Quantum Dot Arrays Research Articles

Модель быстродействия волноводного фотодиода с квантовыми точками

A model is proposed that makes it possible to analytically analyze the speed performance of a waveguide p-i-n photodiode with a light-absorbing region representing a multilayered array of quantum dots separated by undoped spacers. It is shown that there is an optimal number of layers of quantum dots, as well as an optimal thickness of the spacers, which provide the widest bandwidth. The possibility of achieving a frequency range (at the level of -3 dB) above 20 GHz for waveguide photodiodes based on InGaAs/GaAs quantum well-dots is shown.

Ultrahigh-resolution quantum dot patterning for advanced optoelectronic devices.

Quantum dots have attracted significant scientific interest owing to their optoelectronic properties, which are distinct from their bulk counterparts. In order to fully utilize quantum dots for next generation devices with advanced functionalities, it is important to fabricate quantum dot colloids into dry patterns with desired feature sizes and shapes with respect to target applications. In this review, recent progress in ultrahigh-resolution quantum dot patterning technologies will be discussed, with emphasis on the characteristic advantages as well as the limitations of diverse technologies. This will provide guidelines for selecting suitable tools to handle quantum dot colloids throughout the fabrication of quantum dot based solid-state devices. Additionally, epitaxially fabricated single-particle level quantum dot arrays are discussed. These are extreme in terms of pattern resolution, and expand the potential application of quantum dots to quantum information processing.

Multi-Dimensional Quantum Capacitance of the Two-Site Hubbard Model: The Role of Tunable Interdot Tunneling.

Few-electron states confined in quantum-dot arrays are key objects in quantum computing. The discrimination between these states is essential for the readout of a (multi-)qubit state, and can be achieved through a measurement of the quantum capacitance within the gate-reflectometry approach. For a system controlled by several gates, the dependence of the measured capacitance on the direction of the oscillations in the voltage space is captured by the quantum capacitance matrix. Herein, we apply this tool to study a double quantum dot coupled to three gates, which enable the tuning of both the bias and the tunneling between the two dots. Analytical solutions for the two-electron case are derived within a Hubbard model, showing the overall dependence of the quantum capacitance matrix on the applied gate voltages. In particular, we investigate the role of the tunneling gate and reveal the possibility of exploiting interdot coherences in addition to charge displacements between the dots. Our results can be directly applied to double-dot experimental setups, and pave the way for further applications to larger arrays of quantum dots.

Engineering quantum states and electronic landscapes through surface molecular nanoarchitectures

Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scattering and confinement. Both atomic manipulation and supramolecular principles provide access to custom-designed molecular superlattices, which tailor the surface electronic landscape and influence fundamental chemical and physical properties at the nanoscale. Herein, we review the confinement of surface state electrons focusing on their interaction with molecule-based scaffolds created by molecular manipulation and self-assembly protocols under ultrahigh vacuum conditions. Starting from the quasi-free 2D electron gas present at the (111)-terminated surface planes of noble metals, we illustrate the enhanced molecule-based structural complexity and versatility compared to simple atoms. We survey low-dimensional confining structures in the form of artificial lattices, molecular nanogratings or quantum dot arrays, which are constructed upon appropriate choice of their building constituents. Whenever the realized (metal-)organic networks exhibit long-range order, modified surface band structures with characteristic features emerge, revealing intriguing physical properties, such as discretization, quantum coupling or energy and effective mass renormalization. Such collective electronic states can be additionally modified by positioning guest species at the voids of open nanoarchitectures [...].

