arXiv subjects

Armando Rastelli

Publications and source records attributed to Armando Rastelli.

At least 73 records · Page 4Linked to original sources

arXiv2022

SUPER Scheme in Action: Experimental Demonstration of Red-detuned Excitation of a Quantum Dot

The quest for the perfect single-photon source includes finding the optimal protocol for exciting the quantum emitter. Based on a recently proposed, so-called SUPER (swing-up of quantum emitter population) scheme, we demonstrate experimentally that two red-detuned laser pulses, neither of which could yield a significant upper-level population individually, lead to the coherent excitation of a semiconductor quantum dot. We characterize the emitted single photons and show that they have properties comparable to those achieved under resonant excitation schemes.

cond-mat.mes-hall↗

arXiv2022

Electric field induced tuning of electronic correlation in weakly confining quantum dots

We conduct a combined experimental and theoretical study of the quantum-confined Stark effect in GaAs/AlGaAs quantum dots obtained with the local droplet etching method. In the experiment, we probe the permanent electric dipole and polarizability of neutral and positively charged excitons weakly confined in GaAs quantum dots by measuring their light emission under the influence of a variable electric field applied along the growth direction. Calculations based on the configuration-interaction method show excellent quantitative agreement with the experiment and allow us to elucidate the role of Coulomb interactions among the confined particles and -- even more importantly -- of electronic correlation effects on the Stark shifts. Moreover, we show how the electric field alters properties such as built-in dipole, binding energy, and heavy-light hole mixing of multiparticle complexes in weakly confining systems, underlining the deficiencies of commonly used models for the quantum-confined Stark effect.

cond-mat.mes-hall↗

arXiv2022

Surface passivation and oxide encapsulation to improve optical properties of a single GaAs quantum dot close to the surface

Epitaxial GaAs quantum dots grown by droplet etching have recently shown excellent properties as sources of single photons as well as entangled photon pairs. Integration in some nanophotonic structures requires surface-to-dot distances of less than 100 nm. This demands a surface passivation scheme, which could be useful to lower the density of surface states. To address this issue, sulphur passivation with dielectric overlayer as an encapsulation is used for surface to QD distances of 40 nm, which results in the partial recovery of emission linewidths to bulk values as well as in the increase of the photoluminescence intensity.

physics.app-ph↗

arXiv2021

Quantum violation of local causality in urban network with hybrid photonic technologies

Quantum networks play a crucial role for distributed quantum information processing, enabling the establishment of entanglement and quantum communication among distant nodes. Fundamentally, networks with independent sources allow for new forms of nonlocality, beyond the paradigmatic Bell's theorem. Here we implement the simplest of such networks -- the bilocality scenario -- in an urban network connecting different buildings with a fully scalable and hybrid approach. Two independent sources using different technologies, respectively a quantum dot and a nonlinear crystal, are used to share photonic entangled state among three nodes connected through a 270 m free-space channel and fiber links. By violating a suitable non-linear Bell inequality, we demonstrate the nonlocal behaviour of the correlations among the nodes of the network. Our results pave the way towards the realization of more complex networks and the implementation of quantum communication protocols in an urban environment, leveraging on the capabilities of hybrid photonic technologies.

quant-ph↗

arXiv2021

GaAs quantum dots grown by droplet etching epitaxy as quantum light sources

This paper presents an overview and perspectives on the epitaxial growth and optical properties of GaAs quantum dots obtained with the droplet etching method as high-quality sources of quantum light. We illustrate the recent achievements regarding the generation of single photons and polarization entangled photon pairs and the use of these sources in applications of central importance in quantum communication, such as entanglement swapping and quantum key distribution.

quant-ph↗

arXiv2021

Entanglement-based quantum key distribution with a blinking-free quantum dot operated at a temperature up to 20 K

Entanglement-based quantum key distribution promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multi-photon emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures between 4 and 8 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryo-coolers could facilitate the widespread deployment of quantum dots, e.g. in satellite-based quantum communication. Here we demonstrate blinking-free emission of highly entangled photon pairs from GaAs quantum dots embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based quantum key distribution between two building with an average key rate of 55 bits/s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, quantum dots will keep approaching the ideal source of entangled photons for real world applications.

quant-ph↗

arXiv2021

Quantum dot technology for quantum repeaters: from entangled photon generation towards the integration with quantum memories

The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as among one of the most promising solutions. In this work, we focus on memory-based quantum-repeater schemes that rely on semiconductor quantum dots for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural-artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with quantum dots and highlight the next challenges ahead for the implementation of practical quantum networks.

