Novel approach of performing elementary quantum operations using spatial phase

We have developed a theory of a transient intraband light absorption by semiconductor quantum dots. This absorption plays an important role in the two-pulse pump-probe method, which enables determining the energy relaxation rates of electron-hole excited states. We have considered all possible schemes of this process wherein the carrier frequency of optical pump pulses is close to the resonance with the interband transition of the quantum-dot electronic subsystem, while the carrier frequency of probe pulses is resonant to the intraband transition. For ensembles of identical and size-distributed quantum dots, the probe pulse energy absorption induced by the pump pulse is analyzed in relation to the delay time between the pulses. We have found that, under certain conditions, this dependence can be described by a single, two, or three exponentials. The exponents of the exponentials are proportional to the energy relaxation rates of electron-hole excited states.

A review devoted to the theory of excitonic quasimolecules (Biexcitons) (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric quantum dots synthesized in a dielectric matrix.It is shown that exciton quasimolecules formation is of the threshold character and possible in nanosystems, in with the spacing between the quantum dots surfaces is larger than a certain critical spacing.It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor quantum dots is a significant large values, larger than the binding energy of the biexciton in a semiconductor single crystal almost two orders of magnitude.

Traditional methods for fabricating nanoscale arrays are usually based on lithographic techniques. Alternative new approaches rely on the use of nanoscale templates made of synthetic or biological materials. Some proteins, for example, have been used to form ordered two-dimensional arrays. Here, we fabricated nanoscale ordered arrays of metal and semiconductor quantum dots by binding preformed nanoparticles onto crystalline protein templates made from genetically engineered hollow double-ring structures called chaperonins. Using structural information as a guide, a thermostable recombinant chaperonin subunit was modified to assemble into chaperonins with either 3 nm or 9 nm apical pores surrounded by chemically reactive thiols. These engineered chaperonins were crystallized into two-dimensional templates up to 20 microm in diameter. The periodic solvent-exposed thiols within these crystalline templates were used to size-selectively bind and organize either gold (1.4, 5 or 10nm) or CdSe-ZnS semiconductor (4.5 nm) quantum dots into arrays. The order within the arrays was defined by the lattice of the underlying protein crystal. By combining the self-assembling properties of chaperonins with mutations guided by structural modelling, we demonstrate that quantum dots can be manipulated using modified chaperonins and organized into arrays for use in next-generation electronic and photonic devices.

Systems of confined electrons are found everywhere in nature in the form of atoms where the orbiting electrons are confined by the Coulomb attraction of the nucleus. Advancement of nanotechnology has, however, provided us with an alternative way to confine electrons by using artificial confining potentials. A typical structure of this nature is the quantum dot, a nanoscale system which consists of few confined electrons. There are many types of quantum dots ranging from self-assembled to miniaturized semiconductor quantum dots. In this work we are interested in electrostatically confined semiconductor quantum dot systems where the electrostatic confining potential that traps the electrons is generated by external electrodes, doping, strain or other factors. A large number of semiconductor quantum dots of this type are fabricated by applying lithographically patterned gate electrodes or by etching on two-dimensional electron gases in semiconductor heterostructures. Because of this, the whole structure can be treated as a confined two-dimensional electron system. Quantum confinement profoundly affects the way in which electrons interact with each other, and external parameters such as a magnetic field. Since a magnetic field affects both the orbital and the spin motion of the electrons, the interplay between quantum confinement, electron–electron correlation effects and the magnetic field gives rise to very interesting physical phenomena. Thus, confined systems of electrons in a semiconductor quantum dot represent a unique opportunity to study fundamental quantum theories in a controllable atomic-like setup. In this work, we describe some common theoretical models which are used to study confined systems of electrons in a two-dimensional semiconductor quantum dot. The main emphasis of the work is to draw attention to important physical phenomena that arise in confined two-dimensional electron systems under various quantum regimes.

Large-scale synthesis of semiconductor nanocrystals or quantum dots (QDs) with high concentration and high yield through simultaneously increasing the precursor concentration was introduced. This synthetic route conducted in diesel has produced gram-scale CdSe semiconductor quantum dots (In optimal scale-up synthetic condition, the one-pot yield of QDs is up to 9.6g). The reaction has been conducted in open air and at relatively low temperature at 190-230 degrees C in the absence of expensive organic phosphine ligands, aliphatic amine and octadecene, which is really green chemistry without high energy cost for high temperature reaction and unessential toxic chemicals except for Cd, which is the essential building block for QDs.

