Isolated Quantum Dots Research Articles

Fe4 cluster as the smallest 3D Fe cluster with unique quantum magnetic levitation effect on graphene

The unique quantum magnetic levitation effect of small metallic clusters adsorbed on graphene has been studied for their minimal energy configurations in this work. Using first-principles calculations, we specially focus on the tetrahedral Fe4 clusters in a corn-like configuration for the induced magnetic moments in the Fe4 cluster and graphene. Firstly, the 3D magnetic Fe4 cluster spontaneously induced a diamagnetic moment in graphene in the absence of external energy injection. Additionally, as the smallest 3D particle (among Fen clusters), the Fe4 cluster can be levitated by the repulsive magnetic interaction between the Fe cluster and graphene. The maximum energy barrier between different adsorption configurations of the Fe4 cluster on graphene is as low as 0.8 eV, making the Fe4 cluster diffusible between sites with a minimal external force. The conversion energy barrier is relatively low, resulting in low energy consumption. Importantly, the repulsion between Fe clusters may lead to the formation of an ultrahigh-density (1.18 × 1014/cm2) isolated quantum dot array and avoids the formation of larger clusters. These quantum states might be usable for nanoscale data storage devices for next-generation computing applications, and the small diffusion barrier might offer potential applications in quantum mechanical sensing and nanoscale magnetic levitation.

Spin-dependent tunneling in a double quantum dot in the "slow" evolution regime

The tunneling and spin dynamics is studied for the hole states in a GaAs-based double quantum dot in the presence of strong spin-orbit coupling and periodic electric field. The regimes of tunneling with the spin flip are considered for the "slow" evolution when the field frequency is lower than the other energy parameters of the stationary part of the Hamiltonian. It is found that the under such conditions the spin flip tunneling may take place at both resonant and non-resonant regimes with respect to the Zeeman level splitting. In the latter case the driving frequency may be lower compared to the resonance one, and the system dynamics resembles the Landau-Zener-Stuckelberg-Majorana interference effects arising during the dynamic level passage in isolated quantum dots. Keywords: double quantum dot, spin-orbit interaction, Zeeman splitting, tunneling, electric dipole spin resonance.

Спин-зависимое туннелирование в двойной квантовой точке в режиме "медленной" эволюции

The tunneling and spin dynamics is studied for the hole states in a GaAs-based double quantum dot in the presence of strong spin-orbit coupling and periodic electric field. The regimes of tunneling with the spin flip are considered for the “slow” evolution when the field frequency is lower than the other energy parameters of the stationary part of the Hamiltonian. It is found that the under such conditions the spin flip tunneling may take place at both resonant and non-resonant regimes with respect to the Zeeman level splitting. In the latter case the driving frequency may be lower compared to the resonance one, and the system dynamics resembles the Landau-Zener-Stückelberg-Majorana interference effects arising during the dynamic level passage in isolated quantum dots.

Efficient Biexciton Preparation in a Quantum Dot-Metal Nanoparticle System Using On-Off Pulses.

We consider a hybrid nanostructure composed by semiconductor quantum dot coupled to a metallic nanoparticle and investigate the efficient creation of biexciton state in the quantum dot, when starting from the ground state and using linearly polarized laser pulses with on-off modulation. With numerical simulations of the coupled system density matrix equations, we show that a simple on-off-on pulse-sequence, previously derived for the case of an isolated quantum dot, can efficiently prepare the biexciton state even in the presence of the nanoparticle, for various interparticle distances and biexciton energy shifts. The pulse durations in the sequence are obtained from the solution of a transcendental equation.

Helicity-dependent all-optical switching based on the self-trapped triplet excitons

Triplet excitons in organic materials are nonradiative and tightly bound as self-trapped ones due to the strong electron-lattice coupling. They exhibit a finite radius and long lifetime and so can be regarded as the isolated quantum dots. In this work, we theoretically demonstrate that the spin polarization of self-trapped triplet excitons can be coherently controlled by the short circularly polarized laser pulses, acting as a helicity-dependent all-optical switching effect. Such a switching can be achieved within dozens of femtoseconds and conducted in a reproducible manner. The pulse parameter dependence on the switching ratio is further investigated. Our calculation provides a theoretical foundation for exploring the ultrafast all-optical recording and information processing technique based on the organic materials.

Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot

Combining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

A two-dimensional array of single-hole quantum dots

Quantum dots fabricated using methods compatible with semiconductor manufacturing are promising for quantum information processing. In order to fully utilize the potential of this platform, scaling quantum dot arrays along two dimensions is a key step. Here, we demonstrate a two-dimensional quantum dot array where each quantum dot is tuned to single-charge occupancy, verified by simultaneous measurements using two integrated radio frequency charge sensors. We achieve this by using planar germanium quantum dots with low disorder and a small effective mass, allowing the incorporation of dedicated barrier gates to control the coupling of the quantum dots. We measure the hole charge filling spectrum and show that we can tune single-hole quantum dots from isolated quantum dots to strongly exchange coupled quantum dots. These results motivate the use of planar germanium quantum dots as building blocks for quantum simulation and computation.

Electronic properties of \u03b1 \u2212 \U0001d4af3 quantum dots in magnetic fields

We address the electronic properties of quantum dots in the two-dimensional α − \U0001d4af3 lattice when subjected to a perpendicular magnetic field. Implementing an infinite mass boundary condition, we first solve the eigenvalue problem for an isolated quantum dot in the low-energy, long-wavelength approximation where the system is described by an effective Dirac-like Hamiltonian that interpolates between the graphene (pseudospin 1/2) and Dice (pseudospin 1) limits. Results are compared to a full numerical (finite-mass) tight-binding lattice calculation. In a second step we analyse charge transport through a contacted α − \U0001d4af3 quantum dot in a magnetic field by calculating the local density of states and the conductance within the kernel polynomial and Landauer-Büttiker approaches. Thereby the influence of a disordered environment is discussed as well.Graphical abstract

Femto-Second Carrier and Photon Dynamics in Site Controlled Hexagonal InGaN/GaN Isolated Quantum Dots: Natural Radial Potential Well and Its Dynamic Modulation

Quantum dot (QD) is slated to play a significant role in quantum technologies through its usage as a single-photon source. It also has promising applications as efficient light emitters through strain-relaxed structures. This work demonstrates the fabrication of high-quality site controlled InGaN/GaN QDs through a large contribution of chemical etching in an otherwise reactive ion etching process. Uniform sized QDs with a hexagonal base and radii of 15 and 6 nm are fabricated and characterized by photoluminescence and femtosecond transient absorption spectroscopy. The natural exposition of the inherent hexagonal base is a signature of the reduced defects in these dots. The QDs show the intuitive additional quantum confinement due to size reduction, and the photoluminescence peaks manifest a blue shift. The absorption spectra indicate that the QDs are heavily strain-relaxed at smaller radii, and their characteristics as a function of size and pump power are investigated. The degree of strain relaxation and change in energy-band diagram as a function of position along the radius are also determined. The changes are prominent near the periphery, which creates a natural potential well for both electrons and holes, restricting the carriers from reaching the sidewalls. Femtosecond carrier dynamics indicate a lower capture rate for the smaller size of the dots, indicating excessive electrical or optical pumping will not necessarily lead to increased luminescence. The radiative decay of carriers is faster for strain-relaxed QDs due to higher oscillator strength originating from increased electron–hole wave function overlap. Higher pump power is found to decrease the radiative rate due to the increased effective size of the QDs and thereby reducing carrier injection density. This is in stark contrast to the frequently observed increased decay rate due to Auger recombination. Dynamic modification of the size of the QDs by pump power may find potential use in optoelectronic applications.

