Coupled Quantum Dots Research Articles

Compute-in-Memory SRAM Cell Using Multistate Spatial Wavefunction Switched (SWS)-Quantum Dot Channel (QDC) FET

This paper presents multistate spatial wavefunction switched (SWS)-quantum dot channel (QDC) field-effect transistor (FET) static random access memory (SRAM)-based Compute-in-Memory (CIM) cell. The SWS-QDC FETs have two or more vertically stacked coupled quantum dot channels, and the spatial location of carriers within these channels is governed by the applied gate voltage. The location of the carriers can be utilized to encode multiple logic levels within a single device. The utilization of SWS-QDC FETs in CIM cell increases the data storage and energy-efficient computation in the memory. CIM reduces the data access time and improves performance for energy-efficient artificial intelligence (AI) edge devices.

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules.

Coupled colloidal quantum dot molecules (CQDMs) are an emerging class of nanomaterials, manifesting two coupled emission centers and thus introducing additional degrees of freedom for designing quantum-dot-based technologies. The properties of multiply excited states in these CQDMs are crucial to their performance as quantum light emitters, but they cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations essentially categorized as either segregated or localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve the different biexciton species in the prototypical CdSe/CdS CQDM system. By comparing CQDMs with single quantum dots and with nonfused quantum dot pairs, we uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly interacting species corresponding to two nearly independent excitons. The two biexciton types are consistent with numerical simulations, assigning the strongly interacting species to two excitons localized at one side of the quantum dot molecule and the weakly interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

Evolution of a natural TetR protein and development of a Fe3O4 assisted semi-homogeneous fluorescent method for determination of tetracyclines in milk

Compared with antibody, the recognition spectrum of a receptor is broader, and its recognition ability can be improved using simple mutagenesis technique. Compared with conventional immunoassay, the magnetic bead based immunoassay is simpler and can be recycled. Compared with colorimetric and luminescent immunoassays, fluoroimmunoassay is simpler because it does not require a substrate. So a method combines these merits is desirable. In this study, two amino acids in the binding pocket of a natural Escherichia coli TetR protein were mutated to produce a mutant, and the molecular docking showed the binding energies and the numbers of contact acid for 10 tetracyclines all increased. The mutant was coupled with Fe3O4 to synthesize a magnetic complex, and a fluorescent tracer was synthesized by coupling quantum dot and minocycline with bovine serum albumin. Under the assistance of 96-well bottom magnet, a semi-homogeneous method based on the two materials was developed on conventional microplate for determination of the 10 tetracyclines in milk. Results showed once assay was finished within 20 min, the limits of detection (drug concentration showing 10% inhibition) for the 10 drugs were in the range of 0.32–0.94 ng/mL, and the magnetic complex could be regenerated for 6 times. Furthermore, the sensitivities were improved for 4–6 folds in comparison with the use of natural TetR. Therefore, this method is simple, sensitive, time-saving and recyclable, and it can be used for routine screening of the 10 tetracyclines in milk.

Enhancing emission in a QD–nanodisk system via the alignment of the orientation of excitons with the polarization of Mie modes

A Ge quantum dot (QD)–resonator system is an attractive solution for the Si-based emitter applied in monolithic optical–electronic integrated circuits, in which efficient coupling of QDs and cavity modes is crucial for achieving significant emission enhancement. Here, we present unique emission characteristics of a Ge QD–nanodisk array system based on the spatial matching of the two components. In this system, the two-order-of-magnitude emission enhancement is derived from self-assembled QDs, which is related to the orientation of excitons in QDs coinciding with a vertical-polarized Mie mode. In addition, the emission efficiency is also strongly dependent on the position of the QDs in nanodisks. As the orientation of the emitter is matched with the polarized mode, and the QD is located at the maximum of the field, optimal spatial matching can be achieved. These experimental and simulated results demonstrate the importance of spatial matching between emitters with proper polarization and cavity modes, which promises the realization of efficient on-chip light sources.