Charge transport in single polymer fiber transistors in the sub-100 nm regime: temperature dependence and Coulomb blockade

Even though charge transport in semiconducting polymers is of relevance for a number of potential applications in (opto-)electronic devices, the fundamental mechanism of how charges are transported through organic polymers that are typically characterized by a complex nanostructure is still open. One of the challenges which we address here, is how to gain controllable experimental access to charge transport at the sub-100 nm lengthscale. To this end charge transport in single poly(diketopyrrolopyrrole-terthiophene) fiber transistors, employing two different solid gate dielectrics, a hybrid Al2O3/self-assembled monolayer and hexagonal boron nitride, is investigated in the sub-50 nm regime using electron-beam contact patterning. The electrical characteristics exhibit near ideal behavior at room temperature which demonstrates the general feasibility of the nanoscale contacting approach, even though the channels are only a few nanometers in width. At low temperatures, we observe nonlinear behavior in the current–voltage characteristics in the form of Coulomb diamonds which can be explained by the formation of an array of multiple quantum dots at cryogenic temperatures.

Autonomous Estimation of High-Dimensional Coulomb Diamonds from Sparse Measurements

Quantum dot arrays possess ground states governed by Coulomb energies, utilized prominently by singly occupied quantum dots, each implementing a spin qubit. For such quantum processors, the controlled transitions between ground states are of operational significance, as these allow the control of quantum information within the array such as qubit initialization and entangling gates. For few-dot arrays, ground states are traditionally mapped out by performing dense raster-scan measurements in control-voltage space. These become impractical for larger arrays due to the large number of measurements needed to sample the high-dimensional gate-voltage hypercube and the comparatively little information extracted. We develop a hardware-triggered detection method based on reflectometry, to acquire measurements directly corresponding to transitions between ground states. These measurements are distributed sparsely within the high-dimensional voltage space by executing line searches proposed by a learning algorithm. Our autonomous software-hardware algorithm accurately estimates the polytope of Coulomb blockade boundaries, experimentally demonstrated in a 2$\times$2 array of silicon quantum dots.

Micromagnet design for addressable fast spin manipulations in a 2 × 2 quantum dot array

The scaling up of semiconductor quantum dots to two-dimensional arrays is attracting considerable interest for use in large-scale spin-based quantum computation. One of the fundamental technologies to be realized in the two-dimensional arrays of quantum dots is the coherent manipulations of individual electron spins. In this work, we report on a micromagnet design that generates stray magnetic fields that can be used for spin manipulations via electric dipole spin resonance in a 2 × 2 quantum dot array. We consider a micromagnet with lower symmetry than that typically used in the case of linear dot arrays and optimize its dimensions to produce a maximum stray field gradient while maintaining Zeeman splitting differences sufficiently large among the four dots to permit qubit addressability. The optimized field gradient is around 60% smaller than that used in linear dot arrays, but it is still large enough for spin manipulation experiments. Our result represents an important step toward the experimental realization of spin-qubit operations in two-dimensional arrays of quantum dots.

Ligand-Assisted Coupling Manipulation for Efficient and Stable FAPbI3 Colloidal Quantum Dot Solar Cells.

For emerging perovskite quantum dots (QDs), understanding the surface features and their impact on the materials and devices is becoming increasingly urgent. In this family, hybrid FAPbI3 QDs (FA: formamidium) exhibit higher ambient stability, near-infrared absorption and sufficient carrier lifetime. However, hybrid QDs suffer from difficulty in modulating surface ligand, which is essential for constructing conductive QD arrays for photovoltaics. Herein, assisted by an ionic liquid formamidine thiocyanate, we report a facile surface reconfiguration methodology to modulate surface and manipulate electronic coupling of FAPbI3 QDs, which is exploited to enhance charge transport for fabricating high-quality QD arrays and photovoltaic devices. Finally, a record-high efficiency approaching 15 % is achieved for FAPbI3 QD solar cells, and they retain over 80 % of the initial efficiency after aging in ambient environment (20-30 % humidity, 25 °C) for over 600 h.

