Broadband parametric amplification for multiplexed SiMOS quantum dot signals

Silicon-coated CdZnSeS/ZnS quantum dots contribute to great performance white light-emitting diodes

Quantum dots (QDs) have extraordinary strong light absorption and size tunable bandgap. However, QD films are typically limited to ~200-300 nm due to their poor charge mobility. This severely limits the quantum efficiency of QD devices for λ <750 nm (infrared). Herein, we report a record 1 μm thick QD film using intercalated graphene layers as transparent current extractors. This overcomes QD poor mobility, ensuring both effective light absorption and charge extraction towards the near-infrared reaching quantum efficiency (EQE) of 90%. The short diffusion length (LD<200 nm) of QDs limits their useful thickness to ~200-300 nm1–4 , resulting in poor infrared light absorption. To overcome this limitation, we have built a 1 µm thick QD film with intercalated transparent graphene electrodes that keep high charge collection efficiency. As a result, the 1 µm intercalated devices show a superior EQE reaching 90% at λ ~800 nm without the drop of quantum efficiency at λ ~700 nm observed in most QD devices. The EQE of intercalated devices improves over the entire λ~ 600-1100 nm spectrum as the thickness increases from 100 nm to 1 μm, clearly breaking the restriction that the diffusion length of QDs imposes on the film thickness. This improves absorption and charge collection in the infrared.

QmeQ 1.0: An open-source Python package for calculations of transport through quantum dot devices

Understanding the electrical and optical properties of InAs/GaAs(Sb) quantum dots (QDs) is essential for designing and improving the performance of related semiconductor QD devices. In this paper, the effects of In segregation, QD size, capping layer thickness and Sb composition on the electrical and optical properties of type I and type II InAs/GaAs1−x Sb x ( 0⩽x⩽1 ) QDs are systematically investigated and general regularities are summarized. The comparisons of electron distribution probabilities, interband and intraband transition energies and absorption coefficients show that In segregation and QD size have a strong influence on the electron energy level positions of InAs QDs and that the trade-offs between absorption peak energy, absorption intensity and radiative lifetime of InAs/GaAs1−x Sb x QDs can be optimized by adjusting the thickness and Sb composition of the GaAs1−x Sb x capping layer. The results obtained are promising for the applications in the pre-design and performance feedback of the QD devices such as QD lasers, QD infrared detectors and intermediate band solar cells.

