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We show that the axial symmetry of the Bychkov&ndash, Rashba interaction can be exploited to produce electron spin-flip in a circular quantum dot, without lifting the time reversal symmetry. In order to elucidate this effect, we consider ballistic electron transmission through a two-dimensional circular billiard coupled to two one-dimensional electrodes. Using the tight-binding approximation, we derive the scattering matrix and the effective Hamiltonian for the considered system. Within this approach, we found the conditions for the optimal realization of this effect in the transport properties of the quantum dot. Numerical analysis of the system, extended to the case of two-dimensional electrodes, confirms our findings. The relatively strong quantization of the quantum dot can make this effect robust against the temperature effects.

Quantum repeater composed of a quantum memory and an interface between photon qubits and memory qubits is indispensable for long-distance quantum communication. Gate-defined lateral quantum dots (QDs) can be a suitable platform for such quantum repeaters because of its aptitude for spin qubit and feasibility of quantum state transfer from photon polarization to electron spin. So far, the reported photoelectron excitation probabilities in such a QD are not high enough to implement practical repeater protocols. To improve the photoexcitation probability, we combine a surface plasmon antenna (SPA) with QDs. We fabricated a SPA designed to enhance the optical transmission to the QDs in a practical illumination setup in a refrigerator and characterized the fabricated antenna by measuring photocurrents at room temperature.

Electron spins in gate-defined lateral quantum dots (QDs) are a suitable platform for qubits and have the capacity for coherent coupling to free-space photons. Superposition state transfer from single photon polarizations to single spins has been realized using a GaAs gate-defined QD. However, its efficiency is limited by small size of the QDs. Here, we use a surface plasmon antenna to improve the single photoelectron trapping probability in a GaAs gate-defined QD and demonstrate that the photoelectron detection probability is enhanced by 5.3–9.2 times. This result contributes to the development of photon–spin quantum interface using lateral QDs.

We show that in-plane-magnetic-field assisted spectroscopy allows extraction of the in-plane orientation and full 3D shape of the quantum mechanical orbitals of a single electron GaAs lateral quantum dot with sub-nm precision. The method is based on measuring orbital energies in a magnetic field with various strengths and orientations in the plane of the 2D electron gas. As a result, we deduce the microscopic quantum dot confinement potential landscape, and quantify the degree by which it differs from a harmonic oscillator potential. The spectroscopy is used to validate shape manipulation with gate voltages, agreeing with expectations from the gate layout. Our measurements demonstrate a versatile tool for quantum dots with one dominant axis of strong confinement., Comment: 4 pages, 3 color figures, including supplementary on arXiv

In this work, we have investigated the exchange interaction of two electrons in double lateral quantum dot (which is the base of quantum gate) under effect of external magnetic field, electric field and inter dot distance between double dots. From similarity between double quantum dots and molecule, we have used molecular physics approaches (Hitler – London and Hund – Mullikan approximation method) to investigate our system. We also show the magnetization behavior as function of magnetic field.

A quantum dot embedded in a trumpet nanowire has been shown to be a good candidate for realising an efficient on-demand single-photon source [1, 2]. At the lateral quantum dot position the diameter of the nanowire only supports two guided modes — the HE11 and TE01 modes. These modes are excited by the quantum dot after recombination of an electron-hole pair. The slow expansion of the nanowire diameter secures an adiabatic propagation of the modes through the taper meaning that the occupation of the HE11 and TE01 modes will remain the same at the top of the trumpet nanowire (see Fig. 1 right). It is for now not possible to control the radial position of the quantum dot in the nanowires and thus it is important to experimentally determine this after fabrication.

[Image: see text] Single-monolayer transition metal dichalcogenides exhibit exceptionally strong photoluminescence (PL), dominated by a combination of distinct neutral and charged exciton contributions. We show here that the surface charge associated with ferroelectric domains patterned into a lead zirconium titanate film with an atomic force microscope laterally controls the spatial distribution of neutral and charged exciton populations in an adjacent WS(2) monolayer. This is manifested by the intensity and spectral composition of the PL measured in air at room temperature from the areas of WS(2) over a ferroelectric domain with a polarization dipole pointed either out of the surface plane or into the surface plane. This approach enables spatial modulation of PL intensity and trion/neutral exciton populations and fabrication of lateral quantum dot arrays in any geometry, with potential applications in nonvolatile optically addressable memory or optical quantum computation.