Ligand‐Assisted Coupling Manipulation for Efficient and Stable FAPbI3 Colloidal Quantum Dot Solar Cells

AbstractFor emerging perovskite quantum dots (QDs), understanding the surface features and their impact on the materials and devices is becoming increasingly urgent. In this family, hybrid FAPbI3 QDs (FA: formamidium) exhibit higher ambient stability, near‐infrared absorption and sufficient carrier lifetime. However, hybrid QDs suffer from difficulty in modulating surface ligand, which is essential for constructing conductive QD arrays for photovoltaics. Herein, assisted by an ionic liquid formamidine thiocyanate, we report a facile surface reconfiguration methodology to modulate surface and manipulate electronic coupling of FAPbI3 QDs, which is exploited to enhance charge transport for fabricating high‐quality QD arrays and photovoltaic devices. Finally, a record‐high efficiency approaching 15 % is achieved for FAPbI3 QD solar cells, and they retain over 80 % of the initial efficiency after aging in ambient environment (20–30 % humidity, 25 °C) for over 600 h.

Photoluminescence Properties of InAs Quantum Dots Overgrown by a Low-Temperature GaAs Layer under Different Arsenic Pressures

We studied the influence of the arsenic pressure during low-temperature GaAs overgrowth of InAs quantum dots on their optical properties. In the photoluminescence spectrum of quantum dots overgrown at a high arsenic pressure, we observed a single broad line corresponding to unimodal size distribution of quantum dots. Meanwhile, two distinct peaks (~1080 and ~1150 nm) at larger wavelengths are found in the spectra of samples with quantum dots overgrown at a low arsenic pressure. We attributed this phenomenon to the high-pressure suppression of atom diffusion between InAs islands at the overgrowth stage, which makes it possible to preserve the initial unimodal size distribution of quantum dots. The same overgrowth of quantum dots at the low arsenic pressure induces intensive mass transfer, which leads to the formation of arrays of quantum dots with larger sizes. Integrated photoluminescence intensity at 300 K is found to be lower for quantum dots overgrown at the higher arsenic pressure. However, a difference in the photoluminescence intensity for the high- and low-pressure overgrowths is not so significant for a temperature of 77 K. This indicates that excess arsenic incorporates into the capping layer at high arsenic pressures and creates numerous nonradiative recombination centers, diminishing the photoluminescence intensity.

Quantum-dot-like electrical transport of free-standing reduced graphene oxide paper at liquid helium temperatures

We present the results of a study of the electrical properties of macroscopic free-standing reduced graphene oxide paper (RGOP) with a 60 % fraction of sp 2 ‑carbons at liquid helium temperatures. Graphene domains form an array of quantum dots that determine material properties. At low temperatures, the electrical conductivity of the RGOP can be described by the model of tunneling of charge carriers through a disordered quasi-2D quantum dot array. In the free-standing RGOP at the temperature of 4.2 K, the charge carriers are localized in the quantum dots due to the Coulomb blockade. The effective localization length (~5–8 nm) of charge carriers coincides with the average size of the sp 2 ‑carbon regions (~6–8 nm). The transfer of charge carriers in the RGOP at liquid helium temperatures occurs due to tunneling between energetically close graphene domains, which are not nearest neighbors in space and are rather far from each other (up to ~100 nm). • Quantum-dot-like charge transfer in macroscopic free-standing RGO paper • Graphene domains array determines electrical properties of macroscopic RGO material. • Strong localization of charge carriers in free-standing RGO paper at low temperatures • Tunneling between energetically close graphene domains that are not nearest neighbors.

B-doped graphene quantum dots array as fluorescent sensor platforms for plasticizers detection

Human health and environmental safety are imperceptibly disturbed by illegally used plasticizers. A fluorescence sensor array based on B-doped graphene quantum dots was designed to detect six plasticizers. Specifically, the dexterous design is the first to disperse B-doped graphene quantum dots in two solvents of different polarities (N,N-Dimethylformamide and Cyclohexane), select three characteristic fluorescence peaks of B-doped graphene quantum dots in the above two solvents, and construct a 3 × 2 array, with a total of 6 array points. The basis of the detection is that the plasticizers affect the dispersion of B-doped graphene quantum dots in different solutions, which are accompanied by the change of its fluorescence signals. Or the plasticizers affect the electron transfer process of B-doped graphene quantum dots in different solvents, resulting in fluorescence quenching. Fluorescence information output by the sensor array showed a good linear relationship with the concentrations of the six plasticizers. The quantitative detection capability can be specified as: the detection range is 100 ng/mL-195 mg/mL, and the detection limit is 1.05–11.58 ng/mL. The fluorescence sensor array also showed good analytical performance in real samples, with recoveries ranging from 95% to 106%. This method has satisfactory specificity, reproducibility, stability and practicality, making it potential for application.

