Quantum Dot System Research Articles

Engineering the Kitaev Spin Liquid in a Quantum Dot System.

The Kitaev model on a honeycomb lattice may provide a robust topological quantum memory platform, but finding a material that realizes the unique spin-liquid phase remains a considerable challenge. We demonstrate that an effective Kitaev Hamiltonian can arise from a half-filled Fermi-Hubbard Hamiltonian where each site can experience a magnetic field in a different direction. As such, we provide a method for realizing the Kitaev spin liquid on a single hexagonal plaquette made up of 12 quantum dots. Despite the small system size, there are clear signatures of the Kitaev spin-liquid ground state, and there is a range of parameters where these signatures are predicted, allowing a potential platform where Kitaev spin-liquid physics can be explored experimentally in quantum dot plaquettes.

A 4.7‐inch 650 PPI AMQLED display prepared by direct photolithography

AbstractIn this work, a highly efficient photosensitive quantum dot (QD) system was designed. The optimized photosensitive QD system had high photoluminescence quantum yield and colloidal stability. By direct photolithography, RGB pixel arrays with a single sub‐pixel size of 39 μm × 5 μm were successfully prepared. Further, the full‐color QLED device was realized. There were no residual QD emission peaks from neighboring sub‐pixels observed in the electroluminescence spectra. Experience on the full‐color QLED device guided the successful preparation of a 4.7‐inch 650 PPI active matrix QLED prototype. The active matrix QLED prototype could display clear and complete pictures. The color gamut reached 85% of the BT2020 standard. This is the first active matrix QLED prototype prepared with a record‐high resolution by direct photolithography, which promoted the development of QLED display technology.

Location Qubits in a Multiple-Quantum-Dot System.

A physical platform for nodes of the envisioned quantum Internet is long-sought. Here we propose such a platform, along with a conceptually simple and experimentally uncomplicated quantum information processing scheme, realized in a system of multiple crystal-phase quantum dots. We introduce novel location qubits, describe a method to construct a universal set of all-optical quantum gates, and simulate their performance in realistic structures, including decoherence sources. Our results show that location qubits are robust against the main decoherence mechanisms, and realistic single-qubit gate fidelities exceed 99.9%. Our scheme paves a clear way toward constructing multiqubit solid-state quantum registers with a built-in photonic interface─a key building block of the forthcoming quantum Internet.

Nonlinear optical properties of tunable spherical quantum dots under like-screened Kratzer potential

Using the effective mass approximation and the iterative procedure, we study the optical rectification (OR) coefficient of a spherical quantum dot (QD) system with like-screened Kratzer potential (LSKP), taking into account the influence of the confinement potential depth, quantum size and external environment. Our results show that the magnitude of the OR coefficient is strongly dependent on the magnitude of the tunable factor, whose peak value will be red-shifted or blue-shifted. Interestingly, the limiting potential depth and temperature have opposite effects on the OR coefficient in terms of peak location and size. Therefore, it is necessary to pay more attention to the influence of internal and external parameters on nonlinear optical effects, and apply the theory to practical experiments and the manufacture of optoelectronic devices.

Visual explanations of machine learning model estimating charge states in quantum dots

Charge state recognition in quantum dot devices is important in the preparation of quantum bits for quantum information processing. Toward auto-tuning of larger-scale quantum devices, automatic charge state recognition by machine learning has been demonstrated. For further development of this technology, an understanding of the operation of the machine learning model, which is usually a black box, will be useful. In this study, we analyze the explainability of the machine learning model estimating charge states in quantum dots by gradient weighted class activation mapping. This technique highlights the important regions in the image for predicting the class. The model predicts the state based on the change transition lines, indicating that human-like recognition is realized. We also demonstrate improvements of the model by utilizing feedback from the mapping results. Due to the simplicity of our simulation and pre-processing methods, our approach offers scalability without significant additional simulation costs, demonstrating its suitability for future quantum dot system expansions.