So far, all reviews and control approaches of spin relaxation have been done on lateral single electron quantum dots. In such structures, many efforts have been done, in order to eliminate spin-lattice relaxation, to obtain equal Rashba and linear Dresselhaus parameters. But, ratio of these parameters can be adjustable up to 0.7 in a material like GaAs under high- electric field magnitudes. In this article we have proposed a single electron QD structure, where confinements in all of three directions are considered to be almost identical. In this case the effect of cubic Dresselhaus interaction will have a significant amount, which undermines the linear effect of Dresselhaus while it was destructive in lateral QDs. Then it enhances the ratio of the Rashba and Dresselhaus parameters in the proposed structure as much as required and decreases the spin states up and down mixing and the deviation angle from the net spin-down As a result to the least possible value.

At the Air Force Research Laboratory, Space Vehicles Directorate, we are interested in improving the performance of or modifying the capabilities of infrared detectors in order to locate and identify dim and/or distant objects in space. One characteristic we are very interested in is multicolor detection. To this end, we have turned to a novel detector design that we have come to call a Lateral Quantum Dot Infrared Photodetector (LQDIP). In this design, InAs quantum dots are buried in a GaAs quantum well, which in turn is tunnel-coupled to another GaAs quantum well. Photoexcited electrons from the quantum dots tunnel over to the second well and are then swept out via a lateral (perpendicular to the growth direction) bias voltage. This architecture should exhibit the ability to tune to select infrared frequencies with reduced dark current and unity gain. The lateral photocurrent is directed by a vertical (parallel to the growth direction) gate voltage. We will discuss this detector architecture and the LQDIP operating principles and conditions, and we will present some preliminary results of current–voltage, photocurrent, differential conductance, and spectral measurements.

A new infrared detector design, henceforth referred to as a lateral quantum dot infrared photodetector (LQDIP), with the potential for a tunable internal spectral response was investigated. In this design, InAs quantum dots are buried in a GaAs quantum well, which is in turn tunnel-coupled to a second GaAs quantum well. Photoexcited electrons from the quantum dots are expected to tunnel over to the second well, where they are then swept out via a lateral (perpendicular to the growth direction) bias voltage. The lateral photocurrent is in part directed to tunnel into the second quantum well by the depletion field of a narrow pinch-off gate, applied vertically (parallel to the growth direction). Under a proper biasing arrangement, this detector architecture is expected to exhibit the ability to tune to select infrared frequencies as well as operate with reduced dark currents and unity gain in the second well. The LQDIP detector architecture, operating principles and conditions, and preliminary results of I–V, photocurrent, and differential conductance measurements are all discussed.

We present an overview and a comparison between three different methods of creating low density lateral In(Ga)As quantum dot molecules (QDMs) embedded in a GaAs matrix. Each of them is based on the use of nanoholes to control the dot nucleation site and generate the QDMs. The three methods used to create suitable nanoholes are: (1) In situ excess gallium droplet etching, where the nanohole shape is modified by overgrowth of a thin GaAs buffer to give QDM nucleation. (2) Ex situ electron-beam lithographic patterning and wet-etching, where the patterned nanohole size is critical for formation of QDMs. (3) In situ strain-selective etching of buried InAs quantum dots by AsBr3. The mechanisms of QDM formation, dependence on growth parameters, advantages and disadvantages of each technique and future challenges are discussed.

We present here a model for the non-Born-Oppenheimer description of the biexcitonic complex X2(eehh) trapped in laterally-coupled quantum dot system. We define a zeroth-order Hamiltonian which allows us, under certain conditions on the masses and coupling constants, to decouple the problem. We show that the zeroth-order wavefunctions and energies can be described using the exact solutions for the Hookean model for the H2 molecule [Ludena et al., J Chem Phys 2005, 123, 024102]. We also apply this Hookean model to the description of the excitonic complexes X(ehh), X−(eeh), and to the single exciton X(eh) and analyze the dependence of the total non-Born-Oppenheimer zeroth-order energies, and binding energies for these sytems with the mass ratio σ. The general features of the results obtained using this Hookean model agree quite well with those of more elaborate calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

