Multiple Quantum Dots Research Articles

Detuning Axis Pulsed Spectroscopy of Valley-Orbital States in Si / Si - Ge Quantum Dots

Silicon-quantum-dot qubits must contend with low-lying valley excited states that are sensitive functions of the quantum-well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated $\mathrm{Si}$/$\mathrm{Si}$-$\mathrm{Ge}$ double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited-state energies spanning multiple wafers, quantum dots, and orbital states, which are crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for increasing valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley-splitting measurements for guiding the development of $\mathrm{Si}$ qubits.

Filter-Assisted Separation of Multiple Nanomaterials: Mechanism and Application in Atomic/Mass Spectrometry/Fluorescence Label-Free Multimode Bioassays.

Atomic spectrometry (AS) has been widely used in bioassay, but it requires steps to immobilize or separate the signal molecules. In this work, based on the phenomenon that the filter membrane can selectively separate multiple nanomaterials (nanoparticles (NPs) and quantum dots (QDs)) and its related ions, including poly(thymine)-templated Cu NPs and free Cu2+, Ag NPs and free Ag+, CdTe QDs and Cd2+, we constructed multimode and label-free biosensors by chemical vapor generation-atomic fluorescence spectrometry (CVG-AFS), inductively coupled plasma mass spectrometry (ICP-MS), and fluorescence. In this strategy, terminal deoxynucleotidyl transferase (TdT) and polynucleotide kinase (PNK), H2O2, and mucin 1 can be sensitively detected using Cu2+, Ag+, and Cd2+ as the signal probe, respectively. As a result, TdT and T4 PNK in single cells level can be accurately quantified. In addition, the possible separation mechanism of filter membrane was proposed, both Donnan repulsion by charged functional layer and entrapment effect by nanomaterials size contributed to the outstanding separation performance. Subsequently, on the basis that CdTe QDs can selectively identify Cu NPs/Cu2+, Ag NPs/Ag+, and C-Ag+-C/Ag+, cation-exchange reaction (CER) was introduced in this platform due to its unique advantages, including improving the sensitivity of the above system (an order of magnitude), converting the non-CVG metal elements into CVG elements, and using low-cost AFS to substitute the high-cost ICP-MS. In addition, we performed theoretical calculations of the selective CER using density functional theory (DFT). Therefore, this label-free and simple separation AS/ICP-MS sensing platform shows great potential for biomarker analysis.

Erratum: “Individual two-axis control of three singlet-triplet qubits in a micromagnet integrated quantum dot array” [Appl. Phys. Lett. 117, 234001 (2020)

We report individual confinement and two-axis qubit operations of two electron spin qubits in GaAs gate-defined sextuple quantum dot array with integrated micro-magnet. As a first step toward multiple qubit operations, we demonstrate coherent manipulations of three singlet-triplet qubits showing underdamped Larmor and Ramsey oscillations in all double dot sites. We provide an accurate measure of site site-dependent field gradients and rms electric and magnetic noise, and we discuss the adequacy of simple rectangular micro-magnet for practical use in multiple quantum dot arrays. We also discuss current limitations and possible strategies for realizing simultaneous multi multi-qubit operations in extended linear arrays.

A highly tunable quadruple quantum dot in a narrow bandgap semiconductor InAs nanowire.

Quantum dots (QDs) made from semiconductors are among the most promising platforms for the development of quantum computing and simulation chips, and they have the advantages of high density integration and compatibility with the standard semiconductor chip fabrication technology compared to other platforms. However, the development of a highly tunable semiconductor multiple QD system still remains a major challenge. Here, we demonstrate the realization of a highly tunable linear quadruple QD (QQD) in a narrow bandgap semiconductor InAs nanowire via a fine finger gate technique. The QQD is studied by electron transport measurements in the linear response regime. Characteristic two-dimensional charge stability diagrams containing four groups of resonant current lines of different slopes are obtained for the QQD. It is shown that these current lines arise from and can be individually assigned to resonant electron transport through the energy levels of different QDs. Benefitting from the excellent gate tunability, we also demonstrate the tuning of the QQD to regimes where the energy levels of two QDs, three QDs and all four QDs are energetically in resonance, respectively, with the Fermi level of the source and drain contacts. A capacitance network model is developed for the linear QQD and the simulated charge stability diagrams based on this model show good agreement with the experiments. Our work provides solid experimental evidence that narrow bandgap semiconductor nanowire multiple QDs could be used as a versatile platform to achieve integrated qubits for quantum computing and to perform quantum simulations of complex many-body systems.