Probing resonating valence bonds on a programmable germanium quantum simulator

Simulations using highly tunable quantum systems may enable investigations of condensed matter systems beyond the capabilities of classical computers. Quantum dots and donors in semiconductor technology define a natural approach to implement quantum simulation. Several material platforms have been used to study interacting charge states, while gallium arsenide has also been used to investigate spin evolution. However, decoherence remains a key challenge in simulating coherent quantum dynamics. Here, we introduce quantum simulation using hole spins in germanium quantum dots. We demonstrate extensive and coherent control enabling the tuning of multi-spin states in isolated, paired, and fully coupled quantum dots. We then focus on the simulation of resonating valence bonds and measure the evolution between singlet product states which remains coherent over many periods. Finally, we realize four-spin states with s-wave and d-wave symmetry. These results provide means to perform non-trivial and coherent simulations of correlated electron systems.

Offline Coupling of CdS Quantum Dots with Ion Mobility Spectrometry: Simultaneous Determination of Pantoprazole and Domperidone

Offline Coupling of CdS Quantum Dots with Ion Mobility Spectrometry: Simultaneous Determination of Pantoprazole and Domperidone

Effects of metallic electrodes on the thermoelectric properties of zigzag graphene nanoribbons with periodic vacancies

We theoretically analyze the thermoelectric properties of graphene quantum dot arrays (GQDAs) with line- or surface-contacted metal electrodes. Such GQDAs are realized as zigzag graphene nanoribbons (ZGNRs) with periodic vacancies. Gaps and minibands are formed in these GQDAs, which can have metallic and semiconducting phases. The electronic states of the first conduction (valence) miniband with nonlinear dispersion may have long coherent lengths along the zigzag edge direction. With line-contacted metal electrodes, the GQDAs have the characteristics of serially coupled quantum dots (SCQDs) if the armchair edge atoms of the ZGNRs are coupled to the electrodes. By contrast, the GQDAs have the characteristics of parallel quantum dots if the zigzag edge atoms are coupled to the electrodes. The maximum thermoelectric power factors of SCQDs with line-contacted electrodes of Cu, Au, Pt, Pd, or Ti at room temperature were similar or greater than 0.186 nW K−1; their figures of merit were greater than three. GQDAs with line-contacted metal electrodes have much better thermoelectric performance than surface contacted metal electrodes.

3D two-electron double quantum dot: Comparison between the behaviour of some physical quantities under two different confinement potentials in the presence of a magnetic field

We have considered a system consisting of two coupled quantum dots containing two electrons, i.e., two quantum dots next to each other with one excess electron each, subjected to an uniform magnetic field perpendicular to the quantum dots plane. The effect of different confinement potential profiles under which the electrons are subjected is studied. This study has been performed in the light of interest in fundamental logical quantum-gate operations: we have been concerned in analysing the behaviour of the main physical quantities which should be involved in a two-qubit quantum-gate operation for two different profiles of confinement found in literature. Our purpose was to establish how sensitive the physical quantities are to the confinement profile and in which range of magnetic field this issue can be critical.

Interplay of Pauli blockade with electron-photon coupling in quantum dots

Both quantum transport measurements in the Pauli blockade regime and microwave cavity transmission measurements are important tools for spin-qubit readout and characterization. Based on a generalized input-output theory we derive a theoretical framework to investigate how a double quantum dot (DQD) in a transport setup interacts with a coupled microwave resonator while the current through the DQD is rectified by Pauli blockade. We show that the output field of the resonator can be used to infer the leakage current and thus obtain insight into the blockade mechanisms. In the case of a silicon DQD, we show how the valley quasidegeneracy can impose limitations on this scheme. We also demonstrate that a large number of unknown DQD parameters including (but not limited to) the valley splitting can be estimated from the resonator response simultaneous to a transport experiment, providing more detailed knowledge about the microscopic environment of the DQD. Furthermore, we describe and quantify a back action of the resonator photons on the steady-state leakage current.

Extended dimensionality of the density of states in InGaAs coupled quantum dot-ring structures as evidenced by radiative decay times