In this paper, we report progress in the development of a detector for photons in the terahertz region consisting of a lateral quantum dot (QD), defined in a semiconductor heterostructure by mesa patterning and three negatively biased metallic gates, and a single-electron transistor (SET) on top of the mesa and, hence, capacitively coupled to the QD. We study the behavior of the QD as a function of the potential applied to the gates using the SET as a sensitive charge detector and identify the bias region of the device, where it is sensitive to incident terahertz radiation. The QD converts incident photons into charge excitations, which can be detected by the SET, resulting in a signal of the order 10 8 electrons for each absorbed photon. Based on the dark count rate and an estimate of the quantum efficiency, the detector should enable low-power measurements in the terahertz region with noise-equivalent power ~10-19 W/Hz1/2 exceeding the sensitivity of commercially available bolometers by two orders of magnitude

Self-assembled lateral InAs quantum dot molecules (QDMs) are grown by solid-source molecular beam epitaxy (MBE) using the thin-capping-and-regrowth MBE process. Thin capping of GaAs on as-grown InAs quantum dots (QDs) at low temperatures leads to nanohole templates. The shape, size and depth of nanoholes are controlled by capping thickness. Subsequent regrowth with different amounts of InAs on the templates result in nano-propeller QDs with different blades’ dimensions. We showed that the length of the propeller blades is controlled by either the capping temperature or the thickness of capping layer. When the regrowth thickness reaches 1.2 ML, different self-assembled lateral QDMs are formed depending on the shape of nanoholes. The dot uniformity and the dot size of all QDM samples are confirmed by photoluminescence (PL) measurements at 77 K. In addition, by ramping the regrowth temperature while the In shutter is open after the deposition of the capping layer, we are able to investigate the formation of nano-propeller QDs at the beginning phases and to propose a model to explain the evolution.

InAs lateral quantum dot molecules (QDMs) are grown on (001)-GaAs substrates. The self-assembled QDMs are formed in one continuous molecular beam epitaxial (MBE) growth via a thin-capping-and-regrowth technique. Lateral QDMs, each with 10-12 dots arranged in a specific pattern, are determined by the shapes of the underlying nanopropeller quantum dots (QDs). The nanopropeller QDs in turn are obtained by regrowth on nano-holes which have been previously created by capping the first InAs QD layer grown on (001)-GaAs substrate with a thin GaAs layer. The length of the propeller directly influences the number of QDs in a QDM. By varying the conditions for thin-capping, shorter or longer propellers can be achieved, allowing the number of QDs in each QDM to be controlled.

InAs quantum dot molecules (QDMs) formed by molecular-beam epitaxy on GaAs (311)B substrates through self-organized anisotropic strain engineering are studied by excitation-power-density- and temperature-dependent macro- and microphotoluminescence (PL). An unusual asymmetric broadening, together with a continuous shift toward higher energies of the PL peak position, most prominent for the $p$-type modulation-doped QDMs, is observed with increasing excitation power density. The $n$-type modulation-doped QDMs exhibit a square-shaped PL spectrum, resembling that of modulation-doped quantum wells. In temperature-dependent macro-PL, two distinct minima of the full width at half maximum are observed, indicating thermally activated carrier redistribution within the QDMs through two different channels at lower and higher temperatures. The micro-PL spectra of the $p$-type modulation-doped QDMs exhibit discrete sets of sharp peaks on top of broad PL bands. The number and intensity of the sharp peaks increase with excitation power density. With increasing temperature, the number and intensity of the sharp peaks decrease while the intensity of the broad PL bands increases, in agreement with the carrier redistribution at lower temperatures. Only broad PL bands are observed for the $n$-type modulation-doped QDMs with similar behavior. These results are explained by state filling in the presence of extended electron states formed due to lateral electronic coupling of the quantum dots within the QDMs.

Strain feld distribution at the surface of SiGe nanomounds formed by heteroepitaxy is exploited to obtain a positional ordering of the closely spaced Ge quantum dots (quantum dot molecules). We demonstrated, that a low density of the lateral quantum dot molecules (up to 107 cm−2) can be achieved by tuning of the growth conditions. We present a growth model that provide physical insights into possible mechanisms underlying the formation of lateral SiGe quantum dot molecules. The electronic band structure of the molecules was calculated by 6-band kp method. The results of theoretical study are in a good agreement with experimental measurements of photoluminescence spectra from the samples with quantum dot molecules.

