Spectral broadening in self-assembled GaAs quantum dots with narrow size distribution

The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size distribution below 15%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling allow us to deconvolve the contribution of phonon broadening in the ensemble photoluminescence and study it in a wide temperature range.Comment: 9 pages, 4 figure

High-temperature droplet epitaxy of symmetric GaAs/AlGaAs quantum dots

We introduce a high-temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self-assemble with the desired emission wavelength and highly symmetric shape while maintaining a high optical quality. The role of the growth parameters during the droplet arsenization is discussed and modelled.Comment: 18 pages, 5 figure

Optically controlled dual-band quantum dot infrared photodetector

We present the design for a novel type of dual-band photodetector in the thermal infrared spectral range, the Optically Controlled Dual-band quantum dot Infrared Photodetector (OCDIP). This concept is based on a quantum dot ensemble with a unimodal size distribution, whose absorption spectrum can be controlled by optically-injected carriers. An external pumping laser varies the electron density in the QDs, permitting to control the available electronic transitions and thus the absorption spectrum. We grew a test sample which we studied by AFM and photoluminescence. Based on the experimental data, we simulated the infrared absorption spectrum of the sample, which showed two absorption bands at 5.85 um and 8.98 um depending on the excitation power

High-yield fabrication of entangled photon emitters for hybrid quantum networking using high-temperature droplet epitaxy

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations.Comment: 14 pages, 3 figure

Entanglement swapping with photons generated on-demand by a quantum dot

Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of non-classical light can be used for entanglement swapping, but quantum communication technologies with device-independent functionalities demand for push-button operation that, in principle, can be implemented using single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent requirements on the efficiency and purity of generation of entangled states. Here we tackle this challenge and show that pairs of polarization-entangled photons generated on-demand by a GaAs quantum dot can be used to successfully demonstrate all-photonic entanglement swapping. Moreover, we develop a theoretical model that provides quantitative insight on the critical figures of merit for the performance of the swapping procedure. This work shows that solid-state quantum emitters are mature for quantum networking and indicates a path for scaling up.Comment: The first four authors contributed equally to this work. 17 pages, 3 figure

Hyperfine-interaction limits polarization entanglement of photons from semiconductor quantum dots

Excitons in quantum dots are excellent sources of polarization-entangled photon pairs, but a quantitative understanding of their interaction with the nuclear spin bath is still missing. Here we investigate the role of hyperfine energy shifts using experimentally accessible parameters and derive an upper limit to the achievable entanglement fidelity. Our results are consistent with all available literature, indicate that spin-noise is often the dominant process limiting the entanglement in InGaAs quantum dots, and suggest routes to alleviate its effect

Experimental Multi-state Quantum Discrimination in the Frequency Domain with Quantum Dot Light

The quest for the realization of effective quantum state discrimination strategies is of great interest for quantum information technology, as well as for fundamental studies. Therefore, it is crucial to develop new and more efficient methods to implement discrimination protocols for quantum states. Among the others, single photon implementations are more advisable, because of their inherent security advantage in quantum communication scenarios. In this work, we present the experimental realization of a protocol employing a time-multiplexing strategy to optimally discriminate among eight non-orthogonal states, encoded in the four-dimensional Hilbert space spanning both the polarization degree of freedom and photon energy. The experiment, built on a custom-designed bulk optics analyser setup and single photons generated by a nearly deterministic solid-state source, represents a benchmarking example of minimum error discrimination with actual quantum states, requiring only linear optics and two photodetectors to be realized. Our work paves the way for more complex applications and delivers a novel approach towards high-dimensional quantum encoding and decoding operations

Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly-Pumped Quantum Dots

Two-photon resonant excitation of the biexciton-exciton cascade in a quantum dot generates highly polarization-entangled photon pairs in a near-deterministic way. However, there are still open questions on the ultimate level of achievable entanglement. Here, we observe the impact of the laser-induced AC-Stark effect on the spectral emission features and on entanglement. A shorter emission time, longer laser pulse duration, and higher pump power all result in lower values of concurrence. Nonetheless, additional contributions are still required to fully account for the observed below-unity concurrence.Comment: 7 pages, 3 figure

A multipair-free source of entangled photons in the solid state

Unwanted multiphoton emission commonly reduces the degree of entanglement of photons generated by non-classical light sources and, in turn, hampers their exploitation in quantum information science and technology. Quantum emitters have the potential to overcome this hurdle but, so far, the effect of multiphoton emission on the quality of entanglement has never been addressed in detail. Here, we tackle this challenge using photon pairs from a resonantly-driven quantum dot and comparing quantum state tomography and second-order coherence measurements as a function of the excitation power. We observe that the relative (absolute) multiphoton emission probability is as low as $p_m= (5.6 \pm 0.6)10^{-4}$ ($p_2= (1.5 \pm 0.3)10^{-6}$) at the maximum source brightness, values that lead to a negligible effect on the degree of entanglement. In stark contrast with probabilistic sources of entangled photons, we also demonstrate that the multiphoton emission probability and the degree of entanglement remain practically unchanged against the excitation power for multiple Rabi cycles, despite we clearly observe oscillations in the second-order coherence measurements. Our results, explained by a theoretical model that we develop to estimate the actual multiphoton contribution in the two-photon density matrix, highlight that quantum dots can be regarded as a multipair-free source of entangled photons in the solid state