A supramolecular single-site photocatalyst based on multi-to-one Förster resonance energy transfer.

A multi-to-one supramolecular photocatalyst is fabricated by aqueous electrostatic self-assembly of multiple graphene quantum dot (GQD) antennas onto a single-Pt-site porphyrin unimolecular micelle (PtTHPD-UM) catalytic center. Light energy is transferred from GQDs to PtTHPD-UM to catalyze water splitting into hydrogen up to 57 190 μmol g(Pt)-1 h-1 under visible light.

A charge sensor integration to tunable double quantum dots on two neighboring InAs nanowires.

A single quantum dot serving as a charge sensor is integrated to scalable double quantum dots using local top finger-gate techniques on two neighboring pure-phase InAs nanowires. The single dot built on one nanowire capacitively couples one of the double dots constructed on another nanowire via a metal bridge gate. The charge occupation states of double quantum dots can be accurately monitored by the sensor even in a few-electron regime in which transport tunneling current through the double dots vanishes. In the tunneling spectroscopy of double dots, electron inter dot tunneling process is absent; however, it can be illustrated by the sensor in terms of a transconductance line between the two closest triple points. Thus, tunnel coupling strength between the double dots is quantitatively extracted from the detectable charge transition. The highly tunable multiple quantum dots with integrated charge sensors on InAs nanowires could be an essential building block for quantum information processing technology.

Atomistic modeling of InGaN/GaN quantum dots-in-nanowire for graded surface-emitting low-threshold, blue exciton laser

While the development of the quantum photonics field, including emitter–light coupling using photonic system, has given a lot of attention, it offers quantum control of light. However, the main challenge is coupling to strongly photonic mode localization with nanosystem accuracy. This paper highlights the numerical simulations employed by atomically spin tight-binding model of realistically sized InGaN/GaN dots-in nanowire (NW) architectures. The effects of active region size/shapes, crystal growth directions, graded surface/interface properties, strain relaxation distributions, and polarization-induced potentials on the oscillator strengths of InGaN-based lasers have been investigated. The 3D nanofocused photonic modes, which have been deterministically coupled to multiple quantum dots (QDs) through graded surface/interface technique, have been demonstrated. By using a thin graded layers of a site-controlled pyramidal QDs, the photonic nanofocusing on these QDs at the nonpolar pyramid apex has been geometrically accomplished and successfully leads to stronger characteristics in terms of exciton bandgap energy and polarized emission rate compared to its polar counterpart. For optimum coupling, the nonpolar NW, with intrinsically lower built-in field, exhibits an enhancement of the QDs emission rate as high as 0.98, which is 12% greater than that in a recently reported semipolar MQD structure. The atomistic simulated emission rate for the core QDs buried in NW structure is then incorporated into a TCAD simulator to obtain laser device characteristics. Here, the achievement of truncated pyramid–electrically injected graded surface-emitting laser by nanofocusing the photonic modes formed in InGaN NW has been reported. The nonpolar laser device operates at ~ 402 nm and exhibits a threshold current of ~ 391 A/cm2, which is lower nine order of magnitude lower compared to recently reported semipolar green laser diodes. Our model benchmarking has been done against a reported experiment of polar InGaN disks in GaN NW. Significantly, this engineering innovation proves the viability of InGaN nonpolar quantum dots-in-nanowire architecture as low threshold, high polarized, coherent blue nanoscale lasing emitters, and opens future trends toward a next-generation of electrically injected and interfacial grading-emitting nanolasers operating at high-frequency up to a GHz range.

Individual two-axis control of three singlet-triplet qubits in a micromagnet integrated quantum dot array

The individual confinement and two-axis qubit operations of two-electron spin qubits in a GaAs gate-defined sextuple quantum dot array with an integrated micromagnet are reported in this study. As a first step toward multiple-qubit operations, we demonstrate coherent manipulations of three singlet-triplet qubits showing underdamped Larmor and Ramsey oscillations in all double dot sites. An accurate measurement of site-dependent field gradients as well as root-mean-squared electric and magnetic noise is provided, which is followed by a discussion of the adequacy of a simple rectangular micromagnet for practical use in multiple quantum dot arrays. Current limitations and possible strategies for achieving simultaneous multi-qubit operations in extended linear arrays are also presented.