We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response. NSF DMR-0844747 DMR 1309989 GV VALi+d Grant APOSTD/2013/052 NRF of Korea 2011-C0030821/2013R1A1A1007118 MINECO Project CTQ2011-27324

We study self-assembled lateral quantum dot molecules (LQDMs) under varying electric and magnetic fields and deduce the electronic structure, charge configurations and delocalized states that lead to the observed photoluminescence spectra.

A split gate technique is used to form a lateral quantum dot in a two-dimensional electron gas of a modulation-doped silicon/silicon-germanium heterostructure. e-beam lithography was employed to produce split gates. By applying negative voltages to these gates the underlying electron gas is depleted and a lateral quantum dot is formed, the size of which can be adjusted by the gate voltage. We observe single-electron operation with Coulomb blockade behavior below 1K. Gate leakage currents are well controlled, indicating that the recently encountered problems with Schottky gates for this type of application are not an inherent limitation of modulation-doped Si/SiGe heterostructures, as had been speculated., 3 pages

Excitation power density dependent photoluminescence (PL) measurements of an ensemble of ordered lateral quantum dot (QD) molecules containing on average 4 and 8 QDs are carried out. Asymmetric broadening of the PL spectra is observed with increasing excitation power density which is stronger for 8 QDs per molecule than for 4 QDs. In addition, a shift of the PL peak to higher energies occurs. In p-type modulation-doped QD molecules the asymmetric broadening and high-energy shift are enhanced. N-type modulation-doped QD molecules exhibit a square-shaped PL spectrum, resembling that of modulation-doped quantum wells. These findings are consistently explained by state filling in the presence of extended electron states due to lateral electronic coupling of the QDs within the molecules forming a quasi two-dimensional density of states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InAs/GaAs quantum dots (QDs) with ordered structure, due to their peculiar properties, open new way to design novel semiconductor devices such as single-electron transistors or highly parallel computing architectures. The lateral quantum dot alignment achieved during the selfassembly process is not well understood heretofore. The reason is, that quantum structures areusually small and studied at low temperatures. Conversely, the Scanning near-field optical microscopy (SNOM) allows study nanometer details in the non-offensive manner, in the room temperature with high spatial and temporal resolution. The first results of near-field optical study on aligned QDs are presented.

We review mechanisms of low-temperature electronic transport through a quantum dot weakly coupled to two conducting leads. Transport in this case is dominated by electron-electron interaction. At temperatures moderately lower than the charging energy of the dot, the linear conductance is suppressed by the Coulomb blockade. Upon further lowering of the temperature, however, the conductance may start to increase again due to the Kondo effect. We concentrate on lateral quantum dot systems and discuss the conductance in a broad temperature range, which includes the Kondo regime.

We performed low-temperature conductance measurements on a lateral quantum dot device which is electrostatically defined by a split-gate and two overlaying finger gates which introduce tunnel barriers. The observed abrupt change in the measured Coulomb oscillations period is ascribed to the presence of the surface states. Using a simple model, we estimate the surface density of states in our system to be 10 12 cm −2 eV −1 . This value is an order of magnitude lower than that in a GaAs/AlGaAs two-dimensional electron system. Our studies provide the first non-invasive measurements of the surface density of states in a GaAs system.

A surface nanomachining fabrication process using electron beam cross-linked poly(methyl) methacrylate (PMMA) has been developed and characterized. PMMA with different molecular weights (MW 100 K, MW 495 K, MW 950 K) in anisole casting solvent has been crosslinked with different electron beam irradiation levels ranging from 20 C/m/sup 2/ to 240 C/m/sup 2/. This is to investigate the quantifiable relationship between electron dose and its submicrometer remaining thickness after dissolving in acetone. This technique which uses electron beam lithography, offers a high resolution semi-three-dimensional (3-D) nanomachining of the sacrificial layer in a single run. Because of its low Young's modulus, it has been successfully integrated with nanoelectromechanical systems processing and has the advantage of producing low-stress submicrometer thick structures with lateral dimensions as low as, but not limited to 1 /spl mu/m. A fast dry release time from 55 to 100 s using oxygen plasma ashing has been demonstrated for a sacrificial layer aspect ratio of 125. This corresponds to an etch rate of about 0.6 /spl mu/m/s at an average temperature of 40/spl deg/C. The success of using cross-linked PMMA as a gate dielectric is demonstrated by the fabrication of multilayered gated lateral quantum dot devices. Periodic and continuous conductance oscillations arising from Coulomb charging effects are clearly observed in the transport properties at 50 mK.