Resonance fluorescence of a hybrid semiconductor-quantum-dot–metal-nanoparticle system driven by a bichromatic field

We study the spectrum and statistical properties of photons scattered from a semiconductor-quantum-dot--metal-nanoparticle system under monochromatic and bichromatic excitations. We rely on the Bloch equation to describe the evolution of the density matrix of the quantum dot. We pay attention to the self-interaction of the quantum dot in the presence of the nanoparticle. Going beyond the dipole approximation, we show that the system exhibits optical responses of different character in different regions of the quantum dot dipole moment versus the nanoparticle radius phase diagram. In the strong transition and bistability regions, upon changing the initial state, a pronounced fluorescence spectrum may become a faint one, and an oscillatory intensity-intensity correlation may become a monotonically increasing one. The amplitudes, frequencies, and phases of the laser fields tailor the number, position, height, and width of the peaks of the fluorescence spectrum. The antibunched light as well as the sub-Poissonian light can be generated. Our results suggest that in view of solid-state-based sources of nonclassical light, a hybrid quantum-dot--nanoparticle system may be superior to an isolated quantum dot.

Charge Reconfiguration in Isolated Quantum Dot Arrays

AbstractA high level of tunability and control over arrays of quantum dots are key ingredients toward the goal of scalable‐based qubit architectures. Increasing array size simultaneously increases the parameter space and therefore the tuning complexity. The electron reconfiguration behavior of quantum dot arrays isolated from the electron reservoirs is studied experimentally. Isolating a quantum dot array from the reservoirs does not only enable a high degree of control over the tunnel couplings but at the same time drastically simplifies the stability diagrams for small numbers of electrons trapped in the array. Experimental results on double, triple, and quadruple quantum dot arrays are presented and complementary model calculations allow the identification of all transitions observed in the experiment. Highly tunable long‐range transitions are observed in isolated triple quantum dots and evidence of higher‐order cotunneling is found for the quadruple quantum dot array.

Plasmon-assisted two-photon Rabi oscillations in a semiconductor quantum dot – metal nanoparticle heterodimer

Tho-photon Rabi oscillations hold potential for quantum computing and quantum information processing, because during a Rabi cycle a pair of entangled photons may be created. We theoretically investigate the onset of this phenomenon in a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. Two-photon Rabi oscillations in this system occur due to a coherent two-photon process involving the ground-to-biexciton transition in the quantum dot. The presence of a metal nanoparticle nearby the quantum dot results in a self-action of the quantum dot via the metal nanoparticle, because the polatization state of the latter depends on the quantum state of the former. The interparticle interaction gives rise to two principal effects: (i) - enhancement of the external field amplitude and (ii) - renormalization of the quantum dot's resonance frequencies and relaxation rates of the off-diagonal density matrix elements, both depending on the populations of the quantum dot's levels. Here, we focus on the first effect, which results in interesting new features, in particular, in an increased number of Rabi cycles per pulse as compared to an isolated quantum dot and subsequent growth of the number of entangled photon pairs per pulse. We also discuss the destructive role of radiative decay of the excitonic states on two-photon Rabi oscillations for both an isolated quantum dot and a heterodimer.

Gate-Tunable Fano Resonances in Parallel-Polyacene-Bridged Carbon Nanotubes

A nanoscale device functioning in electronic transport via electrically tunable Fano resonances has huge potential for applications but is still rarely available to date. Using first-principles calculations, we show that a double-path molecular junction under an applied gate voltage can realize such a goal. It turns out that the crosstalk between two paths can be mapped to an ideal Fano–Anderson model, a single-path junction coupling to an isolated quantum dot. Its line shape of Fano resonance progressively evolves from an asymmetric to a symmetric Breit–Wigner peak when the gate voltage increases moderately. The significance of this system is illustrated by the sizable coupling strength that scales linearly with the gate voltage. On the basis of this scheme, we propose that these tunable Fano molecular junctions can serve as efficient transistors and thermoelectric energy conversion devices.

Polarization Spectroscopy of an Isolated Quantum Dot and an Isolated Quantum Wire

Photoluminescence spectra of an isolated GaAs quantum dot within an AlGaAs quantum wire are studied. The examination of behavior of spectra in a magnetic field provided an opportunity to estimate the exciton binding energy in the quantum dot, which turned out to be 10 times higher than the bulk exciton binding energy in GaAs. It is demonstrated that the signal of exciton photoluminescence from the quantum dot emitted along the nanowire axis is linearly polarized. At the same time, the photoluminescence signal propagating in the direction perpendicular to the nanowire axis is almost unpolarized. This may be attributed to the nonaxial dot positioning in the wire under a giant increase in the binding energy of an exciton affected by an image potential.