Using Kondo entanglement to induce spin correlations between disconnected quantum dots

We investigate the entanglement between the spins of two quantum dots that are not simultaneously connected to the same system. Quantum entanglement among localized spins is a crucial property for the advancement of quantum computing and quantum information. Generating and controlling an entangled state between quantum dots have garnered significant attention in recent years for this reason. In this study, we demonstrate that information about the spin orientation of a quantum dot can be preserved, utilizing Kondo entanglement, within a reservoir of electrons. Subsequently, this information can be transmitted to another dot after the initial dot has been decoupled from the reservoirs. We employ a double quantum dot system in a parallel geometry to establish the initial state, where each dot interacts with reservoirs of different symmetries. A specific phase in the couplings is chosen to induce antiferromagnetic spin correlation between the dots. The time evolution of the initial state is analyzed using the time-dependent density matrix renormalization group method. Our findings reveal that a partially entangled state between the dots can be achieved, even when they are not simultaneously connected. This entangled state arises transiently and dissipates in the stationary state. The stability of the state observed during the transient phase is demonstrated. To comprehend the details of these phenomena, we employ a canonical transformation of real space.

The photon blockade in a three-wave mixing system coupled with a quantum dot

In this paper, the photon blockade effect in a three-wave mixing coupling system with a quantum dot has been studied. By using analytical calculation and numerical analysis, we find that both the conventional photon blockade and the unconventional photon blockade effects could be realized in this system in strong-coupling regime just by one driving. Besides, compared with Jaynes–Cummings model, this hybrid system shows other obvious advantages in realizing the photon blockade, like blockading photon of more different frequencies, stronger antibunching effect. All the results may provide useful theoretical references for the single-photon devices design by using quantum dot and three-wave mixing system in future experiments and applications.

Eu3+-based InP/ZnS quantum dot fluorescence platform for multi-color and sensitive visualization of tetracycline

A turn-on type ratiometric fluorescence sensing system of blue quantum dot Eu-MPA-InP/ZnS was established for multi-color visualization determination of tetracycline (TC). Mercaptopropionic acid (MPA)-capped InP/ZnS quantum dots (MPA-InP/ZnS QDs) both modify the hydrophilicity of InP/ZnS QDs and serve as a scaffold for coordinating of Eu3+ ions. The blue fluorescence of Eu-MPA-InP/ZnS at 478 nm is reduced by the TC through the inner filter effect (IFE) under a single excitation wavelength of 365 nm. Rich colour gradients and a highly discriminative colour change were features of this multicolour response to TC, which allowed visual quantification of TC in a dose-dependent manner. Furthermore, by cross-linking Eu-MPA-InP/ZnS with agarose (Aga.), a mouldable Eu-MPA-InP/ZnS@Aga 96-well gel sensing device was designed to serve as a handheld sensor for on-site detection of TC. This probe expands the use of InP QDs in analytical sensing and has been effectively applied to the visual detection of tetracycline in milk and the environment.

The magneto thermoelectric coefficients of double quantum dots in series connected to ferromagnetic electrodes

In this paper, the charge and spin magneto coefficients of a double quantum dots system coupled in series connected to ferromagnetic electrodes with collinear magnetic alignments at finite bias voltage are theoretically investigated based on Green’s functions in a linear response regime. Magneto and spin magneto thermopower are estimated as a function of the thermal gradient and quantum dot energy levels. Magneto conductance and spin magneto conductance are also studied. The results showed that the positions of the peaks can be controlled for each value of polarization, and the magneto conductance is negative where the charge conductance for parallel alignment is lesser than that for antiparallel alignment except in the case of high spin polarization and ferromagnetic spin exchange. Enhancing the magneto thermoelectric coefficients of double quantum dots is a promising step in creating adaptable magnetic switching for thermoelectric generations, which could find use in thermal energy-based logic circuits or heat control applications.

