Friedrich Sbresny, Katarina Boos, Tobias Simmet, Arash Ahmadi, David Gershoni, Jacek Kasprzak, Arne Ludwig, Stephan Reitzenstein, Andreas Wieck, Dirk Reuter, Kai Muller, Jonathan J. Finley

Abstract: Tunnel-coupled pairs of optically active quantum dots - quantum dot molecules (QDMs) - offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet-triplet ($S-T_0$) qubits having extended coherence times. The $S-T_0$ basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single ga… ▽ More Tunnel-coupled pairs of optically active quantum dots - quantum dot molecules (QDMs) - offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet-triplet ($S-T_0$) qubits having extended coherence times. The $S-T_0$ basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter-dot orbital couplings, operation of the $S-T_0$ qubits under optimal conditions remains challenging. Here, we present an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand. We perform a four-phase optical and electric field control sequence that facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter-dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. We achieve one- and two-hole charging efficiencies of 93.5 $\pm$ 0.8 % and 80.5 $\pm$ 1.3 %, respectively. Combining efficient charge state preparation and precise setting of inter-dot coupling allows control of few-spin qubits, as would be required for on-demand generation of two-dimensional photonic cluster states or quantum transduction between microwaves and photons. △ Less

Submitted 20 May, 2022; originally announced May 2022.

Comments: 4 figures

Journal ref: Adv Quantum Technol. 2022, 5, 2200049

Extending the time of coherent optical response in ensemble of singly-charged InGaAs quantum dots

Authors: A. N. Kosarev, A. V. Trifonov, I. A. Yugova, I. I. Yanibekov, S. V. Poltavtsev, A. N. Kamenskii, S. E. Scholz, C. Sgroi, A. Ludwig, A. D. Wieck, D. R. Yakovlev, M. Bayer, I. A. Akimov

Abstract: The ability to extend the time scale of the coherent optical response from large ensembles of quantum emitters is highly appealing for applications in quantum information devices. In semiconductor nanostructures, spin degrees of freedom can be used as auxiliary, powerful tools to modify the coherent optical dynamics. Here, we apply this approach to negatively charged (In,Ga)As/GaAs self-assembled… ▽ More The ability to extend the time scale of the coherent optical response from large ensembles of quantum emitters is highly appealing for applications in quantum information devices. In semiconductor nanostructures, spin degrees of freedom can be used as auxiliary, powerful tools to modify the coherent optical dynamics. Here, we apply this approach to negatively charged (In,Ga)As/GaAs self-assembled quantum dots which are considered as excellent quantum emitters with robust optical coherence and high bandwidth. We study 3-pulse spin-dependent photon echoes subject to moderate transverse magnetic fields up to 1 T. We demonstrate that the timescale of coherent optical response can be extended by at least an order of magnitude by the field. Without magnetic field, the photon echo decays with $T_ 2$ = 0.45 ns which is determined by the radiative lifetime of trions $T_1$ = 0.27 ns. In the presence of the transverse magnetic field, the decay of the photon echo signal is given by spin dephasing time of the ensemble of resident electrons $T_{2,e}$ ~ 4 ns. We demonstrate that the non-zero transverse g-factor of the heavy holes in the trion state plays a crucial role in the temporal evolution and magnetic field dependence of the long-lived photon echo signal. △ Less

Submitted 23 January, 2022; originally announced January 2022.