Theory of interactions between cavity photons induced by a mesoscopic circuit

We use a quantum path integral approach to describe the behavior of a microwave cavity coupled to a dissipative mesoscopic circuit. We integrate out the mesoscopic electronic degrees of freedom to obtain a cavity effective action at fourth order in the light/matter coupling. By studying the structure of this action, we establish sufficient conditions in which the cavity dynamics can be described with a Lindblad equation. This equation depends on effective parameters set by electronic correlation functions. It reveals that the mesoscopic circuit induces an effective Kerr interaction and two-photon dissipative processes. We use our method to study the effective dynamics of a cavity coupled to a double quantum dot with normal metal reservoirs. If the cavity is driven at twice its frequency, the double dot circuit generates photonic squeezing and non-classicalities visible in the cavity Wigner function. In particular, we find a counterintuitive situation where mesoscopic dissipation enables the production of photonic Schr\"odinger cats. These effects can occur for realistic circuit parameters. Our method can be generalized straightforwardly to more complex circuit geometries with, for instance, multiple quantum dots, and other types of fermionic reservoirs such as superconductors and ferromagnets.

Analysis of a multiple-quantum-dots embedded ring structure for potential optically-controlled quantum switch or spin filter

We theoretically study the average current through a ring embedded with multiple quantum dots in each arm subjected to a time-dependent external field. A current resonance band can be observed in a six-quantum-dot system. In the presence of a time-dependent external field, mutual transformation occurs between the resonance band and antiresonance band, indicating an effective optically-controlled quantum switch can be realized in a wider quantum dot’s energy regime. As the Zeeman effect is introduced, the conversion between 100 and − 100% for spin polarization p can be realized by adjusting the frequency of time-dependent external field, suggesting a physical scheme of an optically-controlled spin filter. The present work sheds lights onto the design and quantum computation of future nano-devices.

Formation of quantum dots in GaN/AlGaN FETs

GaN and the heterostructures are attractive in condensed matter science and applications for electronic devices. We measure the electron transport in GaN/AlGaN field-effect transistors (FETs) at cryogenic temperature. We observe formation of quantum dots in the conduction channel near the depletion of the 2-dimensional electron gas (2DEG). Multiple quantum dots are formed in the disordered potential induced by impurities in the FET conduction channel. We also measure the gate insulator dependence of the transport properties. These results can be utilized for the development of quantum dot devices utilizing GaN/AlGaN heterostructures and evaluation of the impurities in GaN/AlGaN FET channels.

All-optical charging and charge transport in quantum dots

Optically active quantum dots are one of the promising candidates for fundamental building blocks in quantum technology. Many practical applications would comprise of multiple coupled quantum dots, each of which must be individually chargeable. However, the most advanced demonstrations are limited to devices with only a single dot, and individual charging has neither been demonstrated nor proposed for an array of optically active quantum dots. Here we propose and numerically demonstrate a method for controlled charging of multiple quantum dots and charge transport between the dots. We show that our method can be implemented in realistic structures with fidelities greater than 99.9%. The scheme is based on all-optical resonant manipulation of charges in an array of quantum dots formed by a type-II band alignment, such as crystal-phase quantum dots in nanowires. Our work opens new practical avenues for realizations of advanced quantum photonic devices, for instance, solid-state quantum registers with a photonic interface.