Non-adiabatic quantum control of quantum dot arrays with fixed exchange using Cartan decomposition.

In semiconductor spin qubits which typically interact through short-range exchange coupling, shuttling of spin is a practical way to generate quantum operations between distant qubits. Although the exchange is often tunable through voltages applied to gate electrodes, its minimal value can be significantly large, which hinders the applicability of existing shuttling protocols to such devices, requiring a different approach. In this work, we extend our previous results for double- and triple-dot systems, and describe a method for implementing spin state transfer in long chains of singly occupied quantum dots in a non-adiabatic manner. We make use of Cartan decomposition to break down the interacting problem into simpler problems in a systematic way, and use dynamical invariants to design smooth non-adiabatic pulses that can be implemented in devices with modest control bandwidth. Finally, we discuss the extensibility of our results to directed shuttling of spin states on two-dimensional lattices of quantum dots with fixed coupling. This article is part of the theme issue 'Shortcuts to adiabaticity: theoretical, experimental and interdisciplinary perspectives'.

Atomic-scale construction and characterization of quantum dots array and poly-fluorene chains via 2,7-dibromofluorene on Au(1 1 1)

The application of fluorene and its derivatives has been established in last decade, particularly showing the fabrication of blue polymer, organic light-emitting diode (OLED), organic solar cells (OSCs), etc. As low-cost but applied optoelectronic raw material, DBF [2,7-dibromofluorene] is expected to construct intriguing nanostructures on surface, while the related report has remained elusive so far. Here we demonstrate the construction of 2D nanoporous networks and 1D poly-fluorenes by utilizing DBF on Au(111) substrate. Their atomic structures and electronic properties were characterized by a combination of bond-resolved scanning tunneling microscopy (BR-STM), scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. Specifically, nanoporous double-wall networks exhibit confinement state and lower coupling between the neighboring pores, which is assigned to be a uniform quantum dots array. The dynamic synthesis of poly-fluorene was monitored timely during stepwise annealing. STS and DFT results reveal a bandgap of poly-fluorene around 2.25 eV. STS mapping and the simulations evidence the occupied state is mainly located at the armchair-edge. Our work opens an avenue for the extensive investigation of DBF on surface, building a platform for studying fluorene-like molecules and even their derivatives with prospects in functional optoelectronics and nanomaterials.

(Digital Presentation) Quantum Computing in Si/Sige Quantum Dot Arrays

Quantum computation has captivated the minds of many for almost two decades. For much of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer can actually be built. Quantum bits encoded in the spin state of individual electrons in Si/SiGe quantum dot arrays, have emerged as a highly promising direction. Recent highlights from our group include the operation of a device with six quantum bits and the demonstration of quantum operations with a fidelity above 99.5%. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision.

Using intrinsic properties of quantum dots to provide additional security when uniquely identifying devices

Unique identification of optical devices is important for anti-counterfeiting. Physical unclonable functions (PUFs), which use random physical characteristics for authentication, are advantageous over existing optical solutions, such as holograms, due to the inherent asymmetry in their fabrication and reproduction complexity. However, whilst unique, PUFs are potentially vulnerable to replication and simulation. Here we introduce an additional benefit of a small modification to an established model of nanoparticle PUFs by using a second measurement parameter to verify their authenticity. A randomly deposited array of quantum dots is encapsulated in a transparent polymer, forming a tag. Photoluminescence is measured as a function of excitation power to assess uniqueness as well as the intrinsic nonlinear response of the quantum material. This captures a fingerprint, which is non-trivial to clone or simulate. To demonstrate this concept practically, we show that these tags can be read using an unmodified smartphone, with its built-in flash for excitation. This development over constellation-style optical PUFs paves the way for more secure, facile authentication of devices without requiring complex fabrication or characterisation techniques.