Comments: 9 pages, 4 figures, Supplementary material

Post-processing of real-time quantum event measurements for an optimal bandwidth

Authors: Jens Kerski, Hendrik Mannel, Pia Lochner, Eric Kleinherbers, Annika Kurzmann, Arne Ludwig, Andreas D. Wieck, Jürgen König, Axel Lorke, Martin Geller

Abstract: Single electron tunneling and its transport statistics have been studied for some time using high precision charge detectors. However, this type of detection requires advanced lithography, optimized material systems and low temperatures (mK). A promising alternative, recently demonstrated, is to exploit an optical transition that is turned on or off when a tunnel event occurs. High bandwidths shou… ▽ More Single electron tunneling and its transport statistics have been studied for some time using high precision charge detectors. However, this type of detection requires advanced lithography, optimized material systems and low temperatures (mK). A promising alternative, recently demonstrated, is to exploit an optical transition that is turned on or off when a tunnel event occurs. High bandwidths should be achievable with this approach, although this has not been adequately investigated so far. We have studied low temperature resonance fluorescence from a self-assembled quantum dot embedded in a diode structure. We detect single photons from the dot in real time and evaluate the recorded data only after the experiment, using post-processing to obtain the random telegraph signal of the electron transport. This is a significant difference from commonly used charge detectors and allows us to determine the optimal time resolution for analyzing our data. We show how this post-processing affects both the determination of tunneling rates using waiting-time distributions and statistical analysis using full-counting statistics. We also demonstrate, as an example, that we can analyze our data with bandwidths as high as 350 kHz. Using a simple model, we discuss the limiting factors for achieving the optimal bandwidth and propose how a time resolution of more than 1 MHz could be achieved. △ Less

Submitted 14 December, 2021; originally announced December 2021.

Comments: 8 pages, 4 figures

Electron g-factor determined for quantum dot circuit fabricated from (110)-oriented GaAs quantum well

Authors: T. Nakagawa, S. Lamoureux, T. Fujita, J. Ritzmann, A. Ludwig, A. D. Wieck, A. Oiwa, M. Korkusinski, A. Sachrajda, D. G. Austing, L. Gaudreau

Abstract: The choice of substrate orientation for semiconductor quantum dot circuits offers opportunities for tailoring spintronic properties such as g-factors for specific functionality. In this letter, we demonstrate the operation of a few-electron double quantum dot circuit fabricated from a (110)-oriented GaAs quantum well. We estimate the in-plane electron g-factor from the profile of the enhanced inte… ▽ More The choice of substrate orientation for semiconductor quantum dot circuits offers opportunities for tailoring spintronic properties such as g-factors for specific functionality. In this letter, we demonstrate the operation of a few-electron double quantum dot circuit fabricated from a (110)-oriented GaAs quantum well. We estimate the in-plane electron g-factor from the profile of the enhanced inter-dot tunneling (leakage) current near zero magnetic field. Spin-blockade due to Pauli exclusion can block inter-dot tunneling. However, this blockade becomes inactive due to hyperfine interaction mediated spin flip-flop processes between electron spin states and the nuclear spin of the host material. The g-factor of absolute value ~0.1 found for a magnetic field parallel to the direction [11(bar)0], is approximately a factor of four lower than that for comparable circuits fabricated from material grown on widely-employed standard (001) GaAs substrates, and is in line with reported values determined by purely optical means for quantum well structures grown on (110) GaAs substrates. △ Less

Submitted 14 November, 2021; originally announced November 2021.

Comments: 17 pages, 4 figures

Auger and spin dynamics in a self-assembled quantum dot

Authors: Hendrik Mannel, Jens Kerski, Pia Lochner, Marcel Zöllner, Andreas D. Wieck, Arne Ludwig, Axel Lorke, Martin Geller