quant-ph↗

arXiv2021

Droplet Epitaxy of Semiconductor Nanostructures for Quantum Photonic Devices

The long dreamed quantum internet would consist of a network of quantum nodes (solid-state or atomic systems) linked by flying qubits, naturally based on photons, travelling over long distances at the speed of light, with negligible decoherence. A key component is a light source, able to provide single or entangled photon pairs. Among the different platforms, semiconductor quantum dots are very attractive, as they can be integrated with other photonic and electronic components in miniaturized chips. In the early 1990s two approaches were developed to synthetize self-assembled epitaxial semiconductor quantum dots (QDs), or artificial atoms, namely the Stranski-Krastanov (SK) and the droplet epitaxy (DE) method. Because of its robustness and simplicity, the SK method became the workhorse to achieve several breakthroughs in both fundamental and technological areas. The need for specific emission wavelengths or structural and optical properties has nevertheless motivated further research on the DE method and its more recent development, the local-droplet-etching (LDE), as complementary routes to obtain high quality semiconductor nanostructures. The recent reports on the generation of highly entangled photon pairs, combined with good photon indistinguishability, suggest that DE and LDE QDs may complement (and sometime even outperform) conventional SK InGaAs QDs as quantum emitters. We present here a critical survey of the state of the art of DE and LDE, highlighting the advantages and weaknesses, the obtained achievements and the still open challenges, in view of applications in quantum communication and technology.

physics.optics↗

arXiv2021

Cross-plane thermal conductivity of GaN/AlN superlattices

Heterostructures consisting of alternating GaN/AlN epitaxial layers represent the building-blocks of state-of-the-art devices employed for active cooling and energy-saving lightning. Insights into the heat conduction of these structures are essential in the perspective of improving the heat management for prospective applications. Here, the cross-plane (perpendicular to the sample's surface) thermal conductivity of GaN/AlN superlattices as a function of the layers' thickness is established by employing the $3ω$-method. Moreover, the role of interdiffusion at the interfaces on the phonon scattering is taken into account in the modelling and data treatment. It is found, that the cross-plane thermal conductivity of the epitaxial heterostructures can be driven to values as low as 5.9 W/(m$\cdot$K) comparable with those reported for amorphous films, thus opening wide perspectives for optimized heat management in III-nitride-based epitaxial multilayers.

cond-mat.mtrl-sci↗

arXiv2020

Quantum dots as potential sources of strongly entangled photons for quantum networks

The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies, with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow to overcome this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. Despite the impressive progress made so far, a definitive scalable photon source fulfilling such requirements is still being sought for. Semiconductor quantum dots excel in this context as sub-poissonian sources of polarization entangled photon pairs. In this work we present the state-of-the-art set by GaAs based quantum dots and use them as a benchmark to discuss the challenges to overcome towards the realization of practical quantum networks.

quant-ph↗

arXiv2020

A Hybrid (Al)GaAs-LiNbO$_3$ Surface Acoustic Wave Resonator for Cavity Quantum Dot Optomechanics

A hybrid device comprising a (Al)GaAs quantum dot heterostructure and a LiNbO$_3$ surface acoustic wave resonator is fabricated by heterointegration. High acoustic quality factors $Q>4000$ are demonstrated for an operation frequency $f\approx 300$ MHz. The measured large quality factor-frequency products $Q\times f>10^{12}$ ensures the suppression of decoherence due to thermal noise for temperatures exceeding $T>50\,\mathrm{K}$. Frequency and position dependent optomechanical coupling of single quantum dots and the resonator modes is observed.

physics.optics↗

arXiv2020

Large-Range Frequency Tuning of a Narrow-Linewidth Quantum Emitter

A hybrid system of a semiconductor quantum dot single photon source and a rubidium quantum memory represents a promising architecture for future photonic quantum repeaters. One of the key challenges lies in matching the emission frequency of quantum dots with the transition frequency of rubidium atoms while preserving the relevant emission properties. Here, we demonstrate the bidirectional frequency-tuning of the emission from a narrow-linewidth (close-to-transform-limited) quantum dot. The frequency tuning is based on a piezoelectric strain-amplification device, which can apply significant stress to thick bulk samples. The induced strain shifts the emission frequency of the quantum dot over a total range of $1.15\ \text{THz}$, about three orders of magnitude larger than its linewidth. Throughout the whole tuning process, both the spectral properties of the quantum dot and its single-photon emission characteristics are preserved. Our results show that external stress can be used as a promising tool for reversible frequency tuning of high-quality quantum dots and pave the wave towards the realisation of a quantum dot -- rubidium atoms interface for quantum networking.

cond-mat.mes-hall↗

arXiv2020

Quantum cryptography with highly entangled photons from semiconductor quantum dots