We investigate the spin state of a GaAs lateral quantum dot (QD) by using strongly asymmetric tunnel barriers. The saturated current depends on the current polarity, which switches the direction of the dominant tunneling transition that increases or decreases the total spin. With this technique, the spin polarization and the Coulomb interaction are investigated. We find that high spin states appearing in some magnetic field regions can be understood by considering simple Coulomb interactions.

The optical response of an AlGaN/GaN/(InxGa1−xN)n/GaN/AlGaN heterostructure is obtained from precise, and comparatively simple, transition probability calculations. A comprehensive approach to evaluate these quantities from rigorous expressions of the heterostructure's energy eigenvalues and eigenfunctions is given. Taking full account of the longitudinal quantum wells and the lateral quantum dot confinement, quite precise intraband energy levels and wave functions have been obtained. Photoluminescence results for sectionally constant band-edge profile differ from the experimental measurements by approximately 45 meV. We show that the band-edge modulation arising from charge polarization, is responsible for this difference.

We discuss the effects of lateral quantum dot pitch on the formation of vertically aligned InAs site-controlled quantum dots (SCQDs) separated by thin spacers on GaAs substrates. Bilayers of InAs SCQD square arrays of various quantum dot-array pitches (100 nm, 75 nm, 50 nm, and 40 nm) are fabricated based on nanometer-scale site-control techniques and self-organizing epitaxy. In situ scanning tunneling microscope observations reveal that the vertical pairing probability between the two layers of SCQDs decreases with decreasing pitch, with a dramatic reduction when the pitch falls below 75 nm. Moreover, room-temperature μ-photoluminescence measurements show that the linewidth of the bilayers of vertically aligned SCQDs increases with declining pitch. Again, we found a remarkable increase in photoluminescence linewidth when the pitch falls below 75 nm. The observed results are attributed to lateral interactions between elastic strain fields induced by the first layer of buried InAs SCQDs.

We present the review of our work on spin effects in single lateral quantum dots with the emphasis on the results of Coulomb blockade spectroscopy studies. Realization of a spin-based quantum bit proposal in a lateral quantum dot is discussed. Described are the ways of isolating a single electron spin in a dot containing only one as well as many electrons. Demonstrated is a current readout of spin transitions in a dot by means of spin blockade spectroscopy due to spin polarized injection/detection mechanism in a lateral dot. Discussed are transitions induced both by changing a magnetic field and a number of electrons in a dot with the emphasis on the effects observed close to filling factor in a dot ν = 2.

An AlGaAs/GaAs lateral quantum dot of triangular shape with a characteristic size L

We present a detailed investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the nanoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs [001] substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As-GaAs tubes from the nanometer to micrometer range by simply changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology.

The transport properties of a quantum wire interacting with a lateral two-level quantum dot are presented. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact for T = 0. The wire conductance is calculated for all different states of charge and spin of the quantum dot. We find that the Kondo correlation is stronger for a dot spin S D 1 than for S D 1/2. In both cases the wire current is totally quenched by the Kondo effect. When the dot is in the mixed-valence regime and 1/2 ≤ S D ≤ 1, the wire conductance is partially quenched except in a very small region of the gate potential where it reaches the maximum value e 2 /h.

We calculate energy levels, dipole moments and radiative broadening of interface fluctuation quantum dots. For optically allowed states, the dipole moment grows proportionally to the lateral quantum dot radius while the radiative broadening saturates towards the quantum well radiative broadening for large lateral quantum dot radii. This is accompanied by a change in the angular emission pattern, concentrating emission in forward and backward direction. Optically forbidden states do not couple to light propagating in the growth direction yet they may have a considerable radiative broadening due to spontaneous emission in other directions.