Abstract: The Zeeman-split spin states of a single quantum dot can be used together with its optical trion transitions to form a spin-photon interface between a stationary (the spin) and a flying (the photon) quantum bit. Besides long coherence times of the spin state itself, the limiting decoherence mechanisms of the trion states are of central importance. We investigate here in time-resolved resonance flu… ▽ More The Zeeman-split spin states of a single quantum dot can be used together with its optical trion transitions to form a spin-photon interface between a stationary (the spin) and a flying (the photon) quantum bit. Besides long coherence times of the spin state itself, the limiting decoherence mechanisms of the trion states are of central importance. We investigate here in time-resolved resonance fluorescence the electron and trion dynamics in a single self-assembled quantum dot in an applied magnetic field of up to $B = 10\,$T. The quantum dot is only weakly coupled to an electron reservoir with tunneling rates of about $1\,$ms$^{-1}$. Using this sample structure, we can measure, in addition to the spin-flip rate of the electron and the spin-flip Raman rate of the trion transition, the Auger recombination process, that scatters an Auger electron into the conduction band. The Auger effect destroys the radiative trion transition and leaves the quantum dot empty until an electron tunnels from the reservoir into the dot. The Auger recombination rate decreases by a factor of three from $γ_A=3\,μ$s$^{-1}$ down to $1\,μ$s$^{-1}$ in an applied magnetic field of $10\,$T in Faraday geometry. The combination of an Auger recombination event with subsequent electron tunneling from the reservoir can flip the electron spin and thus constitutes a previously unaccounted mechanism that limits spin coherence, an important resource for quantum technologies. △ Less

Submitted 23 October, 2021; originally announced October 2021.

Sensing dot with high output swing for scalable baseband readout of spin qubits

Authors: Eugen Kammerloher, Andreas Schmidbauer, Laura Diebel, Inga Seidler, Malte Neul, Matthias Künne, Arne Ludwig, Julian Ritzmann, Andreas Wieck, Dominique Bougeard, Lars R. Schreiber, Hendrik Bluhm

Abstract: A crucial requirement for quantum computing, in particular for scalable quantum computing and error correction, is a fast and high-fidelity qubit readout. For semiconductor based qubits, one limiting factor for local low-power signal amplification, is the output swing of the charge sensor. We demonstrate GaAs and Si/SiGe asymmetric sensing dots (ASDs) specifically designed to provide a significant… ▽ More A crucial requirement for quantum computing, in particular for scalable quantum computing and error correction, is a fast and high-fidelity qubit readout. For semiconductor based qubits, one limiting factor for local low-power signal amplification, is the output swing of the charge sensor. We demonstrate GaAs and Si/SiGe asymmetric sensing dots (ASDs) specifically designed to provide a significantly improved response compared to conventional charge sensing dots. Our ASD design features a strongly decoupled drain reservoir from the sensor dot, which mitigates negative feedback effects found in conventional sensors. This results in a boosted output swing of $3\,\text{mV}$, which exceeds the response in the conventional regime of our device by more than ten times. The enhanced output signal paves the way for employing very low-power readout amplifiers in close proximity to the qubit. △ Less

Submitted 4 May, 2023; v1 submitted 28 July, 2021; originally announced July 2021.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Appl. 22, 024044 (2024)

In-flight distribution of an electron within a surface acoustic wave

Authors: Hermann Edlbauer, Junliang Wang, Shunsuke Ota, Americ Richard, Baptiste Jadot, Pierre-André Mortemousque, Yuma Okazaki, Shuji Nakamura, Tetsuo Kodera, Nobu-Hisa Kaneko, Arne Ludwig, Andreas D. Wieck, Matias Urdampilleta, Tristan Meunier, Christopher Bäuerle, Shintaro Takada

Abstract: Surface acoustic waves (SAW) have large potential to realize quantum-optics-like experiments with single flying electrons employing their spin or charge degree of freedom. For such quantum applications, highly efficient trapping of the electron in a specific moving quantum dot (QD) of a SAW train plays a key role. Probabilistic transport over multiple moving minima would cause uncertainty in synch… ▽ More Surface acoustic waves (SAW) have large potential to realize quantum-optics-like experiments with single flying electrons employing their spin or charge degree of freedom. For such quantum applications, highly efficient trapping of the electron in a specific moving quantum dot (QD) of a SAW train plays a key role. Probabilistic transport over multiple moving minima would cause uncertainty in synchronisation that is detrimental for coherence of entangled flying electrons and in-flight quantum operations. It is thus of central importance to identify the device parameters enabling electron transport within a single SAW minimum. A detailed experimental investigation of this aspect is so far missing. Here we fill this gap by demonstrating time-of-flight measurements for a single electron that is transported via a SAW train between distant stationary QDs. Our measurements reveal the in-flight distribution of the electron within the moving acousto-electric quantum dots of the SAW train. Increasing the acousto-electric amplitude, we observe the threshold necessary to confine the flying electron at a specific, deliberately chosen SAW minimum. Investigating the effect of a barrier along the transport channel, we also benchmark the robustness of SAW-driven electron transport against stationary potential variations. Our results pave the way for highly controlled transport of electron qubits in a SAW-driven platform for quantum experiments. △ Less