Multiplexed Single Photons from Deterministically Positioned Nanowire Quantum Dots

Solid-state quantum emitters are excellent sources of on-demand indistinguishable or entangled photons and can host long-lived spin memories, crucial resources for photonic quantum information applications. However, their scalability remains an outstanding challenge. Here we present a scalable technique to multiplex streams of photons from multiple independent quantum dots, on-chip, into a fiber network for use off-chip. Multiplexing is achieved by incorporating a multi-core fiber into a confocal microscope and spatially matching the multiple foci, seven in this case, to quantum dots in an array of deterministically positioned nanowires. First, we report the coherent control of the emission of biexciton-exciton cascade from a single nanowire quantum dot under resonant two-photon excitation. Then, as a proof-of-principle demonstration, we perform parallel spectroscopy on the nanowire array to identify two nearly identical quantum dots at different positions which are subsequently tuned into resonance with an external magnetic field. Multiplexing of background-free single photons from these two quantum dots is then achieved. Our approach, applicable to all types of quantum emitters, can readily be scaled up to multiplex $>100$ quantum light sources, providing a breakthrough in hardware for photonic based quantum technologies. Immediate applications include quantum communication, quantum simulation, and quantum computation.

A nonparabolic conduction band study of circular quantum dot optical properties: modeling of surface roughness by using Koch snowflakes

In this work, we investigate the optical properties of single and multiple quantum dots (QDs) based on AlxGa1-xAs/GaAs with two Koch snowflake-shaped and circular geometries. The idea is to determine the impact of the surface roughness of the circular QDs on the mentioned optical properties. This is an important issue in designing optoelectronic devices (e.g., photodetectors or light-emitting systems) because the position and magnitude of the absorption coefficient play important roles in devising them. We have also studied the effects of the conduction band nonparabolicity, QD size, number of QDs, and composition parameter x on the absorption coefficient. For this purpose, we have used an efficient finite difference method to solve the two-dimensional Schrodinger equation. The computational algorithm of the Koch snowflake-shaped QD production is also described.

Sensitive detection of virus with broad dynamic range based on highly bright quantum dot-embedded nanoprobe and magnetic beads

As virus spread can lead to severe epidemics and pandemics associated with high mortality, it is necessary to have a highly sensitive detection method for viruses. Although various detection methods have been developed so far, current methods in detecting a virus require preprocessing and involve quite intricate processes of low sensitivity. Here, we have developed a virus detection method with a broad dynamic range and high sensitivity, based on immuno-complex formation between quantum dot (QD)-embedded silica nanoparticles (QD2) and magnetic beads. The multiple QD- containing QD2s showed 500 times stronger photoluminescence than individual QDs. When biotin was immobilized as a ligand, streptavidin was detected in a range of 10zM to 10nM. The clinical applicability of the QD2-based system was examined using the avian virus (i.e., H1N1 influenza virus), and it showed a detection range of 4.76×10−4< span class="xps_thinspace"> ∼3.2 hemagglutination unit/mL. This result is comparable to the polymerase chain reaction method, and is approximately 2100 times more sensitive than the conventional hemagglutination method. Since the QD2-based system could detect target molecules with high sensitivity without requiring an amplification step, it can be applied in various biomedical and clinical fields.

(Invited) Exploring in the Near Infrared: Multifunctional Nanoplatforms for Biomedical Applications

Near-infrared (NIR) absorbing and emitting nanomaterials attract significant attention in bioimaging, which shows great potential for disease detection due to its high sensitivity at the subcellular level and low cost of related imaging facilities. However, currently available optical probes are mainly based on visible-emitting materials. The tissue-induced optical extinction and autofluorescence in the visible range result in limited penetration depth and ambiguous photoluminescence signal, which restricts their in vivo use. To address this issue, photoluminescent probes, with both absorption and emission wavelengths operating in the biological windows in the NIR range, in which tissues are optically transparent, are highly desired. Their integration with superparamagnetic nanomaterials to make a multifunctional platform further opens a wide range of promising applications, including bimodal imaging (photoluminescence and magnetic resonance imaging), synergistic hyperthermia (magnetothermal and photothermal), magnetic confinement of trace amounts of biospecies for ultra high-sensitivity biodetection, etc. In this talk, I will present our most recent work on the synthesis of NIR-emitting water soluble, stable core/shell/shell quantum dots (QDs) and multifunctional (NIR photoluminescent and superparamagnetic) nanoparticles and their use in biomedicine. For instance, in one case, multifunctional particles contain single superparamagnetic nanoparticles as cores and NIR-luminescent nanomaterials as shells. In another case, the multifunctional nanoplatform is compose of multiple superparamagnetic nanoparticles and NIR quantum dots in single particles. These different types of multifunctional particles are designed for different biomedical applications.

