Search

Conventional quantum transport methods can provide quantitative information on spin, orbital, and valley states in quantum dots, but often lack spatial resolution. Scanning tunneling microscopy, on the other hand, provides exquisite spatial resolution of the local electronic density of states, but often at the expense of speed. Working to combine the spatial resolution and energy sensitivity of scanning probe microscopy with the speed of microwave measurements, we couple a metallic probe tip to a Si/SiGe double quantum dot that is integrated with a local charge detector. We first demonstrate that a dc-biased tip can be used to change the charge occupancy of the double dot. We then apply microwave excitation through the scanning tip to drive photon-assisted tunneling transitions in the double dot. We infer the double dot energy level diagram from the frequency and detuning dependence of the photon-assisted tunneling resonance condition. These measurements allow us to resolve $\sim$65 $\mu$eV excited states, an energy scale consistent with typical valley splittings in Si/SiGe. Future extensions of this approach may allow spatial mapping of the valley splitting in Si devices, which is of fundamental importance for spin-based quantum processors.

We design and analyze a logical qubit composed of a linear array of electron spins in semiconductor quantum dots. To avoid the difficulty of fully controlling a two-dimensional array of dots, we adapt spin control and error correction to a one-dimensional line of silicon quantum dots. Control speed and efficiency are maintained via a scheme in which electron spin states are controlled globally using broadband microwave pulses for magnetic resonance while two-qubit gates are provided by local electrical control of the exchange interaction between neighboring dots. Error correction with two-, three-, and four-qubit codes is adapted to a linear chain of qubits with nearest-neighbor gates. We estimate an error correction threshold of 1e-4. Furthermore, we describe a sequence of experiments to validate the methods on near-term devices starting from four coupled dots., 29 pages, 27 figures, 170 references

Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison with GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. We report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure, forming two levels of a singlet-triplet qubit. We measure a nuclei-induced T2* of 365 {plus minus} 11 ns, an increase over similar measurements in GaAs-based quantum dots by nearly two orders of magnitude. This value for T2* is consistent with theoretical expectations for our estimated dot sizes and a natural abundance of 29Si.

We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function -- control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the (0,2)(1,1) charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage., Comment: 4 pages, 5 figures

We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double-dot symmetrically between the charge-state anti-crossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of five in comparison to operation near a charge-state anti-crossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations., 10 pages, 8 figures

Three coupled quantum dots in isotopically purified silicon enable all-electrical qubit control with long coherence time., Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantum dots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking.

We present a level set based numerical algorithm for simulating a model of epitaxial growth. The island dynamics model is a continuum model for the growth of thin films. In this paper, we emphasize the details of the numerical method used to simulate the island dynamics model.

We show that fast edge diffusion in a simple kinetic Monte Carlo simulation for epitaxial growth enhances surface roughening for a model of irreversible aggregation. The reason for this is a simple, but rather subtle effect: Faster edge diffusion makes the islands more compact, leading to a significant adatom concentration on top of islands at earlier times. As a consequence, nucleation in higher layers occurs faster, increasing the surface roughness. We also demonstrate that fast edge diffusion can effectively contribute to diffusion of small clusters, and can produce unexpected scaling results.

We introduce a method which enables us to isolate different sources of fluctuations during a typical aggregation process. As an example, we focus on the evolution of islands during irreversible submonolayer epitaxy. We show that only spatial fluctuations in island nucleation are required to produce the scaling of their size distribution as determined by Monte Carlo simulations. In particular, once the islands are seeded, their growth can be described in a purely deterministic manner.

A kinetic theory is formulated for the velocity of a step edge in epitaxial growth. The formulation involves kinetic, mean-field equations for the density of kinks and "edge adatoms" along the step edge. Equilibrium and kinetic steady states, corresponding to zero and nonzero deposition flux, respectively, are derived for a periodic sequence of step edges. The theoretical results are compared to results from kinetic Monte Carlo (KMC) simulations of a simple solid-on-solid model, and excellent agreement is obtained. This theory provides a starting point for modeling the growth of two-dimensional islands in molecular-beam epitaxy through motion of their boundaries, as an alternative to KMC simulations.