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies.

Quantum computers could solve problems insurmountable to conventional computers. The missing ingredient for quantum computing with electron spins is now available — the rotation of a single spin. One of the most promising approaches to realizing a practical quantum bit is to use spin states of single electrons in semiconductor quantum dots. A detailed proposal for such spin qubits was published about ten years ago; since then, all the necessary ingredients to realize it, such as long-lived spin states and the initialization of electron spins, have been reported one by one. Now the last, most difficult step has been accomplished, that of driving coherent spin oscillations for individual electrons, which is necessary to carry out quantum operations. This is achieved by applying oscillating magnetic fields that are resonant with the precession frequencies of the electron spins, generated on-chip near a double quantum dot system. All is now ready to design universal quantum logic gates.

Among solid state quantum emitter systems, semiconductor quantum dots are particularly attractive due to their high radiative quantum efficiencies [1], their strong optical coupling enabling fast [2] and arbitrary [3] qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. For applications such as quantum light sources and quantum nodes, it is essential to maximize the fidelity of the optical control process governing quantum state initialization and control. While the dephasing time tied to radiative relaxation is many orders of magnitude longer than control times achievable with subpicosecond laser pulses, resonant coupling of the electron-hole pair to phonons in the solid-state environment can still contribute to decoherence during the optical control process [2]. This decoherence channel is often referred to as excitation-induced dephasing since the impact on fidelity is dictated in part by the characteristics of the driving laser field. Here we report the demonstration of suppression of phonon-mediated decoherence through the application of frequency-swept laser pulses via adiabatic rapid passage in the strong-driving regime [4]. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the quantum dot. Our findings indicate that the use of telecom-compatible quantum dots leads to decoherence suppression at pulse areas comparable to a single Rabi oscillation period. [1] Atature et al. Nat. Rev. Mater. 3, 38 (2018). [2] Mathew et al. Phys. Rev. B 90, 035316 (2014). [3] Mathew et al. Phys. Rev. B 84, 205322 (2011); Gamouras et al. J. Appl. Phys. 112, 014313 (2012); Gamouras et al. Nano Letters 13, 4666 (2013); Mathew et al. Phys. Rev. B 92, 155306 (2015). [4] Ramachandran et al. Opt. Lett. 45, 6498 (2020).

