Quantum repeaters are crucial for extending the limits of fiber-based quantum communication. We focus on quantum-dot molecules (QDMs) as spin-photon interfaces and a promising platform for this technology. Using a fully quantum-mechanical master-equation formalism, we simulate protocol sequences considering the semiconductor material properties of QDMs [Schall et al., Adv. Quant. Technol. 4.6, 2100002 (2021)] and time-dependent electric fields for gate operations. Our findings indicate that typical QDMs, with currently attainable switching speeds, operate near the adiabatic regime, enabling high-fidelity gate operations. Our approach enables the estimation of transfer rates and predicts fabrication parameters for semiconductor QMDs.
The reservoir computing paradigm has proven effective for autonomous learning and time-series prediction. While classical reservoir computers have been extensively studied, quantum counterparts are gaining attention. Quantum reservoir computers (QRCs) offer advantages like exponential phase-space dimension scaling and entanglement as a unique resource. With advancements in semiconductor fabrication techniques for quantum-photonic systems, such as coupled-cavity arrays, QRC realization is imminent. We explore the properties and quantum advantage of QRCs based on the transverse-field Ising model. Using the benchmark of linear short-term memory capacity, we evaluate the QRC's performance in terms of entanglement and covariance dimension. Possible implementations using interconnected nanolasers as a semiconductor-based quantum-photonic neural network are discussed. [Götting et al., arXiv:2302.03595 (2023)].
The absence of strong losses in high-beta nanolasers makes the identification of the onset of lasing difficult to pinpoint, as the input-output characteristics can become almost thresholdless. The second-order photon correlation function g2(0) has become a valuable tool to assess the coherence properties of nanolasers, as its transition to a value of 1 clearly marks the laser threshold. Most measurements of the zero-delay-time autocorrelation function involve temporal averaging over g2(tau) due to the finite time resolution of the photon detectors. In the past, a generalized Siegert relation has been used to approximately obtain g2(tau). Using full quantum-optical two-time calculations, we address the question in how far it can be used in the partially coherent regime of conventional nanolasers that show a soft transition to lasing, and in few-emitter nanolasers that operate close to or in the regime of strong light-matter coupling.
Multi-partite entanglement is a key resource for many applications in quantum information technologies. Based on two material platforms, we consider methods for entanglement generation. In quantum-dot molecules [1], electric-field switching is used, and we characterize the separation between adiabatic and diabatic dynamics in the realization of entangled target states. Our numerical simulations are based on the Bloch-Redfield formalism and are a key step towards the realization of fully quantum-mechanical protocol simulation. As a second platform, we demonstrate the realization of multi-partite entangled states in coupled-cavity arrays and discuss their role in novel quantum machine learning concepts like quantum reservoir computing, to which we provide some insight.
[1] Schall et al., Advanced Quantum Technologies 4, 2100002 (2021).
Atomically thin transition-metal dichalcogenide (TMD) semiconductors possess strong Coulomb interactions due to reduced dielectric screening, leading to the formation of excitons with exceptionally large binding energies. The enhanced stability of excitons in these materials provides a unique platform to investigate excitonic interactions at room temperature and to examine the role of plasma effects and excitonic interactions over a broad range of excitation densities.
We report an excitation-density dependent crossover between two regimes: Using ultrafast absorption spectroscopy, we observe a pronounced red shift of the exciton resonance followed by an anomalous blue shift with increasing excitation density. Using both material-realistic computation and phenomenological modeling, we attribute this observation to long-range Coulomb interaction in the presence of plasma screening in an attraction-repulsion crossover with the short-ranged exciton-exciton interaction that mimics the Lennard-Jones potential between atoms, suggesting a strong analogy between excitons and atoms in respect of inter-particle interaction.
Our findings underline the important role of many-particle renormalizations and screening due to excited carriers in the device-relevant regime of optically or electrically excited TMDs.
The development and physical understanding of high-beta nanolasers operating in regime of cavity-quantum-electrodynamics (cQED) is a highly interdisciplinary field of research, involving important aspects of nanotechnology, quantum optics, and semiconductor physics. Of particular interest is the quantum limit of operation, in which a few or even a single emitter act as gain material.
The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cQED 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.
Based on an analytic approach, we provide a generalized criterion for strong coupling and the corresponding emission spectrum that includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a state-of-the-art few-emitter quantum-dot (QD)–micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model [1]. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved. By using a master-equation approach for up to 8 QDs coupled to the mode, we provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions by identifying signatures in the mean-photon number, the photon-autocorrelation function, and the emission linewidth.
[1] C. Gies et al., accepted for publication in PRA, arxiv:1606.05591
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