arXiv subjects

Christopher Gies

Publications and source records attributed to Christopher Gies.

At least 37 records · Page 2Linked to original sources

arXiv2023

Gate-based protocol simulations for quantum repeaters using quantum-dot molecules in switchable electric fields

Electrically controllable quantum-dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum-repeater networks. We develop a microscopic open-quantum-systems approach based on a time-dependent Bloch-Redfield equation to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled-photon-pair generation that we propose for quantum repeater applications. Our theory takes into account the quantum-dot molecules' electronic properties that are controlled by time-dependent electric fields as well as dissipation due to electron-phonon interaction. We quantify the transition between adiabatic and non-adiabatic regimes, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, we infer the maximum speed of entangled-state preparation under different experimental conditions, which serves as a first step towards simulation of attainable entangled photon-pair generation rates. The developed formalism opens the possibility for device-realistic descriptions of repeater protocol implementations.

quant-ph↗

arXiv2023

Signatures of superradiance as a witness to multipartite entanglement

Generation and detection of entanglement is at the forefront of most quantum information technologies. There is a plethora of techniques that reveal entanglement on the basis of only partial information about the underlying quantum state, including entanglement witnesses. Superradiance refers to the phenomenon of highly synchronized photon emission from an ensemble of quantum emitters that is caused by correlations among the individual particles and has been connected by Dicke himself to the presence of multipartite entangled states. We investigate this connection in a quantitative way and discuss, whether or not signatures of superradiance from semiconductor nanolasers, manifesting themselves as a modification of the spontaneous-emission time, can be interpreted as a witness to detect entanglement in the underlying state of the emitters.

quant-ph↗

arXiv2023

Confined-state physics and signs of fermionization of moiré excitons in WSe$_2$/MoSe$_2$ heterobilayers

We revisit and extend the standard bosonic interpretation of interlayer excitons in the moiré potential of twisted heterostructures of transition-metal dichalcogenides. In our experiments, we probe a high quality MoSe$_2$/WSe$_2$ van der Waals bilayer heterostructure via density-dependent photoluminescence spectroscopy and reveal strongly developed, unconventional spectral shifts of the emergent moiré exciton resonances. The observation of saturating blueshifts of successive exciton resonances allow us to explain their physics in terms of a model utilizing fermionic saturable absorbers. This approach is strongly inspired by established quantum-dot models, which underlines the close analogy of interlayer excitons trapped in pockets of the moiré potential, and quantum emitters with discrete eigenstates.

cond-mat.mes-hall↗

arXiv2023

Exploring quantum mechanical advantage for reservoir computing

Quantum reservoir computing is an emerging field in machine learning with quantum systems. While classical reservoir computing has proven to be a capable concept of enabling machine learning on real, complex dynamical systems with many degrees of freedom, the advantage of its quantum analogue is yet to be fully explored. Here, we establish a link between quantum properties of a quantum reservoir, namely entanglement and its occupied phase space dimension, and its linear short-term memory performance. We find that a high degree of entanglement in the reservoir is a prerequisite for a more complex reservoir dynamics that is key to unlocking the exponential phase space and higher short-term memory capacity. We quantify these relations and discuss the effect of dephasing in the performance of physical quantum reservoirs.

quant-ph↗

arXiv2023

Multipartite entanglement generation in coupled microcavity arrays

We consider photonic arrays made from quantum emitters in optically coupled microcavities as a platform for entanglement generation. These offer a large degree of tunability with the possibility of site-selective optical excitation. Coherent pumping is considered to drive transitions between vacuum and entangled target states both in a time-dependent manner, and in a quantum bath engineering approach to create entanglement in the steady-state. We demonstrate a numerical scheme that allows to generalize the determination of excitation parameters to larger array sizes and different classes of entangled states. This study is a step towards using coupled cavity arrays as a hardware platform in novel quantum-photonic applications in quantum computing and quantum machine learning.