We adapt the level set method to simulate the growth of thin films described by the motion of island boundaries. This island dynamics model involves a continuum in the lateral directions, but retains atomic scale discreteness in the growth direction. Several choices for the island boundary velocity are discussed, and computations of the island dynamics model using the level set method are presented.

We introduce a model for epitaxial phenomena based on the motion of island boundaries, which is described by the level-set method. Our model treats the growing film as a continuum in the lateral direction, but retains atomistic discreteness in the growth direction. An example of such an ‘‘island dynamics’’ model using the level-set method is presented and compared with the corresponding rate equation description. Extensions of our methodology to more general settings are then discussed. @S1063-651X~98!50212-7#

We present experimental data for the morphological evolution of InAs buffer layers which are interpreted using continuum equations of motion and kinetic Monte Carlo simulations. Our analysis reveals the presence of an instability even as an initially rough surface smooths during growth. This instability is due to the step-edge barrier and causes a characteristic length to emerge while the surface roughness is decreasing, well before the formation of the mounds. The smoothing is well described by a linear continuum equation identical to that which describes the early stages of spinodal decomposition and has important practical implications for the growth of device buffer layers.

Monte Carlo simulations and a scaling hypothesis are used to study the distribution of blob masses on two-dimensional finite-mass clusters at the percolation threshold. The exponents associated with this distribution function are a combination of backbone and percolation exponents. This work offers insights into the structure and fragmentation properties of percolation clusters in particular, and provides methods applicable to other fractal distribution problems in general.

Carlo computer simulations of spinodal decomposition to show that the model exhibits "pinning" of the structure factor, a behavior also seen in phase-separating polymer gels and binary alloys with impurities. The rate of strong bond formation depends on an entropic parameter 0, and we fInd both the pinned domain size and the crossover time between "normal" spinodal decomposition and the pinning scale with 0 as power laws with exponents that relate simply to the usual growth exponent. We propose a speci6c mechanism for pinning that permits the prediction of exact values for the pinning exponents. Finally, we discuss applications of the model to binary alloys with quenched disorder and polymer gels.

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si., Comment: 4 pages, 4 figures

We present computer simulation results of a solution of multiblock copolymers, and show that a network can form under certain conditions. This network formation is due to a bridging effect, which exists for multiblock but not for diblock copolymers. Because of this special property, we suggest that multiblock copolymers may be used quite generally to describe physical gels. Further, we find that separation of block types from each other hinders phase separation of polymers from solvent when the solvent is poor for both block types.

We develop a dynamical model for phase separation in a system with two disparate energy scales. Monte Carlo computer simulations of this model reveal a «pinning» of the structure factor during spinodal decomposition that obeys new scaling relations. We propose a mechanism for the pinning which allows us to predict exact values for the pinning exponents. Finally, we discuss our model in light of recent experiments

Realizing the theoretical promise of quantum computers will require overcoming decoherence. Here we demonstrate numerically that high fidelity quantum gates are possible within a framework of quantum dynamical decoupling. Orders of magnitude improvement in the fidelities of a universal set of quantum gates, relative to unprotected evolution, is achieved over a broad range of system-environment coupling strengths, using recursively constructed (concatenated) dynamical decoupling pulse sequences., Slightly expanded form of the journal version. Also includes the supplementary material as an appendix

A scaling theory and simulation results are presented for fragmentation of percolation clusters by random bond dilution. At the percolation threshold, scaling forms describe the average number of fragmenting bonds and the distribution of cluster masses produced by fragmentation. A relationship between the scaling exponents and standard percolation exponents is verified in one dimension, on the Bethe lattice, and for Monte Carlo simulations on a square lattice. These results further describe the structure of percolation clusters and provide kernels relevant to rate equations for fragmentation.