State-of-the-art quantum key distribution systems are based on the BB84 protocol and single photons generated by lasers. These implementations suffer from range limitations and security loopholes, which require expensive adaptation. The use of polarization entangled photon pairs substantially alleviates the security threads while allowing for basically arbitrary transmission distances when embedded in quantum repeater schemes. Semiconductor quantum dots are capable of emitting highly entangled photon pairs with ultra-low multi-pair emission probability even at maximum brightness. Here we report on the first implementation of the BBM92 protocol using a quantum dot source with an entanglement fidelity as high as 0.97(1). For a proof of principle, the key generation is performed between two buildings, connected by 350 metre long fiber, resulting in an average key rate of 135 bits/s and a qubit error rate of 0.019 over a time span of 13 hours, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By embedding them in state-of-the-art photonic structures, key generation rates in the Gbit/s range are at reach.

quant-ph↗

arXiv2020

Reconfigurable quantum photonics with on-chip detectors

Integrated quantum photonics offers a promising path to scale up quantum optics experiments by miniaturizing and stabilizing complex laboratory setups. Central elements of quantum integrated photonics are quantum emitters, memories, detectors, and reconfigurable photonic circuits. In particular, integrated detectors not only offer optical readout but, when interfaced with reconfigurable circuits, allow feedback and adaptive control, crucial for deterministic quantum teleportation, training of neural networks, and stabilization of complex circuits. However, the heat generated by thermally reconfigurable photonics is incompatible with heat-sensitive superconducting single-photon detectors, and thus their on-chip co-integration remains elusive. Here we show low-power microelectromechanical reconfiguration of integrated photonic circuits interfaced with superconducting single-photon detectors on the same chip. We demonstrate three key functionalities for photonic quantum technologies: 28 dB high-extinction routing of classical and quantum light, 90 dB high-dynamic range single-photon detection, and stabilization of optical excitation over 12 dB power variation. Our platform enables heat-load free reconfigurable linear optics and adaptive control, critical for quantum state preparation and quantum logic in large-scale quantum photonics applications.

physics.app-ph↗

arXiv2020

The crux of using the cascaded emission of a 3-level quantum ladder system to generate indistinguishable photons

We investigate the degree of indistinguishability of cascaded photons emitted from a 3-level quantum ladder system; in our case the biexciton-exciton cascade of semiconductor quantum dots. For the 3-level quantum ladder system we theoretically demonstrate that the indistinguishability is inherently limited for both emitted photons and determined by the ratio of the lifetimes of the excited and intermediate states. We experimentally confirm this finding by comparing the quantum interference visibility of non-cascaded emission and cascaded emission from the same semiconductor quantum dot. Quantum optical simulations produce very good agreement with the measurements and allow to explore a large parameter space. Based on our model, we propose photonic structures to optimize the lifetime ratio and overcome the limited indistinguishability of cascaded photon emission from a 3-level quantum ladder system.

cond-mat.mes-hall↗

arXiv2020

Quantum Teleportation with Imperfect Quantum Dots

Efficient all-photonic quantum teleportation requires fast and deterministic sources of highly indistinguishable and entangled photons. Solid-state-based quantum emitters--notably semiconductor quantum dots--are a promising candidate for the role. However, despite the remarkable progress in nanofabrication, proof-of-concept demonstrations of quantum teleportation have highlighted that imperfections of the emitter still place a major roadblock in the way of applications. Here, rather than focusing on source optimization strategies, we deal with imperfections and study different teleportation protocols with the goal of identifying the one with maximal teleportation fidelity. Using a quantum dot with sub-par values of entanglement and photon indistinguishability, we show that the average teleportation fidelity can be raised from below the classical limit to 0.842(14). Our results, which are backed by a theoretical model that quantitatively explains the experimental findings, loosen the very stringent requirements set on the ideal entangled-photon source and highlight that imperfect quantum dots can still have a say in teleportation-based quantum communication architectures.

quant-ph↗

arXiv2020

The Origin of Antibunching in Resonance Fluorescence

Epitaxial quantum dots have emerged as one of the best single-photon sources, not only for applications in photonic quantum technologies but also for testing fundamental properties of quantum optics. One intriguing observation in this area is the scattering of photons with subnatural linewidth from a two-level system under resonant continuous wave excitation. In particular, an open question is whether these subnatural linewidth photons exhibit simultaneously antibunching as an evidence of single-photon emission. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. First, we independently confirm single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our experimental work is consistent with recent theoretical findings, that explain antibunching from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state.

cond-mat.mes-hall↗

arXiv2019

Correlations between Optical Properties and Voronoi-Cell Area of Quantum Dots

A semiconductor quantum dot (QD) can generate highly indistinguishable single-photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nano-holes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture zone-area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD-density in which atoms diffuse over $μ$m-distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved.

cond-mat.mes-hall↗