Robust parallel laser driving of quantum dots for multiplexing of quantum light sources

Deterministic sources of quantum light (i.e. single photons or pairs of entangled photons) are required for a whole host of applications in quantum technology, including quantum imaging, quantum cryptography and the long-distance transfer of quantum information in future quantum networks. Semiconductor quantum dots are ideal candidates for solid-state quantum emitters as these artificial atoms have large dipole moments and a quantum confined energy level structure, enabling the realization of single photon sources with high repetition rates and high single photon purity. Quantum dots may also be triggered using a laser pulse for on-demand operation. The naturally-occurring size variations in ensembles of quantum dots offers the potential to increase the bandwidth of quantum communication systems through wavelength-division multiplexing, but conventional laser triggering schemes based on Rabi rotations are ineffective when applied to inequivalent emitters. Here we report the demonstration of the simultaneous triggering of >10 quantum dots using adiabatic rapid passage. We show that high-fidelity quantum state inversion is possible in a system of quantum dots with a 15 meV range of optical transition energies using a single broadband, chirped laser pulse, laying the foundation for high-bandwidth, multiplexed quantum networks.

Towards spin state tailoring of charged excitons in InGaAs quantum dots using oblique magnetic fields

We investigate the effect of oblique magnetic field configurations on a singly charged self-assembled quantum dot system as a means to tune the spin composition of the ground electron spin eigenstates. Using magneto-optical spectroscopy and Stokes polarimetry techniques, we evaluate the anisotropic g factors and characterize the polarization properties of the charged quantum dot system under oblique fields. We compare the results to a simple model that captures the resulting level structure and polarization selection rules for arbitrary magnetic field orientations. Under oblique magnetic fields, the system's ground spin eigenstates are composed of unequal superpositions of the electron spins. This provides an additional degree of freedom to tailor the composition of the ground spin states in charged quantum dots and based on this we demonstrate spin pumping and initialization of the tailored ground states, confirming that the double-Λ level structure of the charged quantum dot persists in oblique magnetic fields. These combined results show that the uneven weightings of the tailored spin states can yield systems with interesting behaviors, with a potential towards developing spin-selective readout schemes to further enhance the capabilities of spin qubits. Published by the American Physical Society 2024

An “off-on” fluorescent nanosensor for the detection of cadmium ions based on APDC-etched CdTe/CdS/SiO2 quantum dots

In this study, we have developed a novel fluorescent "OFF-ON" quantum dots (QDs) sensor based on CdTe/CdS/SiO2 cores. Ammonium pyrrolidine dithiocarbamate (APDC), ethylenediamine tetraacetic acid (EDTA), and 1,10-phenanthroline (Phen) served as potential chemical etchants. Among these three etchants, APDC exhibited the most pronounced quenching effect (94.06%). The APDC-etched CdTe/CdS/SiO2 QDs demonstrated excellent optical properties: the fluorescence of the APDC-etched CdTe/CdS/SiO2 QDs system (excitation wavelength: 365 nm and emission wavelength: 622 nm) was significantly and selectively restored upon the addition of cadmium ions (Cd2+) (89.22%), compared to 15 other metal ions. The linear response of the APDC-etched CdTe/CdS/SiO2 QDs was observed within the cadmium ion (Cd2+) concentration ranges of 0–20 μmol L−1 and 20–160 μmol L−1 under optimized conditions (APDC: 300 μmol L−1, pH: 7.0, reaction time: 10 min). The detection limit (LOD) of the APDC-etched CdTe/CdS/SiO2 QDs for Cd2+ was 0.3451 μmol L−1 in the range of 0–20 μmol L−1. The LOD achieved by the QDs in this study surpasses that of the majority of previously reported nanomaterials. The feasibility of using APDC-etched CdTe/CdS/SiO2 QDs for Cd2+ detection in seawater, freshwater, and milk samples was verified, with average recoveries of 95.27%–110.68%, 92%–106.47%, and 90.73%–111.60%, respectively, demonstrating satisfactory analytical precision (RSD ≤ 8.26).

Theoretical Analysis of GeSn Quantum Dots for Photodetection Applications.

