arXiv subjects

Arne Ludwig

Publications and source records attributed to Arne Ludwig.

At least 73 records · Page 4Linked to original sources

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

Coherent control of a high-orbital hole in a semiconductor quantum dot

Coherently driven semiconductor quantum dots are one of the most promising platforms for non-classical light sources and quantum logic gates which form the foundation of photonic quantum technologies. However, to date, coherent manipulation of single charge carriers in quantum dots is limited mainly to their lowest orbital states. Ultrafast coherent control of high-orbital states is obstructed by the demand for tunable terahertz pulses. To break this constraint, we demonstrate an all-optical method to control high-orbital states of a hole via stimulated Auger process. The coherent nature of the Auger process is proved by Rabi oscillation and Ramsey interference. Harnessing this coherence further enables the investigation of single-hole relaxation mechanism. A hole relaxation time of 161 ps is observed and attributed to the phonon bottleneck effect. Our work opens new possibilities for understanding the fundamental properties of high-orbital states in quantum emitters and developing new types of orbital-based quantum photonic devices.

quant-ph↗

arXiv2023

Enhanced Electron Spin Coherence in a GaAs Quantum Emitter

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.

quant-ph↗

arXiv2023

Independent operation of two waveguide-integrated quantum emitters

We demonstrate the resonant excitation of two quantum dots in a photonic integrated circuit for on-chip single-photon generation in multiple spatial modes. The two quantum dots are electrically tuned to the same emission wavelength using a pair of isolated $p$-$i$-$n$ junctions and excited by a resonant pump laser via dual-mode waveguides. We demonstrate two-photon quantum interference visibility of $(79\pm2)\%$ under continuous-wave excitation of narrow-linewidth quantum dots. Our work solves an outstanding challenge in quantum photonics by realizing the key enabling functionality of how to scale-up deterministic single-photon sources.

quant-ph↗

arXiv2023

On-chip spin-photon entanglement based on single-photon scattering

The realization of on-chip quantum gates between photons and solid-state spins is a key building block for quantum-information processors, enabling, e.g., distributed quantum computing, where remote quantum registers are interconnected by flying photons. Self-assembled quantum dots integrated in nanostructures are one of the most promising systems for such an endeavor thanks to their near-unity photon-emitter coupling and fast spontaneous emission rate. Here we demonstrate an on-chip entangling gate between an incoming photon and a stationary quantum-dot spin qubit. The gate is based on sequential scattering of a time-bin encoded photon with a waveguide-embedded quantum dot and operates on sub-microsecond timescale; two orders of magnitude faster than other platforms. Heralding on detection of a reflected photon renders the gate fidelity fully immune to spectral wandering of the emitter. These results represent a major step in realizing a quantum node capable of both photonic entanglement generation and on-chip quantum logic, as demanded in quantum networks and quantum repeaters.

quant-ph↗

arXiv2023

Violation of Bell inequality by photon scattering on a two-level emitter

Entanglement, the non-local correlations present in multipartite quantum systems, is a curious feature of quantum mechanics and the fuel of quantum technology. It is therefore a major priority to develop energy-conserving and simple methods for generating high-fidelity entangled states. In the case of light, entanglement can be realized by interactions with matter, although the required nonlinear interaction is typically weak, thereby limiting its applicability. Here, we show how a single two-level emitter deterministically coupled to light in a nanophotonic waveguide is used to realize genuine photonic quantum entanglement for excitation at the single photon level. By virtue of the efficient optical coupling, two-photon interactions are strongly mediated by the emitter realizing a giant nonlinearity that leads to entanglement. We experimentally generate and verify energy-time entanglement by violating a Bell inequality (Clauder-Horne-Shimony-Holt Bell parameter of $S=2.67(16)>2$) in an interferometric measurement of the two-photon scattering response. As an attractive feature of this approach, the two-level emitter acts as a passive scatterer initially prepared in the ground state, i.e., no advanced spin control is required. This experiment is a fundamental advancement that may pave a new route for ultra-low energy-consuming synthesis of photonic entangled states for quantum simulators or metrology.

quant-ph↗

arXiv2023

Direct observation of non-linear optical phase shift induced by a single quantum emitter in a waveguide

Realizing a sensitive photon-number-dependent phase shift on a light beam is required both in classical and quantum photonics. It may lead to new applications for classical and quantum photonics machine learning or pave the way for realizing photon-photon gate operations. Non-linear phase-shifts require efficient light-matter interaction, and recently quantum dots coupled to nanophotonic devices have enabled near-deterministic single-photon coupling. We experimentally realize an optical phase shift of $0.19 π\pm 0.03$ radians ($\approx 34$ degrees) using a weak coherent state interacting with a single quantum dot in a planar nanophotonic waveguide. The phase shift is probed by interferometric measurements of the light scattered from the quantum dot in the waveguide. The nonlinear process is sensitive at the single-photon level and can be made compatible with scalable photonic integrated circuitry. The work may open new prospects for realizing high-efficiency optical switching or be applied for proof-of-concept quantum machine learning or quantum simulation demonstrations.

quant-ph↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

Quantum Key Distribution using Deterministic Single-Photon Sources over a Field-Installed Fibre Link

Quantum-dot-based single-photon sources are key assets for quantum information technology, supplying on-demand scalable quantum resources for computing and communication. However, longlasting issues such as limited long-term stability and source brightness have traditionally impeded their adoption in real-world applications. Here, we realize a quantum key distribution field trial using true single photons across an 18-km-long dark fibre, located in the Copenhagen metropolitan area, using an optimized, state-of-the-art, quantum-dot single-photon source frequency-converted to the telecom wavelength. A secret key generation rate of >2 kbits/s realized over a 9.6 dB channel loss is achieved with a polarization-encoded BB84 scheme, showing remarkable stability for more than 24 hours of continuous operation. Our results highlight the maturity of deterministic single-photon source technology while paving the way for advanced single-photon-based communication protocols, including fully device-independent quantum key distribution, towards the goal of a quantum internet.

quant-ph↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2023

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

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.

quant-ph↗

arXiv2023

Quantum Interference of Identical Photons from Remote GaAs Quantum Dots

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.

quant-ph↗

arXiv2023

Deterministic photon source interfaced with a programmable silicon-nitride integrated circuit

We develop a quantum photonic platform that interconnects a high-quality quantum dot single-photon source and a low-loss photonic integrated circuit made in silicon nitride. The platform is characterized and programmed to demonstrate various multiphoton applications, including bosonic suppression laws and photonic entanglement generation. The results show a promising technological route forward to scale-up photonic quantum hardware.

quant-ph↗

arXiv2023

Cavity-enhanced excitation of a quantum dot in the picosecond regime

A major challenge in generating single photons with a single emitter is to excite the emitter while avoiding laser leakage into the collection path. Ideally, any scheme to suppress this leakage should not result in a loss in efficiency of the single-photon source. Here, we investigate a scheme in which a single emitter, a semiconductor quantum dot, is embedded in a microcavity. The scheme exploits the splitting of the cavity mode into two orthogonally-polarised modes: one mode is used for excitation, the other for collection. By linking experiment to theory, we show that the best population inversion is achieved with a laser pulse detuned from the quantum emitter. The Rabi oscillations have an unusual dependence on pulse power. Our theory describes them quantitatively allowing us to determine the absolute photon creation probability. For the optimal laser detuning, the population innversion is 98\%. The Rabi oscillations depend on the sign of the laser-pulse detuning. We show that this arises from the non-trivial effect of phonons on the exciton dynamics. The exciton-phonon interaction is included in the theory and gives excellent agreement with all the experimental results.

quant-ph↗