The dependence of substitutional C fraction on growth temperature and substrate orientation is measured for Si1−yCy alloy films grown on (001) and (118) Si by molecular-beam epitaxy. Secondary ion mass spectrometry and high-resolution x-ray diffraction were used to measure the total C and the substitutional C concentrations, respectively, in several samples prepared at temperatures between 450 and 650 °C. The substitutional C fraction decreased rapidly with increasing temperature in this range, regardless of orientation, and was slightly lower for growth on (118) Si. Cross-sectional transmission electron microscopy on (118)-oriented samples revealed a tendency for C to concentrate periodically on (001) facets which formed immediately after initiation of Si1−yCy growth. A kinetic Monte Carlo simulation based upon enhanced diffusion of Si dimers in the presence of subsurface C predicted a step instability leading to step bunching and the formation of periodic surface features, as well as the accumulation of...

We report on the fabrication and characterization of a few-electron quantum dot controlled by a single gate electrode. Our device has a double-quantum-well design, in which the doping controls the occupancy of the lower well while the upper well remains empty under the free surface. A small air-bridged gate contacts the surface, and is positively biased to draw laterally confined electrons into the upper well. Electrons tunneling between this accumulation-mode dot and the lower well are detected using a quantum point contact (QPC), located slightly offset from the dot gate. The charge state of the dot is measured by monitoring the differential transconductance of the QPC near pinch-off. Addition spectra starting with N=0 were observed as a function of gate voltage. DC sensitivity to single electrons was determined to be as high as 8.6%, resulting in a signal-to-noise ratio of ~9:1 with an equivalent noise bandwidth of 12.1 kHz. Analysis of random telegraph signals associated with the zero to one electron transition allowed a measurement of the lifetimes for the filled and empty states of the one-electron dot: 0.38 ms and 0.22 ms, respectively, for a device with a 10 nm AlInAs tunnel barrier between the two wells., 3 pages, 3 figures

Gate-tunable high-mobility InSb/In_{1-x}Al_{x}Sb quantum wells (QWs) grown on GaAs substrates are reported. The QW two-dimensional electron gas (2DEG) channel mobility in excess of 200,000 cm^{2}/Vs is measured at T=1.8K. In asymmetrically remote-doped samples with an HfO_{2} gate dielectric formed by atomic layer deposition, parallel conduction is eliminated and complete 2DEG channel depletion is reached with minimal hysteresis in gate bias response of the 2DEG electron density. The integer quantum Hall effect with Landau level filling factor down to 1 is observed. A high-transparency non-alloyed Ohmic contact to the 2DEG with contact resistance below 1{\Omega} \cdot mm is achieved at 1.8K., Comment: 25 pages, 10 figures

The error threshold for fault tolerant quantum computation with concatenated encoding of qubits is penalized by internal communication overhead. Many quantum computation proposals rely on nearest-neighbour communication, which requires excess gate operations. For a qubit stripe with a width of L+1 physical qubits implementing L levels of concatenation, we find that the error threshold of 2.1x10^-5 without any communication burden is reduced to 1.2x10^-7 when gate errors are the dominant source of error. This ~175X penalty in error threshold translates to an ~13X penalty in the amplitude and timing of gate operation control pulses., minor correction

Microscopic mechanisms of the $(2\ifmmode\times\else\texttimes\fi{}4)$ to $(4\ifmmode\times\else\texttimes\fi{}2)$ surface phase transition on InAs(001) are identified using a combination of theoretical and experimental methods. Two distinct transition stages are found, both rate limited by ${\mathrm{As}}_{2}$ desorption. The unusually high prefactors observed experimentally are traced back to the microscopic ${\mathrm{As}}_{2}$ desorption processes. Calculated interactions between As dimers explain the observed disorder of the mixed $\ensuremath{\alpha}2(2\ifmmode\times\else\texttimes\fi{}4)$ and $\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructions and are not responsible for observed first order behavior of the transition.