GeSn alloys have recently emerged as complementary metal-oxide-semiconductor (CMOS)-compatible materials for optoelectronic applications. Although various photonic devices based on GeSn thin films have been developed, low-dimensional GeSn quantum structures with improved efficiencies hold great promise for optoelectronic applications. This study theoretically analyses Ge-capped GeSn pyramid quantum dots (QDs) on Ge substrates to explore their potential for such applications. Theoretical models are presented to calculate the effects of the Sn content and the sizes of the GeSn QDs on the strain distributions caused by lattice mismatch, the band structures, transition energies, wavefunctions of confined electrons and holes, and transition probabilities. The bandgap energies of the GeSn QDs decrease with the increasing Sn content, leading to higher band offsets and improved carrier confinement, in addition to electron-hole wavefunction overlap. The GeSn QDs on the Ge substrate provide crucial type-I alignment, but with a limited band offset, thereby decreasing carrier confinement. However, the GeSn QDs on the Ge substrate show a direct bandgap at higher Sn compositions and exhibit a ground-state transition energy of ~0.8 eV, rendering this system suitable for applications in the telecommunication window (1550 nm). These results provide important insights into the practical feasibility of GeSn QD systems for optoelectronic applications.

Resonance and Anti-resonance in transport through quantum dots-Majorana bound states hybrid structure

We calculate the linear conductance through a double quantum-dot system with a normal metal lead, which is enclosed in a closed shape where quantum dot-1 is connected to Majorana bound states (MBSs). The Quantum dots configuration from parallel to series allows for a transition in the shape of resonant to non-resonant in the spectrum. This is caused by quantum interference between the energy levels. This process results in the formation of bonding and antibonding states that are involved in electron transport. An asymmetric quantum dots configuration can produce a Fano resonance, and its position can be adjusted by altering the energy levels.

Asymmetric 1D and 2D tunneling induced grating in Quantum Dot system

We propose a tunneling-induced grating based on the phenomenon of tunneling-induced transparency. Our method allows us to alternate between zeroth-order diffraction along with completely different higher-order diffraction patterns. This is accomplished by using four laser beams to drive quantum dots via a planar configuration. Specifically, we modulate a standing waves controlling beam which propagates orthogonal with the planar medium, while weaker planar probing beam, generated signal field, and pumping field are applied adjacent towards the medium. We quantitatively analyzed the dynamics for the phase, amplitude modulations, along with probing fields diffraction strengths of various ordering. By altering the relative phase of fields, we expect asymmetric 1D and 2D tunneling induced grating in quantum dot system. Our suggested approach has significant uses in optical memory systems, notably in the storing of information to diffraction orders via tunneling-induced grating.

Photoluminescence switching in quantum dots connected with fluorinated and hydrogenated photochromic molecules.

We investigate switching of photoluminescence (PL) from PbS quantum dots (QDs) crosslinked with two different types of photochromic diarylethene molecules, 4,4'-(1-cyclopentene-1,2-diyl)bis[5-methyl-2-thiophenecarboxylic acid] (1H) and 4,4'-(1-perfluorocyclopentene-1,2-diyl)bis[5-methyl-2-thiophenecarboxylic acid] (2F). Our results show that the QDs crosslinked with the hydrogenated molecule (1H) exhibit a greater amount of switching in photoluminescence intensity compared to QDs crosslinked with the fluorinated molecule (2F). With a combination of differential pulse voltammetry and density functional theory, we attribute the different amount of PL switching to the different energy levels between 1H and 2F molecules which result in different potential barrier heights across adjacent QDs. Our findings provide a deeper understanding of how the energy levels of bridge molecules influence charge tunneling and PL switching performance in QD systems and offer deeper insights for the future design and development of QD based photo-switches.

Field-dependent THz transport nonlinearities in semiconductor nano structures.