&lt;sec&gt;Spin qubits in silicon-based semiconductor quantum dots have become one of the prominent candidates for realizing fault-tolerant quantum computing due to their long coherence time, good controllability, and compatibility with modern advanced integrated circuit manufacturing processes. In recent years, due to the remarkable advances in silicon-based materials, the structure of quantum dot and its fabrication process, and qubit manipulation technology, the great progress of high-fidelity state preparation and readout, single- and two-qubit gates of spin qubits for silicon based semiconductor quantum computation has been achieved. The control fidelities for single- and two-qubit gates all exceed 99%—fault tolerance threshold required by the surface code known for its exceptionally high tolerance to errors. In this paper, we briefly introduce the basic concepts of silicon-based semiconductor quantum dots, discuss the state-of-art technologies used to improve the fidelities of single- and two-qubit gates, and finally highlight the research directions that should be focused on.&lt;/sec&gt;&lt;sec&gt;This paper is organized as follows. Firstly, we introduce three major types of quantum dot (QD) devices fabricated on different silicon-based substrates, including Si/SiGe heterojunction and Si/SiO&lt;sub&gt;2&lt;/sub&gt;. The spin degree of electron or nucleus hosted in QD can be encoded into spin qubits. Electron spin qubits can be thermally initialized to ground state by using an electron reservoirs, and can be read out by spin-charge conversion mechanism: energy-selective readout (Elzerman readout) with reservoirs or Pauli spin blockade (PSB) without reservoirs. Additionally, high fidelity single-shot readout has been demonstrated by using radio-frequency gate reflectometry combined with the PSB, which has unique advantages in large-scale qubit array. To coherently control the spin qubits, electron dipole resonance (ESR) or electron dipole spin resonance (EDSR) for electron and nuclear magnetic resonance (NMR) for nucleus are introduced. With the help of isotope purification greatly improving the dephasing time of qubit and fast single-qubit manipulation based on EDSR, fidelity above 99.9% can be reached. For the two-qubit gates based on exchange interaction between electron spins, the strength of interaction &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$ J $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; combined with Zeeman energy difference &lt;inline-formula&gt;&lt;tex-math id="M2"&gt;\begin{document}$ \Delta {E}_{z} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; determines the energy levels of system, which lead to the different two-qubit gates, such as controlled-Z (CZ), controlled-Rotation (CROT), and the square root of the SWAP gate (&lt;inline-formula&gt;&lt;tex-math id="M3"&gt;\begin{document}$ \sqrt{\rm{S}\rm{W}\rm{A}\rm{P}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M3.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221900_M3.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;) gates. In order to improve the fidelity of two-qubit gates, a series of key technologies is used experimentally, they being isotope purification, symmetry operation, careful Hamiltonian engineering, and gate set tomography. Fidelity of two-qubit gates exceeding 99% has been demonstrated for electron spin qubits in Si/SiGe quantum dots and nuclear spin qubits in donors. These advances have pushed the silicon-based spin qubit platform to become a major stepping stone towards fault-tolerant quantum computation. Finally, we discuss the future study of spin qubits, that is, how to effectively expand the number of qubits, and many other problems to be explored and solved.&lt;/sec&gt;

Lead sulfide quantum dots (QDs) were grown in situ on nanoporous TiO 2 by successive ion layer adsorption and reaction (SILAR) and by atomic layer deposition (ALD), to fabricate solid-state quantum-dot sensitized solar cells (QDSSCs). With the ultimate goal of increasing QD surface coverage, this work compares the impact of these two synthetic routes on the light absorption and electrical properties of devices. A higher current density was observed in the SILAR-grown QD devices under reverse bias, as compared to ALD-grown QD devices, attributed to injection problems of the lower-band-gap QDs present in the SILAR-grown QD device. To understand the effects of QD surface coverage on device performance, particularly interfacial recombination, electron lifetimes were measured for varying QD deposition cycles. Electron lifetimes were found to decrease with increasing SILAR cycles, indicating that the expected decrease in recombination between electrons in the TiO2 and holes in the hole-transport material, due to increased QD surface coverage, is not the dominant effect of increased deposition cycles. © 2013 IEEE.

In this work, we report the fabrication of a charge-tunable GaAs/Al0.25Ga0.75As quantum dot (QD) device containing QDs deposited by modified droplet epitaxy producing almost strain and composition gradient free QDs. We obtained a QD density in the low 109 cm−2 range that enables us to perform spectroscopy on single droplet QDs showing linewidths as narrow as 40 μeV. The integration of the QDs into a Schottky diode allows us to controllably charge a single QD with up to four electrons, while non-classical photoluminescence is proven by photon auto-correlation measurements showing photon-antibunching (g(2)(0) = 0.05).