We observed radiative decay times as a function of temperature by analyzing the thermal relaxation from the excited exciton states in InGaAs coupled quantum dot (QD)–quantum ring (QR) structures. To investigate the confinement of electron-hole pair wavefunctions in the coupled QD-QR structures, we measured radiative decay times with temperature, where the photoluminescence decay time of electron-hole pairs was corrected based on the relative photoluminescence intensity at 4 K to discriminate the radiative processes. For decoupled QRs and QDs, quasi-one dimensional (for QRs) and zero-dimensional (for QDs) densities of states have been reported. However, dimensional densities of states from decoupled QRs and QDs do not apply when the distance between QRs and QDs is sufficiently close in the coupled QD-QR structure owing to coupling interactions (both short- and long-range). In the present coupled structure, QD and QR is connected at the bottom, and the top side peak-to-peak distance is approximately 24 nm. Based on our observations, the power order of temperature dependence increased from 0 to 0.33 (for QDs) and from 0.7 to 1.32 (for QRs), corresponding to the quasi-zero-dimensional (∼Tα=0.33) and quasi-two-dimensional (∼Tα=1.32) confinement state, respectively, owing to amended wavefunctions of electron-hole pairs and dark states effect.

Global Quantum Discord and Entanglement in two Coupled Double Quantum Dots AlGaAs/GaAs

Global Quantum Discord and Entanglement in two Coupled Double Quantum Dots AlGaAs/GaAs

Giant orbital magnetic moments and paramagnetic shift in artificial relativistic atoms and molecules.

Materials such as graphene and topological insulators host massless Dirac fermions that enable the study of relativistic quantum phenomena. Single quantum dots and coupled quantum dots formed with massless Dirac fermions can be viewed as artificial relativistic atoms and molecules, respectively. Such structures offer a unique testbed to study atomic and molecular physics in the ultrarelativistic regime (particle speed close to the speed of light). Here we use a scanning tunnelling microscope to create and probe single and coupled electrostatically defined graphene quantum dots to unravel the magnetic-field responses of artificial relativistic nanostructures. We observe a giant orbital Zeeman splitting and orbital magnetic moment up to ~70 meV T-1 and ~600μB (μB, Bohr magneton) in single graphene quantum dots. For coupled graphene quantum dots, Aharonov-Bohm oscillations and a strong Van Vleck paramagnetic shift of ~20 meV T-2 are observed. Our findings provide fundamental insights into relativistic quantum dot states, which can be potentially leveraged for use in quantum information science.

Witnessing quantum correlations in two coupled quantum dots under intrinsic decoherence

The influences of intrinsic decoherence, tunnelling rates, and Coulomb coupling interaction on the behaviour of negativity, quantum coherence and the measurement-induced disturbance of two coupled quantum dot molecules are discussed. The aforementioned is implemented by obtaining the exact solution of the Milburn equation, where the initial setting state is prepared in an entangled Werner state. Various phenomena are illustrated for three physical measures, namely entanglement, coherence, and quantum correlations. It showed that the intrinsic decoherence strength plays a destructive role for the maximum bounds of all these physical quantities, in which they are decayed to a non-zero fixed point. Altering the tunnelling rates strength increases/ decreases the maximum and minimum bounds of the three physical measurements, whereby increasing the tunnelling rate lessens the entanglement and denigrates the maximum and minimum values of coherence and quantumness. Otherwise, the Coulomb coupling enhances the stability of all physical quantities.

Realization of a minimal Kitaev chain in coupled quantum dots.

Majorana bound states constitute one of the simplest examples of emergent non-Abelian excitations in condensed matter physics. A toy model proposed by Kitaev shows that such states can arise at the ends of a spinless p-wave superconducting chain1. Practical proposals for its realization2,3 require coupling neighbouring quantum dots (QDs) in a chain through both electron tunnelling and crossed Andreev reflection4. Although both processes have been observed in semiconducting nanowires and carbon nanotubes5-8, crossed-Andreev interaction was neither easily tunable nor strong enough to induce coherent hybridization of dot states. Here we demonstrate the simultaneous presence of all necessary ingredients for an artificial Kitaev chain: two spin-polarized QDs in an InSb nanowire strongly coupled by both elastic co-tunnelling (ECT) and crossed Andreev reflection (CAR). We fine-tune this system to a sweet spot where a pair of poor man's Majorana states is predicted to appear. At this sweet spot, the transport characteristics satisfy the theoretical predictions for such a system, including pairwise correlation, zero charge and stability against local perturbations. Although the simple system presented here can be scaled to simulate a full Kitaev chain with an emergent topological order, it can also be used imminently to explore relevant physics related to non-Abelian anyons.