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay. PACS numbers: 78.67.Hc, 78.47.+p, 42.50.Md, 71.38.-k The strong spatial confinement of carriers in semiconductor quantum dots (QDs) leads to striking differences in the carrier-phonon interaction compared with systems of higher dimensionality. In particular, the discrete energy level s tructure in QDs results in long exciton and electron dephasing times [1, 2, 3, 4], making these semiconductor nanostructures highly attractive for implementation in quantum information processing applications. The study of dephasing mechanisms in QDs is commonly carried out using transient four wave mixing (FWM) spectroscopy. Using resonant interband excitation, FWM measurements have revealed the absorption lineshape of single QDs to consist of a narrow zero phonon line (ZPL) and an acoustic phonon-related broadband centred at the same energy. The only intraband FWM study [5] involved resonant excitation of high energy transitions in th e valence band of p-doped QDs yielding dephasing times ∼ 15 ps. However it was not possible to determine the dephasing mechanisms in this case. Intraband studies of the well-resolved lowest energy conduction band electron transitions in InAs/GaAs QDs have provided deep insight into the electron-phonon interaction and carrier relaxation processes in n-doped samples. Clear evidence of strong coupling between electrons and phonons, resulting in polaron formation, has been demonstrated using magneto-transmission measurements [6]. Ultrafast studies [7, 8] of polaron decay have shown that the previously assumed ’phonon bottleneck’ picture is not valid. Compared with semiconductor quantum wells, the intraband population relaxation time in QDs is long (∼ 50 ps) suggesting relatively long dephasing times. However there have been no reports of direct dephasing measurements to date. In the present letter we present the first investigations of i ntraband dephasing in n-doped QDs using degenerate FWM. Our calculations of the absorption lineshape in this case sh ow marked differences in comparison with the interband absorption [9]. The intraband lineshape consists of peaked acoustic phonon sidebands separated by ∼ 1.5 meV from the ZPL, which corresponds to phonons with wavelength close to the dot size, and is reminiscent of the lineshape associated wit h impurity-bound electron transitions [10]. Using pulse durations short enough to excite both the ZPL and acoustic phonon sidebands we find damped oscillations in the FWM signal, indicative of coherent acoustic phonon generation, followed by a single exponential decay. In contrast with the interband case, where the origin of the strong temperature dependence of the excitonic linewidth is still subject to debate, the simple 3 -level structure of the lowest energy conduction band states in InAs QDs permits an accurate simulation of the temperature dependence of the FWM signal. The excellent agreement found between experiment and theory, shows that virtual transitions between the p-states is the dominant dephasing mechanism at high temperature. At low temperature, we have measured an intersublevel dephasing time of T2 ∼ 90 ps. It is also interesting to compare our results with previous intraband dephasing measurements in higher dimensional (quantum well) systems [11]. Here phonon-mediated processes are not significant with the intraband dephasing instead determined by electronelectron interactions, yielding typical dephasing times ∼ 0.3 ps which are approximately 2 orders of magnitude faster than for the QD samples studied here. The relatively long intraband dephasing time in QDs is key to the efficient operation of new types of mid-infrared QD-based devices, such as intersublevel polaron lasers [12] and may be relevant for potential device applications such as qubits for quantum information processors [13]. The investigated samples were grown on (100) GaAs substrates by molecular beam epitaxy in the Stranski-Krastranow mode. They comprise 80 layers of InAs self-assembled QDs separated by 50 nm wide GaAs barriers, thus preventing both structural and electronic coupling between QD layers. The polaron transitions were studied between s-like ground (s) and p-like first excited ( p) states within the conduction band. To populate the s state, the samples were delta-doped with Si 2 nm below each QD layer. The doping density was controlled in such a way that the average doping did not exceed 1 electron per dot (see Ref. [14] for more details). Absorp

The dependence of the directions of polarization of exciton emissions, fine structure splittings (FSS), and polarization anisotropy on the light- and heavy-hole (LH–HH) mixing in semiconductor quantum dots (QDs) is investigated using a mesoscopic model. In general, all QDs have a four-fold exciton ground state. Two exciton states have directions of polarization in the growth-plane, while the other two are along the growth direction of the QD. The LH–HH mixing does affect the FSS and polarization anisotropy of bright exciton states in the growth-plane in the low symmetry QDs (e.g., C2V, CS, C1), while it has no effect on the FSS and polarization anisotropy in high symmetry QDs (e.g., C3V, D2d). When the hole ground state is pure HH or LH, the bright exciton states in the growth-plane are normal to each other. The LH–HH mixing affects the relative intensities and directions of bright exciton states in the growth-plane of the QD. The polarization anisotropy of exciton emissions in the growth-plane of the QD is independent of the phase angle of LH–HH mixing but strongly depends on the magnitude of LH–HH mixing in low symmetry QDs.