Single-photon detection of THz waves is realized by using two kinds of detectors. The first type of detector exploits a semiconductor lateral quantum dot in magnetic fields. The quantum dot is operated as a single electron transistor (SET) to detect individual events of THz-photon absorption (1.5 THz-1.9 THz) due to cyclotron-resonance induced intra-dot polarization. The second type detector utilizes parallel double quantum dots, in which one of the quantum dots serve as an SET while the other quantum dot affects the SET conductance when it is ionized by a THz photon absorption (about 0.5 THz). The detector performance is described. A future perspective for further improvements of the detectors is also briefly given.

We employ pulse excitation measurements in order to investigate spin-dependent energy relaxation inside a quantum dot. A fast pulse signal is applied to a gate electrode in order to excite a quantum dot, and the induced transient current is analyzed in terms of energy relaxation. We describe transient current measurements on a lateral quantum dot, in which the tunneling rates are controllable with gate voltages, and on a vertical quantum dot, in which the total number of electrons and total spin can be well identified. The spin-conservation rule during energy relaxation is clearly identified.

Low-temperature transport properties of a lateral quantum dot formed by overlaying finger gates in a clean one-dimensional channel are investigated. Continuous and periodic oscillations superimposed upon ballistic conductance steps are observed, when the conductance G of the dot changes within a wide range 0 2e^2/h may be due to suppression of inter-1D-subband scattering. Fully transmitted subbands contribute to coherent background of conductance, while sequential tunneling via weakly transmitted subbands leads to Coulomb charging of the dot.

Experimental observation of novel Kondo effects in quantum dots is reviewed with particular emphasis on the effect of magnetic field. The electronic configuration including spins in quantum dots can be modified by even a small magnetic field. We first use a vertical quantum dot with good tunability of orbital degeneracy in the few-electron regime but having a fixed tunnel coupling between the dot and leads. By tuning the spin configuration out of two electrons as a function of magnetic field, we observe a strong Kondo effect when the spin singlet and triplet are degenerate. This is due to the increased number of coherent co-tunneling processes. In the second we use a lateral quantum dot having good tunability of the tunnel coupling but some ambiguity in the electronic configuration. By adjusting both of the tunnel coupling and magnetic field, we observe the Kondo effect in the unitary limit. This is only observed in the presence of a small magnetic field, and can also be due to the change in the spin configuration.

We investigate correlation effects in few-electron lateral quantum dots between filling factor ν=2 and 1. The correlations are the non-trivial spin depolarization events associated with the evolution of the total spin of the droplet with increasing magnetic field. We monitor the spin evolution by measurements of coulomb blockade (CB) peaks on lateral quantum dot devices containing from 5 to 20 electrons. Measurements are made as a function of the magnetic field, temperature, and source–drain bias. The temperature and high source–drain bias dependence of the CB peaks in the vicinity of the spin-flip are consistent with spin depolarization events due to electronic correlations.

Our experiments demonstrate that Schottky-barrier reducing mechanisms can be overcome by adequately designed Si/SiGe heterostructures and that single electron transistor (SET) functionality can be achieved in modulation-doped Si/SiGe heterostructures with a standard split-gate approach that can easily be integrated into an array of coupled SETs

It is known that the local conditions in an electrical circuit can affect a nearby, but electrically isolated circuit. Such a ‘noninvasive’ measurement technique can be employed to study the screening ability (compressibility) of an electron system. In this work, we use a quasi-one-dimensional ballistic channel to screen an electric field between a gate and lateral quantum dot. Our results clearly show that the screening ability decreases with decreasing number of occupied 1D channel. When there are a large number of 1D subbands occupied in the channel, the screening ability tends to saturate, approaching its 2D limit.

A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe.

Growth of InGaAs nanostructures by molecular beam epitaxy using partial-cap and regrowth technique results in an ensemble of lateral quantum dot molecules (QDMs). Each QDM comprises a large, central quantum dot and several small, satellite quantum dots which emit at different energies and exhibit qualitatively different optical behaviors. This chapter reviews, explains, and discusses the various fundamental aspects of lateral QDM single layers: the nucleation mechanism, the photoluminescent spectra, and the unique bimodal optical characteristics. The chapter ends by demonstrating the usefulness of lateral QDM bi-layers as a promising broadband near-infrared material.

