arXiv subjects

Sven Rodt

Publications and source records attributed to Sven Rodt.

At least 37 records · Page 2Linked to original sources

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

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

Scalable deterministic integration of two quantum dots into an on-chip quantum circuit

Integrated quantum photonic circuits (IQPCs) with deterministically integrated quantum emitters are critical elements for scalable quantum information applications and have attracted significant attention in recent years. However, scaling up them towards fully functional photonic circuits with multiple deterministically integrated quantum emitters to generate photonic input states remains a great challenge. In this work, we report on a monolithic prototype IQPC consisting of two pre-selected quantum dots deterministically integrated into nanobeam cavities at the input ports of a 2x2 multimode interference beam-splitter. The on-chip beam splitter exhibits a splitting ratio of nearly 50/50 and the integrated quantum emitters have high single-photon purity, enabling on-chip HBT experiments, depicting deterministic scalability. Overall, this marks a cornerstone toward scalable and fully-functional IQPCs.

quant-ph↗

arXiv2023

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

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.

cond-mat.mes-hall↗

arXiv2022

Numerical optimization of single-mode fiber-coupled single-photon sources based on semiconductor quantum dots

We perform extended numerical studies to maximize the overall photon coupling efficiency of fiber-coupled quantum dot single-photon sources emitting in the near-infrared and telecom regime. Using the finite element method, we optimize the photon extraction and fiber-coupling efficiency of quantum dot single-photon sources based on micromesas, microlenses, circular Bragg grating cavities and micropillars. The numerical simulations which consider the entire system consisting of the quantum dot source itself, the coupling lens, and the single-mode fiber yield overall photon coupling efficiencies of up to 83%. Our work provides objectified comparability of different fiber-coupled single-photon sources and proposes optimized geometries for the realization of practical and highly efficient quantum dot single-photon sources.

quant-ph↗

arXiv2022

A Quantum Key Distribution Testbed using a Plug&Play Telecom-wavelength Single-Photon Source

Deterministic solid-state quantum light sources are considered key building blocks for future communication networks. While several proof-of-principle experiments of quantum communication using such sources have been realized, most of them required large setups often involving liquid helium infrastructure or bulky closed-cycle cryotechnology. In this work, we report on the first quantum key distribution (QKD) testbed using a compact benchtop quantum dot single-photon source operating at telecom wavelengths. The plug\&play device emits single-photon pulses at O-band wavelengths ($1321\,$nm) and is based on a directly fiber-pigtailed deterministically-fabricated quantum dot device integrated into a compact Stirling cryocooler. The Stirling is housed in a 19-inch rack module including all accessories required for stand-alone operation. Implemented in a simple QKD testbed emulating the BB84 protocol with polarization coding, we achieve an antibunching of $g^{(2)}(0) = 0.10\pm0.01$ and a raw key rate of up to $(4.72\pm0.13)\,$kHz using an external pump laser. In this setting, we further evaluate the performance of our source in terms of the quantum bit error ratios, secure key rates, and tolerable losses expected in full implementations of QKD also accounting for finite key size effects. Furthermore, we investigate optimal settings for a two-dimensional temporal acceptance window applied on receiver side, resulting in predicted tolerable losses up to $23.19\,$dB. Not least, we compare our results with previous proof-of-concept QKD experiments using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.

quant-ph↗

arXiv2021

Bright electrically controllable quantum-dot-molecule devices fabricated by in-situ electron-beam lithography