Learning Coulomb Diamonds in Large Quantum Dot Arrays

We introduce an algorithm that is able to find the facets of Coulomb diamonds in quantum dot arrays. We simulate these arrays using the constant-interaction model, and rely only on one-dimensional raster scans (rays) to learn a model of the device using regularized maximum likelihood estimation. This allows us to determine, for a given charge state of the device, which transitions exist and what the compensated gate voltages for these are. For smaller devices the simulator can also be used to compute the exact boundaries of the Coulomb diamonds, which we use to assess that our algorithm correctly finds the vast majority of transitions with high precision.

Dielectric and electrical properties of reduced graphene oxide paper after electron irradiation

We present the results of a study of the dielectric and electrical properties of macroscopic reduced graphene oxide paper (RGOP) before and after 1 MeV electron irradiation with a low dose of 1⋅1016 cm−2. It was found that the Coulomb interaction affects the transport of charge carriers and causes their strong localization within graphene domains. The conductivity of the RGOP can be described by the charge carrier tunneling through a disordered graphene quantum dot array. It was shown that charge carrier localization length decreases from 5–8 nm to 1–2 nm after electron irradiation. Structural and electrical studies revealed that the irradiated sample demonstrates electrical conductivity higher than the initial one, despite the lower sp2-carbon fraction, the lower localization length, unchanged average size of graphene domains, and the higher defect states and functional groups content. The improvement in conductivity is also accompanied by an increase in the dielectric permittivity ɛ from 5 for the initial RGOP to 35 after electron irradiation. The data obtained indicate that disordered regions can affect the electrical properties of reduced graphene oxide.

A shuttling-based two-qubit logic gate for linking distant silicon quantum processors

Control of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qubits via coherent spin shuttling, a key technology for linking distant silicon quantum processors. Coherent shuttling of a spin qubit enables efficient switching of the exchange coupling with an on/off ratio exceeding 1000, while preserving the spin coherence by 99.6% for the single shuttling between neighboring dots. With this shuttling-mode exchange control, we demonstrate a two-qubit controlled-phase gate with a fidelity of 93%, assessed via randomized benchmarking. Combination of our technique and a phase coherent shuttling of a qubit across a large quantum dot array will provide feasible path toward a quantum link between distant silicon quantum processors, a key requirement for large-scale quantum computation.

High-frequency transport and zero-sound in an array of SYK quantum dots

We study an array of strongly correlated quantum dots of complex SYK type and account for the effects of quadratic terms added to the SYK Hamiltonian; both local terms and inter-dot tunneling are considered in the non-Fermi-liquid temperature range T \gg T_{FL}T≫TFL. We take into account soft-mode fluctuations and demonstrate their relevance for physical observables. Electric \sigma(\omega,p)σ(ω,p) and thermal \kappa(\omega,p)κ(ω,p) conductivities are calculated as functions of frequency and momentum for arbitrary values of the particle-hole asymmetry parameter \mathcal{E}ℰ. At low-frequencies \omega \ll Tω≪T we find the Lorenz ratio L = \kappa(0,0)/T\sigma(0,0)L=κ(0,0)/Tσ(0,0) to be non-universal and temperature-dependent. At \omega \gg Tω≫T the conductivity \sigma(\omega,p)σ(ω,p) contains a pole with nearly linear dispersion \omega \approx sp\sqrt{\ln\frac{\omega}{T}}ω≈splnωT reminiscent of the “zero-sound”, known for Fermi-liquids. We demonstrate also that the developed approach makes it possible to understand the origin of heavy Fermi liquids with anomalously large Kadowaki-Woods ratio.