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

Recent advances in nanofabrication technologies have enabled us to fabricate semiconductor quantum dots in which electrons are three-dimensionally confined. These quantum dots are often referred to as artificial atoms since their electronic properties—for example the ionization energy and discrete excitation spectrum—resemble those of real atoms. Electrons bound to a nucleus potential encounter sufficiently strong effects of quantum-mechanical confinement and mutual Coulomb interactions that they are well arranged in ordered states, and this leads to the arrangement of atoms in the periodic table. It is well known in atom physics that the threedimensional spherically symmetric potential around atoms gives rise to the shell structure 1s, 2s, 2p, 3s, 3p,…. The ionization energy has a large maximum for atomic numbers 2, 10, 18,…. Up to atomic number 23, these shells are filled sequentially. Hund's rule determines whether a spin-down or a spin-up electron is added. This article describes how closely we can approach the electronic properties of real atoms through the use of semiconductor quantum dots.Both the effects of quantum confinement and Coulomb interaction become strong in quantum dots when the dot size is comparable to the electron wavelength and contains just a few electrons. The consequence of these factors on transport have only recently been studied in vertical-dot devices, which contain a dot located between source and drain contacts by means of heterostructure tunnel barriers because the few-electron regime is only accessible in the vertical-dot device. Studies include transport measurements through submicron resonant tunneling devices and submicron gated resonant-tunneling devices, and capacitance measurements on submicron double-barrier structures. However quantum-dot devices usually contain some disorder—for instance because of impurities or when the shape of the dot is irregular—which readily causes sample specific inhomogeneity in the electronic properties. Clean quantum dots, in the form of regular disks, have only recently been fabricated in a semiconductor heterostructure (Figure 1), and have been used to study the atomlike properties of artificial atoms.

Self-assembled In(Ga)As/GaAs quantum dots (QDs), known as "artificial atoms" for their fully quantized electronic states, show unique physical properties and new prospects for applications. Based on the QDs, an increasing number of leading laboratories on the world engage in developing novel optoelectronic devices with special advantages over existing devices. This paper reviews the current research developments in self-assembled QD devices in China, covering QD lasers and inter sub-level QD photodetectors. QD devices in China are undergoing a rapid advance and have achieved the world's best results in terms of certain characteristics.

Stranski-Krastanov (SK) Quantum dots (QDs) based lasers and detectors are now used in many fields because of the plentiful advantages they offer, some of which are normal incident absorption and phonon bottleneck. In order to optimize the properties of the devices, growth parameters have to be tuned properly. The effects of growth temperature (GT) and growth rate (GR) variations on the QD devices have been studied here by comparing simulations and experiments. In this study, InAs QDs are grown on GaAs substrates with four different temperatures, from 480&deg;C to 510&deg;C, and with five different GRs, from 0.15 ML/s to 0.025ML/s. To observe the grown heterostructures' structural and optical properties photoluminescence (PL) and PL excitation (PLE) have been performed on the samples. The size, shape, and composition of the QDs ultimately decide the energy levels in the heterostructure. Hence, it determines the optical and electrical properties of the devices. Here we simulated 3-D strain profiles of the QD and compared the results with PL and PLE. The trends in simulated biaxial strain and heavy hole (HH) - light hole (LH) band splitting, observed in the PLE, match pretty well. We observed that the change in GT drastically affects the composition of the dots and the wetting layer, whereas a change in GR only changes the lateral size of the QDs, and do not affect the strain or composition. These studies can be beneficial for p-i-p short-wave infrared (SWIR) detectors since their spectral response is tuned by the HH-LH band splitting.

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (0-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In-GaAsP-InP quantum well (2-D), all of which operate near 1.55-mum wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer.

We develop a model for slow light based on the coherent population oscillation of quantum dots at room temperature. With the absorption dip and corresponding variation of the refractive index due to the coherent population beating induced by the pump and probe signal, quantum dots can be used as a slow-light medium. Our theoretical model matches the experimental results very well. We also experimentally and theoretically demonstrate that both the forward-bias injection current and reverse-bias voltage can change the group index in a semiconductor quantum-dot waveguide. Our results indicate that quantum-dot devices can be potentially used as electrically and optically controllable slow light devices

Carbon nanotubes (CNTs) are attractive building blocks for quantum dots because of their extremely small diameters. In this chapter, typical behaviors of the carbon nanotube quantum dots and their artificial atom features are discussed. Then, some applications to the quantum-dot devices are presented. Some challenges on the necessary technologies toward integrated quantum-dot devices are also described.

We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.