We present a general methodology for developing predictive atomistic models of III-V semiconductor surfaces. We apply this methodology to the development of a kinetic Monte Carlo model that describes both equilibrium and growth of InAs(001) surfaces. Thermodynamic equilibrium behavior is solely based on ab initio density-functional theory calculations. Inclusion of kinetic processes and a large set of atomistic configurations on this highly complex surface enables the correct prediction of several experimental trends, including two thermodynamically stable surface reconstructions in the As-rich regime.

A level-set model for the simulation of epitaxial growth is described. In this model, the motion of island boundaries of discrete atomic layers is determined by the time evolution of a continuous level-set function w. The adatom concentration is treated in a mean-field manner. We use this model to systematically examine the importance of various fluctuations in the submonolayer and multilayer regimes. We find that, in the submonolayer regime for large values of D/F, the dominant fluctuations are associated with the spatial seeding of islands. We also show how different microscopic mechanisms can be included into this formalism. In the multilayer regime, our model exhibits surface roughening.

We introduce a general method for describing the morphological evolution of coherent heteroepitaxial films based on linear elasticity. The substrate and film are modelled as discrete layers of continuous material whose lateral and vertical extents are measured in discrete atomic-scale units. The force-balance equations are obtained from a variational calculation of a suitably discretized form of the total elastic energy. The continuum limit of these equations and the boundary conditions along straight boundaries reproduce standard results of linear elasticity theory, but the boundary conditions at corners have no such analogue. We apply our methodology to the step-bunching instability on a one-dimensional strained vicinal surface and discuss the effects of epilayer thickness, elastic constants, and the misfit strain on the equilibration of the surface toward the bunched state.

The initial stages of InAs(001) homoepitaxial growth are investigated using a combination of kinetic Monte Carlo simulations based on ab initio density functional theory and scanning tunneling microscopy. In the two dimensional island nucleation mode investigated, the island number density is found to decrease with increasing As. This behavior is explained by a suppression of the effective In-adatom density leading to a reduction in island nucleation. The relevant microscopic processes responsible for this reduction are identified.

Growth kinetics and thermodynamic equilibrium can both be determining factors at different stages of III-V semiconductor heteroepitaxy. We study their interplay, employing kinetic Monte Carlo simulations for the InAs(001) surface. The simulation contains atomistic details of both species, including the stability of different reconstructions and their kinetics. The behavior of the surface in thermodynamic equilibrium, including different reconstructions, is determined exclusively by extensive total energy calculations employing ab initio density functional theory. The continuous phase transition between the α2(2x4) and β2(2x4), predicted by theory, is confirmed by experiment. At full layer coverage, a recovery of the stable reconstruction is observed. The different time scales associated with As2 and In are discussed with respect to equilibrium and kinetics.

Molecular Beam Epitaxy is a method for growing atomically thin films of material. During epitaxial growth, atoms are deposited on a surface, where they hop randomly until attaching at the edges of partially completed atomic monolayers. This process has practical application to the fabrication of high speed semiconductor electronic devices.

We study the late stages of spinodal decomposition in a Ginzburg-Landau mean field model with quenched disorder. Random spatial dependence in the coupling constants is introduced to model the quenched disorder. The effect of the disorder on the scaling of the structure factor and on the domain growth is investigated in both the zero temperature limit and at finite temperature. In particular, we find that at zero temperature the domain size, $R(t)$, scales with the amplitude, $A$, of the quenched disorder as $R(t) = A^{-\beta} f(t/A^{-\gamma})$ with $\beta \simeq 1.0$ and $\gamma \simeq 3.0$ in two dimensions. We show that $\beta/\gamma = \alpha$, where $\alpha$ is the Lifshitz-Slyosov exponent. At finite temperature, this simple scaling is not observed and we suggest that the scaling also depends on temperature and $A$. We discuss these results in the context of Monte Carlo and cell dynamical models for phase separation in systems with quenched disorder, and propose that in a Monte Carlo simulation the concentration of impurities, $c$, is related to $A$ by $A \sim c^{1/d}$., Comment: RevTex manuscript 5 pages and 5 figures (obtained upon request via email sth@iris.bu.edu)