Submitted 24 September, 2021; v1 submitted 20 July, 2021; originally announced July 2021.

Comments: Main paper: 11 pages, 3 figures, Supplementary material: 5 pages, 4 figures

Journal ref: Appl. Phys. Lett. 119, 114004 (2021)

Heat-Driven Electron-Motion in a Nanoscale Electronic Circuit

Authors: Shintaro Takada, Giorgos Georgiou, Everton Arrighi, Hermann Edlbauer, Yuma Okazaki, Shuji Nakamura, Arne Ludwig, Andreas D. Wieck, Michihisa Yamamoto, Christopher Bäuerle, Nobu-Hisa Kaneko

Abstract: We study the interaction between two closely spaced but electrically isolated quasi-one-dimensional electrical wires by a drag experiment. In this work we experimentally demonstrate the generation of current in an unbiased (drag) wire, which results from the interactions with a neighboring biased (drive) wire. The direction of the drag current depends on the length of the one-dimensional wire with… ▽ More We study the interaction between two closely spaced but electrically isolated quasi-one-dimensional electrical wires by a drag experiment. In this work we experimentally demonstrate the generation of current in an unbiased (drag) wire, which results from the interactions with a neighboring biased (drive) wire. The direction of the drag current depends on the length of the one-dimensional wire with respect to the position of the barrier in the drag wire. When we additionally form a potential barrier in the drive wire, the direction of the drag current is determined by the relative position of the two barriers. We interpret this behavior in terms of electron excitations by phonon-mediated interactions between the two wires in presence of the electron scattering inside the drive wire. △ Less

Submitted 25 October, 2021; v1 submitted 19 July, 2021; originally announced July 2021.

Comments: Main paper: 10 pages, 4 figures, Supplemental material: 6 pages, 3 figures

Journal ref: J. Phys. Soc. Jpn. 90, 113707 (2021)

Quantum Interference of Identical Photons from Remote GaAs Quantum Dots

Authors: Liang Zhai, Giang N. Nguyen, Clemens Spinnler, Julian Ritzmann, Matthias C. Löbl, Andreas D. Wieck, Arne Ludwig, Alisa Javadi, Richard J. Warburton

Abstract: Photonic quantum technology provides a viable route to quantum communication, quantum simulation, and quantum information processing. Recent progress has seen the realisation of boson sampling using 20 single-photons and quantum key distribution over hundreds of kilometres. Scaling the complexity requires architectures containing multiple photon-sources, photon-counters, and a large number of indi… ▽ More Photonic quantum technology provides a viable route to quantum communication, quantum simulation, and quantum information processing. Recent progress has seen the realisation of boson sampling using 20 single-photons and quantum key distribution over hundreds of kilometres. Scaling the complexity requires architectures containing multiple photon-sources, photon-counters, and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single-photons. For applications, a significant roadblock is the poor quantum coherence upon interfering single photons created by independent quantum dots. Here, we demonstrate two-photon interference with near-unity visibility ($93.0\pm0.8$)\% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero-phonon-line -- only the weak phonon-sideband is rejected -- and temporal post-selection is not employed. Exploiting the quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity ($85.0\pm 1.0$)\% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual-coherence of the photons stems from high-quality materials, a diode-structure, and the relatively large quantum dot size. Our results establish a platform, GaAs QDs, for creating coherent single photons in a scalable way. △ Less

Submitted 14 February, 2023; v1 submitted 7 June, 2021; originally announced June 2021.