Luminescent quantum dots: Synthesis, optical properties, bioimaging and toxicity

Recently, mass production and extensive use of novel engineered nanomaterials (e.g., carbon nanotubes, fullerene, nano-TiO2, nano-Au and quantum dots) raise increasing concerns about their potential harmful effects on our environment and human health. Therefore, for better understanding of their environmental and biological fate upon accidental release during their manufacturing and transport processes, increasing number of studies on the toxicities of engineered nanomaterials were conducted during last few years. However, regarding the toxicity of engineered nanomaterials, contradictory results were frequently found in the current literature, probably due to the widely varying physicochemical properties of engineered nanomaterials and inconsistency of the dosage/exposure conditions. Among various engineered nanoparticles, semiconductor quantum dots (QDs) have received significant attention as a new type of fluorophore for the biological and medical imaging. However, they also induced increasing concern as a potential pollutant, due to its unknown toxicity as a nanometer-scale material as well as the toxic components (i.e., Cd, Zn, Se and etc). But, their colloidal stability, photochemical property, dissolution behavior and overall cytotoxic impact of these QD colloids have not been systematically studied and completely understood yet. In this study, using various analytical tools, we have prepared and characterized water-soluble monodisperse QD nanocolloids with well-defined core and hydrodynamic size, functional groups at the interface, impurities, nanoparticle concentration and impurities in aqueous solution. Then, toxicological impacts of QD nanocolloids collected at different stage of preparation (i.e., different degree of impurity) were tested on gram negative bacterial cells (i.e., Escherichia coli). Preparation, surface modification and characterization of QD nanoparticles. From XRD analysis of QD nanoparticles, we confirmed that the QDs synthesized using SiPOP mothod predominantly have the wurtzite CdSe crystal structure (see supporting information, Figure S1). Additionally, absences of CdO or ZnO peaks in the XRD results indicated that negligible degree of oxidations occurred during the preparation of this QD nanoparticle. Furthermore, application of schrrer equation to the width of CdSe (110) peak resulted in the estimated value of CdSe core size, which is 3.25 nm (see supporting information, Figure S2), while estimation of CdSe core size from the position (559 nm) of the first excitonic absorption peak resulted in very similar value (3.24 nm). To make this QD soluble in aqueous solution, ligand exchange reaction of TOPO capped QD (i.e., QD) was performed using MAA (mercaptoacetic acid) and aqueous colloid of carboxylic acid group terminated QD was prepared. Then, several cycles of wash-out procedure were conducted to remove residual MAA from aqueous solution. To assure the monodispersity of this QDs in aqueous solution, hydrodynamic sizes of QDs were also measured by dynamic light scattering methods (QD = 4.56 nm, see Fig. 1) and confirmed that the individual QD particles are well dispersed in solution with least degree of aggregation. Concentrations of QDs in aqueous solutions were determined from the intensities of the first excitonic absorption peak and extinction coefficients based on the method reported by Yu et al. and presented in the units of moles of QD per liter (i.e., [QD]). Fourier Transform Infrared (FT-IR) spectroscopy was used to monitor changes in surface modifying groups of QDs as well as impurities included during ligand exchange process. During surface modification process, the replacement of TOPO and changes in adsorbed MAA molecules were monitored by FT-IR and corresponding spectra were presented in Fig. S3 (In supporting information). The IR spectrum shown in Fig. S3(e) shows spectrum collected from QD solutions after several cycles of wash-out process and showed quite distinct spectral features with that of Fig. S3(d). First of all, absence of thiol peaks around 2570 cm−1 indicate that there is no free MAA with thiol groups and most of the MAA are already lost their hydrogen and

The influence of strain waves traveling across a semiconductor quantum dot on the optical spectra of the lowest quantum dot transition is analyzed. Pure dephasing interactions between electronic and phononic degrees of freedom in quantum dot systems are considered, which represent the most important type of electron-phonon interaction in strongly confined quantum dots. For the case of excitation by ultrafast laser pulses, a generating function formalism provides analytical results, which are exact within the framework of the model. Two situations are compared: (i) a phonon wave packet is generated by the optical excitation of a single-quantum dot near a surface, which after reflection at the surface reenters the quantum dot and (ii) a phonon wave packet is generated by the excitation of a nearby second dot and then travels across the quantum dot. Although the displacement fields passing the dot are almost identical in these two situations, we find that the real time responses as well as the corresponding spectra exhibit qualitative differences and thus allow for a discrimination of phonon wave packets from different origins.