Unlocking the electronic, optical and transport properties of semiconductor coupled quantum dots using first principles methods

Semiconductor coupled quantum dots provide a unique opportunity for tuning band gaps by tailoring band offsets, making them ideal for photovoltaic and other applications. Here, we have studied stability, trends in the band gap, band offsets, and optical properties for a series of coupled quantum dots comprised of II–VI semiconductors using a hybrid functional method. We have shown how the quantum confinement and interfacial strain considerably affect the band gap and band offsets for these heterostructures at the nanoscale. We show that the trend in band offsets obtained from our first-principles electronic structure calculations agrees with that obtained from the method of average electrostatic potential. It is found that a common anion rule for band offset is followed for these heterostructures at the nanoscale. Further, the calculated optical absorption spectra for these coupled quantum dots reveal that absorption peaks lie in the ultra-violet (UV) region, whereas absorption edges are in the visible region. In addition to electronic and optical properties, we have also explored transport properties for two representative coupled quantum dots, either having common cations or common anions, which revealed asymmetric nature in current–voltage characteristics. Therefore, these semiconductor coupled quantum dots may be useful for photovoltaic, light-emitting diode, and optoelectronic devices.

Optical Gain of Vertically Coupled Cd0.6Zn0.4Te/ZnTe Quantum Dots.

The optical modal gain of Cd0.6Zn0.4Te/ZnTe double quantum dots was measured using a variable stripe length method, where large and small quantum dots are separated with a ZnTe layer. With a large (~18 nm) separation layer thickness of ZnTe, two gain spectra were observed, which correspond to the confined exciton levels of the large and small quantum dots, respectively. With a small (~6 nm) separation layer thickness of ZnTe, a merged single gain spectrum was observed. This can be attributed to a coupled state between large and small quantum dots. Because the density of large quantum dots (4 × 1010 cm-2) is twice the density of small quantum dots (2 × 1010 cm-2), the density of the coupled quantum dots is determined by that of small quantum dots. As a result, we found that the peak gain (123.9 ± 9.2 cm-1) with the 6 nm separation layer is comparable to that (125.2 ± 29.2 cm-1) of the small quantum dots with the 18 nm separation layer.

Förster Blockade and Excitation Transfer Behaviors of Two-Photon Rabi Oscillations of Exciton–Biexciton Levels in Coupled Quantum Dots

We provide a theoretical study for the dynamical behavior of two-photon Rabi oscillations in two coupled quantum dots. Each dot is a three-level quantum system (ground, one-exciton, and biexciton states). We describe the optical dynamics within the density matrix formalism framework. The two-photon Rabi oscillations are found to be significantly affected by the strength of the interdot dipolar interaction. Using a detuned laser pulse with an exciton state of the driven quantum dot in our numerical simulations, a direct coherent excitation between the biexciton states can be established, but whether Rabi oscillations can be observed depend mainly on the strength of the dipolar interaction and bandwidth of the excitation laser. When the strength of the dipolar coupling is less than the excitation laser bandwidth, the time-dependent populations of the various two biexciton states of the system exhibit coherent oscillations with several frequencies. However, as we increase the dipolar coupling strength, the population transfer between the quantum dots levels amazingly becomes forbidden, and the excitation gets trapped at the initial state. The reduction of the influence of the dipolar interaction leads to a manipulation of two-photon Rabi oscillation in an ultrashort timescale (∼ps), opening possibilities to achieve ultrafast quantum state control and quantum logic gate generation.

Narrow Intrinsic Line Widths and Electron–Phonon Coupling of InP Colloidal Quantum Dots

InP quantum dots (QDs) are the material of choice for QD display applications and have been used as active layers in QD light-emitting diodes (QDLEDs) with high efficiency and color purity. Optimizing the color purity of QDs requires understanding mechanisms of spectral broadening. While ensemble-level broadening can be minimized by synthetic tuning to yield monodisperse QD sizes, single QD line widths are broadened by exciton-phonon scattering and fine-structure splitting. Here, using photon-correlation Fourier spectroscopy, we extract average single QD line widths of 50 meV at 293 K for red-emitting InP/ZnSe/ZnS QDs, among the narrowest for colloidal QDs. We measure InP/ZnSe/ZnS single QD emission line shapes at temperatures between 4 and 293 K and model the spectra using a modified independent boson model. We find that inelastic acoustic phonon scattering and fine-structure splitting are the most prominent broadening mechanisms at low temperatures, whereas pure dephasing from elastic acoustic phonon scattering is the primary broadening mechanism at elevated temperatures, and optical phonon scattering contributes minimally across all temperatures. Conversely for CdSe/CdS/ZnS QDs, we find that optical phonon scattering is a larger contributor to the line shape at elevated temperatures, leading to intrinsically broader single-dot line widths than for InP/ZnSe/ZnS. We are able to reconcile narrow low-temperature line widths and broad room-temperature line widths within a self-consistent model that enables parametrization of line width broadening, for different material classes. This can be used for the rational design of more spectrally narrow materials. Our findings reveal that red-emitting InP/ZnSe/ZnS QDs have intrinsically narrower line widths than typically synthesized CdSe QDs, suggesting that these materials could be used to realize QDLEDs with high color purity.