Effects of Geometry on the Electronic Properties of Semiconductor Elliptical Quantum Rings

The electronic states in GaAs-AlxGa1−xAs elliptically-shaped quantum rings are theoretically investigated through the numerical solution of the effective mass band equation via the finite element method. The results are obtained for different sizes and geometries, including the possibility of a number of hill-shaped deformations that play the role of either connected or isolated quantum dots (hills), depending on the configuration chosen. The quantum ring transversal section is assumed to exhibit three different geometrical symmetries - squared, triangular and parabolic. The behavior of the allowed confined states as functions of the cross-section shape, the ring dimensions, and the number of hills-like structures are discussed in detail. The effective energy bandgap (photoluminescence peak with electron-hole correlation) is reported as well, as a function of the Al molar fraction.

Influence of dopant concentration on electrical quantum transport behaviors in junctionless nanowire transistors**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0200503), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province, China (Grant No. 18IRTSTHN016), and the National Natural Science Foundation of China (Grant Nos. 61376096, 61327813, and 61404126).

We discuss the random dopant effects in long channel junctionless transistor associated with quantum confinement effects. The electrical measurement reveals the threshold voltage variability induced by the random dopant fluctuation. Quantum transport features in Hubbard systems are observed in heavily phosphorus-doped channel. We investigate the single electron transfer via donor-induced quantum dots in junctionless nanowire transistors with heavily phosphorus-doped channel, due to the formation of impurity Hubbard bands. While in the lightly doped devices, one-dimensional quantum transport is only observed at low temperature. In this sense, phonon-assisted resonant-tunneling is suppressed due to misaligned levels formed in a few isolated quantum dots at cryogenic temperature. We observe the Anderson-Mott transition from isolate electron state to impurity bands as the doping concentration is increased.

Inducing coherent quantum dot interactions

We measure many-body interactions in isolated quantum dot states using double-quantum multidimensional coherent spectroscopy. Few states are probed in a diffraction limited spot, which is enabled by a novel collinear scheme in which radiated four-wave-mixing signals are measured with phase resolution. Many-body interactions are enhanced by an additional prepulse tuned to the delocalized quasi-continuum states. We propose this effect as a method for controlling coupling between quantum states.

Atomically manufactured nickel\u2013silicon quantum dots displaying robust resonant tunneling and negative differential resistance

Providing a spin-free host material in the development of quantum information technology has made silicon a very interesting and desirable material for qubit design. Much of the work and experimental progress has focused on isolated phosphorous atoms. In this article, we report on the exploration of Ni–Si clusters that are atomically manufactured via self-assembly from the bottom-up and behave as isolated quantum dots. These small quantum dot structures are probed at the atomic-scale with scanning tunneling microscopy and spectroscopy, revealing robust resonance through discrete quantized energy levels within the Ni–Si clusters. The resonance energy is reproducible and the peak spacing of the quantum dot structures increases as the number of atoms in the cluster decrease. Probing these quantum dot structures on degenerately doped silicon results in the observation of negative differential resistance in both I–V and dI/dV spectra. At higher surface coverage of nickel, a well-known √19 surface modification is observed and is essentially a tightly packed array of the clusters. Spatial conductance maps reveal variations in the local density of states that suggest the clusters are influencing the electronic properties of their neighbors. All of these results are extremely encouraging towards the utilization of metal modified silicon surfaces to advance or complement existing quantum information technology.

Guest Editorial – Special Issue on Quantum Circuits

Guest Editorial – Special Issue on Quantum Circuits

On small signal equivalent circuit models for quantum dots

SummaryIn this paper, we introduce small signal equivalent circuit models for quantum dots at equilibrium driven by classical electromagnetic fields. We investigate both an isolated quantum dot and a pair of interacting quantum dots coupled by the electric dipole–dipole interaction. In particular, we derive the impulse responses of the equivalent circuit elements from the density operator of the quantum dots. The use of equivalent circuit elements for complex systems of quantum dots may greatly simplify the analysis and the design of light‐matter interaction processes at the nanoscale. Copyright © 2017 John Wiley & Sons, Ltd.