Self-organized semiconductor quantum dots represent almost ideal two-level systems, which have strong potential to applications in photonic quantum technologies. For instance, they can act as emitters in close-to-ideal quantum light sources. Coupled quantum dot systems with significantly increased functionality are potentially of even stronger interest since they can be used to host ultra-stable singlet-triplet spin qubits for efficient spin-photon interfaces and for a deterministic photonic 2D cluster-state generation. We realize an advanced quantum dot molecule (QDM) device and demonstrate excellent optical properties. The device includes electrically controllable QDMs based on stacked quantum dots in a pin-diode structure. The QDMs are deterministically integrated into a photonic structure with a circular Bragg grating using in-situ electron beam lithography. We measure a photon extraction efficiency of up to (24$\pm$4)% in good agreement with numerical simulations. The coupling character of the QDMs is clearly demonstrated by bias voltage dependent spectroscopy that also controls the orbital couplings of the QDMs and their charge state in quantitative agreement with theory. The QDM devices show excellent single-photon emission properties with a multi-photon suppression of $g^{(2)}(0) = (3.9 \pm 0.5) \cdot 10^{-3}$. These metrics make the developed QDM devices attractive building blocks for use in future photonic quantum networks using advanced nanophotonic hardware.

cond-mat.mes-hall↗

arXiv2020

Thermal stability of emission from single InGaAs/GaAs quantum dots at the telecom O-band

Single-photon sources are key building blocks in most of the emerging secure telecommunication and quantum information processing schemes. Semiconductor quantum dots (QD) have been proven to be the most prospective candidates. However, their practical use in fiber-based quantum communication depends heavily on the possibility of operation in the telecom bands and at temperatures not requiring extensive cryogenic systems. In this paper we present a temperature-dependent study on single QD emission and single-photon emission from metalorganic vapour-phase epitaxy-grown InGaAs/GaAs QDs emitting in the telecom O-band. Micro-photoluminescence studies reveal that trapped holes in the vicinity of a QD act as reservoir of carriers that can be exploited to enhance photoluminescence from trion states observed at elevated temperatures up to at least 80 K. The luminescence quenching is mainly related to the promotion of holes to higher states in the valence band and this aspect must be primarily addressed in order to further increase the thermal stability of emission. Photon autocorrelation measurements yield single photon emission with a purity of $g_{50\mathrm{K}}^{(2)}\left(0\right)=0.13$ up to 50 K. Our results imply that these nanostructures are very promising candidates for single-photon sources at elevated temperatures in the telecom O-band and highlight means for improvements in their performance.

cond-mat.mes-hall↗

arXiv2020

Deterministically fabricated strain-tunable quantum dot single-photon sources emitting in the telecom O-band

Most quantum communication schemes aim at the long-distance transmission of quantum information. In the quantum repeater concept, the transmission line is subdivided into shorter links interconnected by entanglement distribution via Bell-state measurements to overcome inherent channel losses. This concept requires on-demand single-photon sources with a high degree of multi-photon suppression and high indistinguishability within each repeater node. For a successful operation of the repeater, a spectral matching of remote quantum light sources is essential. We present a spectrally tunable single-photon source emitting in the telecom O-band with the potential to function as a building block of a quantum communication network based on optical fibers. A thin membrane of GaAs embedding InGaAs quantum dots (QDs) is attached onto a piezoelectric actuator via gold thermocompression bonding. Here the thin gold layer acts simultaneously as an electrical contact, strain transmission medium and broadband backside mirror for the QD-micromesa. The nanofabrication of the QD-micromesa is based on in-situ electron-beam lithography, which makes it possible to integrate pre-selected single QDs deterministically into the center of monolithic micromesa structures. The QD pre-selection is based on distinct single-QD properties, signal intensity and emission energy. In combination with strain-induced fine tuning this offers a robust method to achieve spectral resonance in the emission of remote QDs. We show that the spectral tuning has no detectable influence on the multi-photon suppression with $g^{(2)}(0)$ as low as 2-4% and that the emission can be stabilized to an accuracy of 4 $μ$eV using a closed-loop optical feedback.

cond-mat.mes-hall↗

arXiv2020

Numerical Investigation of Light Emission from Quantum Dots Embedded into On-Chip, Low Index Contrast Optical Waveguides