In-situ catalytic mechanism coupling quantum dot effect for achieving high-performance sulfide anode in potassium-ion batteries

Though numerous framework structures have been constructed to strengthen the reaction kinetics and durability, the inevitable generation of polysulfide dissolution during conversion-process can cause irreparable destruction to ion-channel and crystal structure integrality, which has become a huge obstacle to the application of metal-sulfide in potassium-ion batteries. Herein, the quantum dot structure with catalytic conversion capability is synchronously introduced into the design of FeS2 anode materials to heighten its K+-storage performance. The constructed quantum dot structure anchored by the graphene with space-confinement effect can shorten the ion diffusion path and enlarge the active area, thus accelerating the K+-ions transmission kinetics and absorption action, respectively. The intermediate phase of formed Fe-nanoclusters possesses high-active catalysis ability, which can effectively suppress the polysulfide shuttle combined with the enhanced absorption effect, fully guaranteeing the structure stability and cycling reversibility. Predictably, the fabricated quantum dot FeS2 can express a prominent advantage in rapid potassiation/depotassiation processes (518.1 mAh g−1, 10 A g−1) and a superior cycling lifespan with gratifying reversible capacity at superhigh rate (177.7 mAh g−1, 9000 cycles, 5 A g−1). Therefore, engineering quantum dot structure with self-induced catalysis action for detrimental polysulfide is an achievable strategy to implement high-performance sulfide anode materials for K-ions accommodation.

Multifunctional Nano-Realgar Hydrogel for Enhanced Glioblastoma Synergistic Chemotherapy and Radiotherapy: A New Paradigm of an Old Drug.

Realgar, as a kind of traditional mineral Chinese medicine, can inhibit multiple solid tumor growth and serve as an adjuvant drug in cancer therapy. However, the extremely low solubility and poor body absorptive capacity limit its application in clinical medicine. To overcome this therapeutic hurdle, realgar can here be fabricated into a nano-realgar hydrogel with enhanced chemotherapy and radiotherapy (RT) ability. Our objective is to evaluate the superior biocompatibility and anti-tumor activity of nano-realgar hydrogel. We have successfully synthesized nano-realgar quantum dots (QDs) coupling with 6-AN molecules (NRA QDs) and further encapsulated with a pH-sensitive dextran hydrogel carrier with hyaluronic acid coating (DEX-HA gel) to promote bioavailability, eventually forming a multifunctional nano-realgar hydrogel (NRA@DH Gel). To better investigate the tumor therapy efficiency of the NRA@DH Gel, we have established the mice in situ bearing GL261 brain glioblastoma as animal models assigned to receive intratumor injection of NRA@DH Gel. The designed NRA@DH Gel as an antitumor drug can not only exert the prominent chemotherapy effect but also as a "sustainable reactive oxygen species (ROS) generator" can inhibit in the pentose phosphate pathway (PPP) metabolism and reduce the production of nicotinamide adenine dinucleotide phosphate (NADPH), thereby inhibiting the conversion of glutathione disulfide (GSSG) to glutathione (GSH), reducing GSH concentrations in tumor cells, triggering the accumulation of ROS, and finally enhancing the effectiveness of RT. Through the synergistic effect of chemotherapy and RT, NRA@DH Gel effectively inhibited the proliferation and migration of tumor cells, suppressed tumor growth, improved motor coordination, and prolonged survival in tumor-bearing mice. Our work aims to improve the NRA@DH Gel-mediated synergistic chemotherapy and RT will endow a "promising future" for the old drug in clinically comprehensive applications.