The work provides computational arguments in support of excitonic approach for the treatment of the photo-induced processes in semiconductor quantum dots. The non-radiative relaxation, non-radiative recombination, and photo-luminescence quantum yield are computed for a range of atomistic models of semiconductor quantum dots (QDs) in the quantum confinement regime. The excitonic (EX) approach is compared to independent orbital approximation (IOA) approach. Both approaches address dissipation of the electronic energy from electronic degrees of freedom to thermal vibrations of the lattice. The difference of two approaches appears in treatment of energies of electronic states and in a way how the electron-phonon interaction is taken into account. IOA approach uses energies of Kohn-Sham orbitals and on the fly non-adiabatic couplings. [1-3] EX approach uses Bethe-Salpeter equation (BSE) for energies.[4-6] The excitonic wavefunctions from BSE is used to construct a linear transformation matrix that transforms IOA-based non-adiabatic couplings into an excitonic basis. Both approaches are compared in application to untrasmall 1 nm diameter Si QD.Results include an evidence that hot excitons relax sooner in the excitonic picture than in the IOA picture. The observed effect is rationalized via smaller subgaps and different available relaxation pathways in the excitonic picture. The most surprising result is found for the simulated emission spectrum. The spectum in the excitonic picture demonstrates intensity in several 5 orders of magnitude higher than in the IOA picture. This observation is related to formation of a bright exciton in the lowest excitation of the ultra-small Si QDs. Obtained evidence favors excitonic approach and promises a reliable interpretation and prediction of time-dependent observables in a range of semiconductor quantum dots of different composition, sizes, and surface environment.[7] Most intriguing results are expected for QDs representing interface between PbSe and CdSe. [8]Support of National Science foundation via NSF CHE-2004197 is gratefully acknowledged.[1] D. S. Kilin and D. A. Micha, “Relaxation of photoexcited electrons at a nanostructured Si(111) surface”, J. Phys. Chem. Lett. 1, 1073-1077 (2010).[2] D. J. Vogel and D. S. Kilin, "First-Principles Treatment of Photoluminescence in Semiconductors" J. Phys. Chem. C 119, 50, 27954–27964 (2015).[3] D. J. Vogel, A. B. Kryjevski, T. M. Inerbaev, and D. S. Kilin, "Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots", J. Phys. Chem. Lett. 8, 13, 3032–3039(2017).[4] A. B. Kryjevski and Dmitri Kilin, "Enhanced multiple exciton generation in amorphous silicon nanowires and films", Molec. Phys. 114, 365-379 (2016).[5] M. Rohlfing and S. G. Louie, "Electron-Hole Excitations in Semiconductors and Insulators", Phys. Rev. Lett. 81, 2312-2315 (1998).[6] T. Sander, G. Kresse, "Macroscopic dielectric function within time-dependent density functional theory—Real time evolution versus the Casida approach", J. Chem. Phys. 146, 064110 (2017).[7] S. V. Kilina, P. K. Tamukong, and D. S. Kilin, "Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives", Acc. Chem. Res. 49, 10, 2127–2135 (2016).[8] H. B. Griffin, A. B. Kryjevski, and Dmitri S. Kilin, "Ab initio calculations of through-space and through-bond charge-transfer properties of interacting Janus-like PbSe and CdSe quantum dot heterostructures", Molec. Phys., e2273415 (2023).

Semiconductors quantum dots (QD) exhibit unique size-dependent optical properties which enable them to be considered for biomedical applications, including diagnosis and cancer treatment. Furthermore, the synthesis of less toxic and Cd-free QDs directly in aqueous phase assures their biocompatibility as required for nanomedicine applications. Accordingly, the present work was focused on the direct synthesis of alloyed ZnSe(S) QDs capped with thioglycolic acid (TGA) or 3-mercaptopropionic acid (MPA) in aqueous medium under microwave irradiation conditions. X-ray diffraction measurements suggested that as-synthesized QDs exhibited a solidsolution like structure with an average crystallite size of 3.8 ± 0.1 nm and 2.1 ± 0.2 nm for TGA- and MPA-capped Zn-based QDs, respectively. HRTEM measurements confirmed the small size and monodispersity of the synthesized QDs. The chemisorption of carboxylate groups in TGA (vas COO at 1577 cm-1 and vs COO at 1388 cm-1) and MPA (vas COO at 1560 cm-1 and vs COO at 1401 cm-1) onto the QDs was confirmed by FT-IR spectroscopy analyses. The band gap energy values were estimated at 3.1eV or 3.3eV, respectively, when the QDs were synthesized in the presence of TGA or MPA species. QDs suspended in biocompatible Phosphate Buffer Saline (PBS) exhibited a remarkable photo-stability; the corresponding photo-luminescence spectra recorded under 302nm excitation evidenced strong emission peaks centered on 390nm and 395nm for TGA- and MPA-capped QDs, respectively. As-synthesized thiol-capped alloyed ZnSe(S) QDs can be considered very promising biocompatible candidates for nanomedicine applications.