Single-photon emitters integrated into quantum optical circuits will enable new, miniaturized quantum optical devices. Here, we numerically investigate semiconductor quantum dots embedded to low refractive index contrast waveguides. We discuss a model to compute the coupling efficiency of the emitted light field to the fundamental propagation mode of the waveguide, and we optimize the waveguide dimensional parameters for maximum coupling efficiency. Further, we show that for a laterally cropped waveguide the interplay of Purcell-enhancement and optimized field profile can enhance the coupling efficiency by a factor of about two.

physics.optics↗

arXiv2020

Quantum dot single-photon emission coupled into single-mode fibers with 3D printed micro-objectives

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications. For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is realized by deterministic fabrication of the QD-microlens via in-situ electron-beam lithography and 3D two-photon laser writing of the on-chip micro-objective and fiber-holder. The QD with microlens is an efficient single-photon source, whose emission is collimated by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in the on-chip fiber chuck, which is precisely aligned to the QD-microlens thanks to the sub-$μ$m processing accuracy of high-resolution two-photon direct laser writing. This way, we obtain a fully integrated high-quality quantum device with broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 26%, a single-photon flux of 1.5 MHz at single-mode fibre output and a multi-photon probability of 13 % under pulsed optical excitation. In addition, the stable design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum applications.

physics.app-ph↗

arXiv2020

Directional single-photon emission from deterministic quantum dot waveguide structures

Chiral light-matter interaction can lead to directional emission of two-level light emitters in waveguides. This interesting physics effect has raised considerable attention in recent years especially in terms of on-chip quantum systems. In this context, our work focuses on tailoring single semiconductor quantum dot-waveguide (QD-WG) systems to emit single photons with high directionality. We use low-temperature in-situ electron-beam lithography enabled by cathodoluminescence mapping to select suitable QDs and to integrate them deterministically into linear waveguide structures at specific chiral points determined by numerical calculations. We observe excitonic and biexcitonic emission from the fabricated QD-WG structure in a confocal microPL setup enabling the optical characterization in terms of directional emission of circularly polarized photons emitted by integrated QDs. Our results show a high degree of anisotropy on the level of 54% for directional QD emission and antibunching in autocorrelation experiment confirming the fabricated QD-WG system, which is a prerequisite for using this effect in advanced applications in integrated quantum circuits.

cond-mat.mes-hall↗

arXiv2020

Entanglement robustness to excitonic spin precession in a quantum dot

A semiconductor quantum dot (QD) is an attractive resource to generate polarization-entangled photon pairs. We study the excitonic spin precession (flip-flop) in a family of QDs with different excitonic fine-structure splitting (FSS) and its impact on the entanglement of photons generated from the excitonic-biexcitonic radiative cascade. Our results reveal that coherent processes leave the time post-selected entanglement of QDs with finite FSS unaffected while changing the eigenstates of the system. The flip-flop's precession is observed via quantum tomography through anomalous oscillations of the coincidences in the rectilinear basis. A theoretical model is constructed with the inclusion of an excitonic flip-flop rate and is compared with a two-photon quantum tomography measurement on a QD exhibiting the spin flip-flop mechanism. A generalization of the theoretical model allows estimating the degree of entanglement as a function of the FSS and the spin-flip rate. For a finite temporal resolution, the negativity is found to be oscillating with respect to both the FSS and the spin-flip rate. This oscillatory behavior disappears for perfect temporal resolution and maximal entanglement is retrieved despite the flip-flop process.

cond-mat.mes-hall↗

arXiv2020

Interplay between emission wavelength and s-p splitting in MOCVD-grown InGaAs/GaAs quantum dots emitting above 1.3 μm

The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. Observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between s-shell and p-shell shows an inverse dependence on the emission energy. The experimental data are compared with the results of confined states calculations, where the impact of the size and composition in the investigated structures is simulated within the 8-band $\mathbf{k}\cdot\mathbf{p}$ model. On this basis, the experimental observation is attributed mainly to changes in indium content within individual quantum dots, indicating a way of engineering and selecting a desired quantum dot, whose electronic structure is the most suitable for a given nanophotonic application.