The spin-dependent dynamical blockade was investigated in a lateral quantum dot in a magnetic field. Spin-polarized edge channels in the two-dimensional leads and the spatial distribution of Landau orbitals in the dot modulate the tunnel coupling of the quantum dot level spectrum. In a measurement of the electron shot noise we observe a pattern of super-Poissonian noise which is correlated to the spin-dependent competition between different transport channels. © 2013 American Physical Society.

The strain field distribution at the surface of a multilayer structure with disklike SiGe nanomounds formed by heteroepitaxy is exploited to arrange the symmetric quantum dot molecules typically consisting of four elongated quantum dots ordered along the [010] and [100] directions. The morphological transition from fourfold quantum dot molecules to continuous fortresslike quantum rings with an increasing amount of deposited Ge is revealed. We examine key mechanisms underlying the formation of lateral quantum dot molecules by using scanning tunneling microscopy and numerical calculations of the strain energy distribution on the top of disklike SiGe nanomounds. Experimental data are well described by a simple thermodynamic model based on the accurate evaluation of the strain dependent part of the surface chemical potential. The spatial arrangement of quantum dots inside molecules is attributed to the effect of elastic property anisotropy.

Spin qubits have been successfully realized in electrostatically defined, lateral few-electron quantum dot circuits. Qubit readout typically involves spin to charge information conversion, followed by a charge measurement made using a nearby biased quantum point contact. It is critical to understand the back-action disturbances resulting from such a measurement approach. Previous studies have indicated that quantum point contact detectors emit phonons which are then absorbed by nearby qubits. We report here the observation of a pronounced back-action effect in multiple dot circuits where the absorption of detector-generated phonons is strongly modified by a quantum interference effect, and show that the phenomenon is well described by a theory incorporating both the quantum point contact and coherent phonon absorption. Our combined experimental and theoretical results suggest strategies to suppress back-action during the qubit readout procedure., 25 pages, 8 figures

We present a lateral quantum dot device which has a tunable number of quantum dots. Depending on easily tunable gate voltages, one, two or three quantum dots are found. They are investigated in transport and charge detection. © 2013 AIP Publishing LLC.

We present a unified theoretical approach, the finite-U slave-boson mean-field approach developed initially by Kotliar and Ruckenstein (1986 Phys. Rev. Lett. 57 1362), for the description of the linear and nonlinear Kondo-type transport through coupled double quantum dots (DQDs) connected in series to leads and through a multi-level lateral quantum dot. We find that, with increasing inter-dot hopping, the DQDs manifest three distinct physical scenarios: the Kondo singlet state of each dot with its adjacent lead, the spin singlet state consisting of local spins on each dot and the doubly occupied bonding orbital of the coupled dots. For a two-level quantum dot, our calculation demonstrates a substantial conductance enhancement near the degeneracy point of the spin singlet and triplet states, a non-monotonic temperature dependence of conductance and a sharp dip and non-zero bias maximum of the differential conductance. These agree well with recent experimental observations.

We fabricate groups of closely spaced self-assembled InAs quantum dots (QDs)—termed lateral QD molecules—on GaAs (001) by a combination of molecular-beam epitaxy and AsBr3 in situ etching. An initial array of homogeneously sized nanoholes is created by locally strain-enhanced etching of a GaAs cap layer above InAs QDs. Deposition of InAs onto the nanoholes causes a preferential formation of the InAs QD molecules around the holes. The number of QDs per QD molecule ranges from 2 to 6, depending on the InAs growth conditions. By decreasing the substrate temperature, the number of QDs per QD molecule increases, but the statistical distribution is wider due to a reduced In atom diffusion length. Our photoluminescence investigation documents the nanohole and QD molecule formation step by step and confirms the high crystal quality of these structures. An analysis of the nanohole geometry as a function of annealing time and InAs filling allows us to propose a model for the QD molecule formation process.

We use photoluminescence spectroscopy to investigate the ground state of single self-assembled InGaAs lateral quantum dot molecules. We apply a voltage along the growth direction that allows us to control the total charge occupancy of the quantum dot molecule. Using a combination of computational modeling and experimental analysis, we assign the observed discrete spectral lines to specific charge distributions. We explain the dynamic processes that lead to these charge configurations through electrical injection and optical generation. Our systemic analysis provides evidence of inter-dot tunneling of electrons as predicted in previous theoretical work., 9 pages, 4 figures