The fragmentation properties of percolation clusters yield information about their structure. Monte Carlo simulations and exact cluster enumeration for a square bond lattice and exact calculations for the Bethe lattice are used to study the fragmentation probability ${\mathrm{a}}_{\mathrm{s}}$(p) of clusters of mass s at an occupation probability p and the likelihood ${\mathrm{b}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$(p) that fragmentation of an s cluster will result in a daughter cluster of mass s\ensuremath{'}. Evidence is presented to support the scaling laws ${\mathrm{a}}_{\mathrm{s}}$(${\mathrm{p}}_{\mathrm{c}}$)\ensuremath{\sim}s and ${\mathrm{b}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$(${\mathrm{p}}_{\mathrm{c}}$)=${\mathrm{s}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\varphi}}}$g(s\ensuremath{'}/s), with \ensuremath{\varphi}=2-\ensuremath{\sigma} given by the standard cluster-number scaling exponent \ensuremath{\sigma}. Simulations for d=2 verify the finite-size-scaling form ${\mathrm{c}}_{\mathrm{s}\ensuremath{'}\mathrm{sL}}$(${\mathrm{p}}_{\mathrm{c}}$)=${\mathrm{s}}^{1\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\varphi}}}$g\ifmmode \tilde{}\else \~{}\fi{}(s\ensuremath{'}/s,s/${\mathrm{L}}^{{\mathrm{d}}_{\mathrm{f}}}$) of the product ${\mathrm{c}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$(${\mathrm{p}}_{\mathrm{c}}$)=${\mathrm{a}}_{\mathrm{s}}$(${\mathrm{p}}_{\mathrm{c}}$)${\mathrm{b}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$(${\mathrm{p}}_{\mathrm{c}}$), where L is the lattice size and ${\mathrm{d}}_{\mathrm{f}}$ is the fractal dimension. Exact calculations of the fragmentation probability ${\mathrm{f}}_{\mathrm{st}}$ of a cluster of mass s and perimeter t indicate that branches are important even on the maximum perimeter clusters. These calculations also show that the minimum of ${\mathrm{b}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$(p) near s\ensuremath{'}=s/2, where the two daughter masses are comparable, deepens with increasing p.

Combining theoretical and experimental methods, we investigate the influence of group V fluxes of As2 and Sb2 on the InAs(001) surface. We find that equilibrated surfaces under As2 flux change their surface stoichiometry continuously for InAs in the technologically relevant α2(2×4)-β2(2×4) reconstruction region and that the As-dimer density increases with increasing As2 flux. The change of the surface morphology under Sb2 exposure is also studied and discussed specifically in the context of interface formation. The existence of a common α2(2×4) reconstruction allows for the possibility of keeping the In sublattice unchanged when switching from As2 to Sb2 flux.

Coherent charge-photon and spin-photon coupling has recently been achieved in silicon double quantum dots (DQDs). Here, we demonstrate a versatile split-gate cavity-coupler that allows more than one DQD to be coupled to the same microwave cavity. Measurements of the cavity transmission as a function of level detuning yield a charge cavity coupling rate of g c / 2 π = 58 MHz, a charge decoherence rate of γ c / 2 π = 36 MHz, and a cavity decay rate of κ / 2 π = 1.2 MHz. The charge cavity coupling rate is in good agreement with device simulations. Our coupling technique can be extended to enable simultaneous coupling of multiple DQDs to the same cavity mode, opening the door to long-range coupling of semiconductor qubits using microwave frequency photons.