cond-mat.mes-hall↗

arXiv2020

Tools for the Performance Optimization of Single-Photon Quantum Key Distribution

Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for four-state polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of $g^{(2)}(0)$, inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including measurement-device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources.

quant-ph↗

arXiv2019

Plug&play fibre-coupled 73 kHz single-photon source operating in the telecom O-band

A user-friendly fibre-coupled single-photon source operating at telecom wavelengths is a key component of photonic quantum networks providing long-haul ultra-secure data exchange. To take full advantage of quantum-mechanical data protection and to maximize the transmission rate and distance, a true quantum source providing single-photons on demand is highly desirable. We tackle this great challenge by developing a ready to use semiconductor quantum dot (QD)-based device that launches single photons at a wavelength of 1.3 um directly into a single-mode optical fibre. In our approach the QD is deterministically integrated into a nanophotonic structure to ensure efficient on-chip coupling into a fibre. The whole arrangement is integrated into a 19" compatible housing to enable stand-alone operation by cooling via a compact Stirling cryocooler. The realized source delivers single photons with multiphoton events probability as low as 0.15 and single-photon emission rate up to 73 kHz into a standard telecom single-mode fibre.

physics.app-ph↗

arXiv2019

Deterministically Fabricated Solid-State Quantum-Light Sources

This topical review focuses on solid-state quantum-light sources which are fabricated in a deterministic fashion. In this framework we cover quantum emitters represented by semiconductor quantum dots, colour centres in diamond, and defect-/strain-centres in two-dimensional materials. First, we introduce the topic of quantum-light sources and non-classical light generation for applications in photonic quantum technologies, motivating the need for the development of scalable device technologies to push the field to real-world applications. In the second part, we summarize material systems hosting quantum emitters in the solid-state. The third part reviews deterministic fabrication techniques and comparatively discusses their advantages and disadvantages. The techniques are classified in bottom-up approaches, exploiting the site-controlled positioning of the quantum emitters themselves, and top-down approaches, allowing for the precise alignment of photonic microstructures to pre-selected quantum emitters. Special emphasis is put on the progress achieved in the development of in-situ techniques, which significantly pushed the performance of quantum-light sources towards applications. Additionally we discuss hybrid approaches, exploiting pick-and-place techniques or wafer-bonding. The fourth part presents state-of-the-art quantum-dot quantum-light sources based on the fabrication techniques presented in the previous sections, which feature engineered functionality and enhanced photon collection efficiency. The article closes by highlighting recent applications of deterministic solid-state-based quantum-light sources in the fields of quantum communication, quantum computing, and quantum metrology, and discussing future perspectives in the field of solid-state quantum-light sources.

physics.app-ph↗

arXiv2019

Optimized Designs for Telecom-Wavelength Quantum Light Sources Based on Hybrid Circular Bragg Gratings

We present a design study of quantum light sources based on hybrid circular Bragg Gratings (CBGs) for emission wavelengths in the telecom O-band. The evaluated CBG designs show photon extraction efficiencies > 95% and Purcell factors close to 30. Using simulations based on the finite element method, and considering the influence of possible fabrication imperfections, we identify optimized high-performance CBG designs which are robust against structural aberrations. In particular, full 3D simulations reveal that the designs show robustness regarding deviations of the emitter position in the device well within reported positioning accuracies of deterministic fabrication technologies. Furthermore, we investigate the coupling of the evaluated hybrid CBG designs to single-mode optical fibers, which is particularly interesting for the development of practical quantum light sources. We obtain coupling efficiencies of up to 77% for off-the-shelf fibers, and again proof robustness against fabrication imperfections. Our results show prospects for the fabrication of close-to-ideal fiber-coupled quantum light sources for long distance quantum communication.

physics.app-ph↗