Enhancing electrostatic coupling in silicon quantum dot array by dual gate oxide thickness for large-scale integration

We propose a structure with word/bit line control for a two-dimensional quantum dot array, which allows random access for arbitrary quantum dots with a small number of control signals. To control multiple quantum dots with a single signal, every quantum dot should have a wide operating voltage allowance to overcome the property variations. We fabricate two-dimensional quantum dot arrays using silicon-complementary-metal-oxide-semiconductor technology with an alternating dual-standard gate oxide thickness. The quantum dots are designed to have an allowable operating voltage window of 0.2 V to control the number of electrons, which is a window one order of magnitude wider than that of previous works. The proposed structure enables both easy fabrication and operation for multiple quantum dots and will pave the way for practical use of large-scale quantum computers.

Multiplexed Single-Photon Source Based on Multiple Quantum Dots Embedded within a Single Nanowire.

Photonics-based quantum information technologies require efficient, high emission rate sources of single photons. Position-controlled quantum dots embedded within a broadband nanowire waveguide provide a fully scalable route to fabricating highly efficient single-photon sources. However, emission rates for single-photon devices are limited by radiative recombination lifetimes. Here, we demonstrate a multiplexed single-photon source based on a multidot nanowire. Using epitaxially grown nanowires, we incorporate multiple energy-tuned dots, each optimally positioned within the nanowire waveguide, providing single photons with high efficiency. This linear scaling of the single-photon emission rate with number of emitters is demonstrated using a five-dot nanowire with an average multiphoton emission probability of <4% when excited at saturation. This represents the first ever demonstration of multiple single-photon emitters deterministically incorporated in a single photonic device and is a major step toward achieving GHz single-photon emission rates from a scalable multi-quantum-dot system.

Controlled growth of InGaN quantum dots on photoelectrochemically etched InGaN quantum dot templates

Controlled growth of InGaN quantum dots (QDs) using photoelectrochemically (PEC) etched InGaN QD templates is demonstrated. The InGaN QDs are grown by a self-assembly (SA) method using metal-organic chemical vapor deposition on templates consisting of planar GaN and PEC etched InGaN QDs for comparison. The InGaN QD templates are formed using quantum-size-controlled PEC etching of planar InGaN layers on GaN, which produces controlled QD radiuses with a statistical mean (μ) of 17.3 nm and standard deviation (σ) of 6.2 nm, and densities of 1.2 × 1010 cm−2. The PEC etched QDs are capped with an AlGaN interlayer and GaN barrier layer to recover a planar surface morphology for subsequent SA growth of QDs. The PEC QD templates behave as seeds via localize strain near the PEC QDs which provide improved control of the SA QD growth. The SA grown QDs on PEC QD templates are smaller and have controlled radiuses with μ = 21.7 nm and σ = 11.7 nm compared to the SA QDs on planar GaN templates with radiuses of μ = 37.8 nm and σ = 17.8 nm. Additionally, the dot densities of the SA QDs on PEC QD templates are ~3 times higher and more closely match the underlying densities of the template (8.1 × 109 cm−2). Multiple quantum dots (MQDs) are also grown on both templates that consist of 4 periods of SA QDs and AlGaN/GaN interlayer/barrier layers. The MQDs grown on PEC QD templates better retain their planarized smooth surfaces after barrier layer growth, and exhibit ~3 times stronger PL intensity at room temperature compared to MQDs grown on planar GaN.

Internal polarization electric field effects on the efficiency of InN/In[formula omitted]N multiple quantum dot solar cells

In this work, we investigate the influence of the internal electric field induced by the polarization inside the active region of the p-i-n photodiode on the characteristics of InN/InxGa1-xN quantum dots intermediate band solar cell. Considering the conduction and valence band offsets, the electron and hole energy levels have been determined by solving analytically the corresponding Schrödinger equations. The hole level, usually neglected in similar studies, is taken into account to determine all the intermediate transitions. All parameters of multiple quantum dot solar cells such as open-circuit voltage, short-circuit current density and photovoltaic conversion efficiency are determined as functions of the indium content, the internal electric field, inter-dot distances and dot sizes. Our calculations show that determining the photovoltaic conversion efficiency (η) without taking into account the internal electric field leads to an overestimation of η.