quant-ph↗

arXiv2022

Revisiting the Siegert relation for the partially coherent regime of nanolasers

The Siegert relation connects the first- and second-order coherence properties of light. While strictly valid only in the thermal regime and in the absence of correlations, this relation is routinely extended to the partially coherent regime in the study of high-$β$ nanolasers, where it aids in the identification of the lasing threshold. We re-evaluate the use of a generalized Siegert relation in different device regimes. A full two-time quantum-optical theory is derived as a reference for obtaining first- and second-order correlation functions $g^{(1)}(τ)$ and $g^{(2)}(τ)$ in the steady state. We find that even in the partially coherent regime the generalized Siegert relation is well suited as an approximation to $g^{(2)}(τ)$ as long as emitter correlation are negligible, but does not apply well in the quantum regime of few-emitter nanolasers, or to devices featuring sub- and superradiant emission.

physics.optics↗

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

Observation of two-dimensional Anderson localisation of ultracold atoms

Anderson localisation -- the inhibition of wave propagation in disordered media -- is a surprising interference phenomenon which is particularly intriguing in two-dimensional (2D) systems. While an ideal, non-interacting 2D system of infinite size is always localised, the localisation length-scale may be too large to be unambiguously observed in an experiment. In this sense, 2D is a marginal dimension between one-dimension, where all states are strongly localised, and three-dimensions, where a well-defined phase transition between localisation and delocalisation exists as the energy is increased. Here we report the results of an experiment measuring the 2D transport of ultracold atoms between two reservoirs, which are connected by a channel containing pointlike disorder. The design overcomes many of the technical challenges that have hampered observation of localisation in previous works. We experimentally observe exponential localisation in a 2D ultracold atom system.

cond-mat.quant-gas↗

arXiv2019

Thresholdless transition to coherent emission at telecom wavelengths from coaxial nanolasers

The ongoing miniaturization of semiconductor lasers has enabled ultra-low threshold devices and even provided a path to approach thresholdless lasing with linear input-output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator and mode-density enhancement. Here, we investigate high-$β$ metal-clad coaxial nanolasers, which facilitate thresholdless lasing. We experimentally and theoretically investigate both the conventional lasing characteristics, as well as the photon statistics of the emitted light. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. Our work clearly highlights an important and often misunderstood aspect of high-$β$ lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero threshold laser.

physics.app-ph↗

arXiv2018

Delayed transition to coherent emission in nanolasers with extended gain media

The realization of high-$β$ lasers is one of the prime applications of cavity-QED promising ultra-low thresholds, integrability and reduced power consumption in the field of \textit{green photonics}. In such nanolasers spontaneous emission can play a central role even above the threshold. By going beyond rate-equation approaches, we revisit the definition of a laser threshold in terms of the input-output characteristics and the degree of coherence of the emission. We demonstrate that there are new regimes of cavity-QED lasing, realized e.g. in high-$Q$ nanolasers with extended gain material, for which the two can differ significantly such that coherence is reached at much higher pump powers than required to observe the thresholdlike intensity jump. Against the common perception, such devices do not benefit from high-$β$ factors in terms of power reduction, as a significant amount of stimulated emission is required to quieten the spontaneous emission noise.

cond-mat.mes-hall↗

arXiv2018

Prospects and limitations of transition-metal dichalcogenide laser gain materials

Nanolasers operate with a minimal amount of active material and low losses. In this regime, single layers of transition-metal dichalcogenides (TMDs) are being investigated as next generation gain materials due to their high quantum efficiency. We provide results from microscopic gain calculations of highly excited TMD monolayers and specify requirements to achieve lasing with four commonly used TMD semiconductors. Our approach includes band-structure renormalizations due to excited carriers that trigger a direct-to-indirect band-gap transition. As a consequence, we predict a rollover for the gain that limits the excitation regime where laser operation is possible. A parametrization of the peak gain is provided that is used in combination with a rate-equation theory to discuss consequences for experimentally accessible laser characteristics.

cond-mat.mes-hall↗

arXiv2018

Observation of exciton redshift-blueshift crossover in monolayer WS2

We report a rare atom-like interaction between excitons in monolayer WS2, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to inter-particle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2018