Comments: 20 pages, 9 figures, supplementary information included in a separate PDF

Real-time observation of charge-spin cooperative dynamics driven by a nonequilibrium phonon environment

Authors: Kazuyuki Kuroyama, Sadashige Matsuo, Jo Muramoto, Shunsuke Yabunaka, Sasha R. Valentin, Arne Ludwig, Andreas D. Wieck, Yasuhiro Tokura, Seigo Tarucha

Abstract: Quantum dots are recognized as a suitable platform for studying thermodynamic phenomena involving single electronic charges and spins in nano-scale devices. However, such a thermodynamic system is usually driven by electron reservoirs at different temperatures, not by a lattice temperature gradient. We report on experimental observations of charge-spin cooperative dynamics in transitions of two-el… ▽ More Quantum dots are recognized as a suitable platform for studying thermodynamic phenomena involving single electronic charges and spins in nano-scale devices. However, such a thermodynamic system is usually driven by electron reservoirs at different temperatures, not by a lattice temperature gradient. We report on experimental observations of charge-spin cooperative dynamics in transitions of two-electron spin states in a GaAs double quantum dot located in a non-equilibrium phonon environment. Enhancements in the spin-flip processes are observed, originating from phonon excitation combined with the spin-orbit interaction. In addition, due to the spatial gradient of phonon density between the dots, the spin-flip rate during an inter-dot electron tunnel from a hot to a cold dot is more enhanced than in the other direction, resulting in accumulation of parallel spin states in the double dot. △ Less

Submitted 2 June, 2021; originally announced June 2021.

Optically driving the radiative Auger transition

Authors: Clemens Spinnler, Liang Zhai, Giang N. Nguyen, Julian Ritzmann, Andreas D. Wieck, Arne Ludwig, Alisa Javadi, Doris E. Reiter, Paweł Machnikowski, Richard J. Warburton, Matthias C. Löbl

Abstract: In a radiative Auger process, optical decay is accompanied by simultaneous excitation of other carriers. The radiative Auger process gives rise to weak red-shifted satellite peaks in the optical emission spectrum. These satellite peaks have been observed over a large spectral range: in the X-ray emission of atoms; close to visible frequencies on donors in semiconductors and quantum emitters; and a… ▽ More In a radiative Auger process, optical decay is accompanied by simultaneous excitation of other carriers. The radiative Auger process gives rise to weak red-shifted satellite peaks in the optical emission spectrum. These satellite peaks have been observed over a large spectral range: in the X-ray emission of atoms; close to visible frequencies on donors in semiconductors and quantum emitters; and at infrared frequencies as shake-up lines in two-dimensional systems. So far, all the work on the radiative Auger process has focussed on detecting the spontaneous emission. However, the fact that the radiative Auger process leads to photon emission suggests that the transition can also be optically excited. In such an inverted radiative Auger process, excitation would correspond to simultaneous photon absorption and electronic de-excitation. Here, we demonstrate optical driving of the radiative Auger transition on a trion in a semiconductor quantum dot. The radiative Auger and the fundamental transition together form a $Λ$-system. On driving both transitions of this $Λ$-system simultaneously, we observe a reduction of the fluorescence signal by up to $70\%$. Our results demonstrate a type of optically addressable transition connecting few-body Coulomb interactions to quantum optics. The results open up the possibility of carrying out THz spectroscopy on single quantum emitters with all the benefits of optics: coherent laser sources, efficient and fast single-photon detectors. In analogy to optical control of an electron spin, the $Λ$-system between the radiative Auger and the fundamental transitions allows optical control of the emitters' orbital degree of freedom. △ Less

Submitted 7 May, 2021; originally announced May 2021.

Comments: 8 pages, 6 figures

Journal ref: Nature Communications 12, 6575 (2021)