Charge transport nonlinearities in semiconductor quantum dots and nanorods are studied. Using a density matrix formalism, we retrieve the field-dependent nonlinear mobility and show the possibility of intra-pulse gain. We further demonstrate that the dynamics of master equations can be captured in an analytical formula for the field-dependent charge carrier mobility, e.g. for two-level systems. This equation extends the linear response theory based Kubo-Greenwood result to nonlinear processes at elevated field strength, easily reached in THz transport spectroscopy. With these tools we analyze the field strength, chirp, temperature and dephasing dependence of the charge carrier mobility in the model system of CdSe quantum dots and wires. Stark broadening and Rabi splitting result in strong alterations of the mobility spectra, pronounced at low temperatures. The mobility spectra are strongly temperature and pulse shape dependent in the nonlinear regime. The findings are of immediate interest e.g. for nonlinear THz generation, conversion and amplification in 6G technology and nano electronics. Our results further enable experimentalists to fit and understand measured charge transport nonlinearities with analytical expressions and to design nanosystems with engineered material properties.

Triplet energy transfer from quantum dots increases Ln(iii) photoluminescence, enabling excitation at visible wavelengths.

Europium(iii) complexes are promising for bioimaging because of their long-lived, narrow emission. The photoluminescence (PL) from europium(iii) complexes is usually low. Thus, the effective utilization of low-energy light >400 nm and enhancement of PL are long-standing goals. Here, we show for the first time that 1-naphthoic acid triplet transmitter ligands bound to CdS quantum dots (QDs) and europium(iii) complexes create an energy transfer cascade that takes advantage of the strong QD absorption. This is confirmed by transient absorption spectroscopy, which shows hole mediated triplet energy transfer from QDs to 1-NCA, followed by triplet transfer from 1-NCA to europium(iii) complexes with an efficiency of 65.9 ± 7.7%. Smaller CdS QDs with a larger driving force lead to higher triplet transfer efficiency, with Eu(iii) PL intensity enhanced up to 21.4 times, the highest value ever reported. This hybrid QD system introduces an innovative approach to enhance the brightness of europium complexes.

Fano and Dicke Effects in Parallel-Coupled Quantum Dots Embedded Between Two Leads

In this article, electronic transport through a double quantum dot attached in parallel to a metal lead and with the presence of a magnetic flux is studied with the help of the Green-function formalism and the equation of motion approach. A detail formula is obtained for the density of states and the conductance. The Fano and Dicke effects in the conductance spectra of the double quantum dots system DQD is found and controlled by tuning the coupling (Γ) of the two dots to right (left) Leads and by changing the external magnetic flux.

Bright photoluminescence from CdSe quantum dots conjugated with metal phthalocyanines

Engineering organic/inorganic nanohybrids is one of the emerging tracks in material science to search for new nanomaterials with tailored and/or enhanced properties. In this work, we report the synthesis of CdSe quantum dots (QDs) conjugated with selected metal phthalocyanine (MPc), e.g., ZnPc and CuPc, utilizing the hot-injection organometallic method. The prepared pristine and conjugated QD systems exhibit practically identical particle size and spherical morphology and feature the characteristic CdSe excitonic peaks in their optical absorption spectra. The successful incorporation of ZnPc or CuPc molecules is ensured by the appearance of their sharp Q-band peaks in the optical absorption and the Zn2p or Cu2p core levels in the X-ray photoemission spectroscopy (XPS) measurements. The pure CdSe QDs sample of selected size (∼4.0 nm) exhibits well-defined photoluminescence (PL) peak at the limit of the green range, which is remarkably enhanced for the conjugated QD systems, reaching ∼250 % for the CuPc/CdSe QDs. Such prominent PL enhancement is attributed to Förster resonance energy transfer stimulated by deep B-band emissions from CuPc molecules combined with charge transfer from MPc molecules to CdSe QDs. Furthermore, the emission is greatly amplified upon prolonged exposure to laser beam, exceeding 400 % of the initial CuPc/CdSe emission after only 40 min of exposure time, yet without significant changes in their emission color. Thus, these MPc/CdSe conjugated QD systems are promising nanomaterials for intense white light sources.