Polariton hyperspectral imaging of two-dimensional semiconductor crystals

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form quasiparticles known as polaritons up to room temperature. Here, we explore the two-dimensional nature of TMD polaritons with cavity-assisted hyperspectral imaging. Using extended WS$_2$ monolayers, we establish the regime of strong coupling with a scanning microcavity to map out polariton properties and correlate their spatial features with intrinsic and extrinsic effects. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-perturbative polariton-phonon coupling. Our measurements reveal a promisingly consistent polariton landscape, and highlight the importance of phonons for future polaritonic devices.

physics.optics↗

arXiv2017

On thresholdless lasing features in high-$β$ nitride nanobeam cavities: a quantum optical study

Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. These studies are accompanied by a vivid debate on how to prove and interpret lasing in the high-$β$ regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Due to their high optical quality, such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-$β$ lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power dependent interplay between 0D and 2D gain contributions.

cond-mat.mes-hall↗

arXiv2017

Strong light-matter coupling in the presence of lasing

The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cavity-QED systems, strong coupling may persevere even at elevated excitation levels sufficient to cross the threshold to lasing. In the presence of stimulated emission, the vacuum-Rabi doublet in the emission spectrum is modified and the established criterion for strong coupling no longer applies. We provide a generalized criterion for strong coupling and the corresponding emission spectrum, which includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a few-emitter quantum-dot--micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved, and provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions.

physics.optics↗

arXiv2017

Non-invasive control of excitons in two-dimensional materials

We investigate how external screening shapes excitons in two-dimensional (2d) semiconductors embedded in laterally structured dielectric environments. An atomic scale view of these elementary excitations is developed using models which apply to a variety of materials including transition metal dichalcogenides (TMDCs). We find that structured dielectrics imprint a peculiar potential energy landscape on excitons in these systems: While the ground-state exciton is least influenced, higher excitations are attracted towards regions with high dielectric constant of the environment. This landscape is "inverted" in the sense that low energy excitons are less strongly affected than their higher energy counterparts. Corresponding energy variations emerge on length scales of the order of a few unit cells. This opens the prospect of trapping and guiding of higher excitons by means of tailor-made dielectric substrates on ultimately small spatial scales.

cond-mat.mtrl-sci↗

arXiv2016

Nonequilibrium Carrier Dynamics in Transition Metal Dichalcogenide Semiconductors

When exploring new materials for their potential in (opto)electronic device applications, it is important to understand the role of various carrier interaction and scattering processes. Research on transition metal dichalcogenide (TMD) semiconductors has recently progressed towards the realisation of working devices, which involve light-emitting diodes, nanocavity lasers, and single-photon emitters. In these two-dimensional atomically thin semiconductors, the Coulomb interaction is known to be much stronger than in quantum wells of conventional semiconductors like GaAs, as witnessed by the 50 times larger exciton binding energy. The question arises, whether this directly translates into equivalently faster carrier-carrier Coulomb scattering of excited carriers. Here we show that a combination of ab-initio band-structure and many-body theory predicts carrier relaxation on a 50-fs time scale, which is less than an order of magnitude faster than in quantum wells. These scattering times compete with the recently reported sub-ps exciton recombination times, thus making it harder to achieve population inversion and lasing.

cond-mat.mtrl-sci↗

arXiv2013

Coulomb-assisted cavity feeding in the non-resonant optical emission from a quantum dot

Recent experiments have demonstrated that for a quantum dot in an optical resonator off-resonant cavity mode emission can occur even for detunings of the order of 10 meV. We show that Coulomb mediated Auger processes based on additional carriers in delocalized states can facilitate this far off-resonant emission. Using a novel theoretical approach for a non-perturbative treatment of the Auger-assisted quantum-dot carrier recombination, we present numerical calculations of the far off-resonant cavity feeding rate and cavity mean photon number confirming efficient coupling at higher densities of carriers in the delocalized states. In comparison to fast Auger-like intraband scattering processes, we find a reduced overall efficiency of Coulomb-mediated interband transitions due the required electron-hole correlations for the recombination processes.

cond-mat.mes-hall↗