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Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantum wells require large or tunable energy splittings of the valley states associated with degenerate conduction band minima. Existing proposals to deterministically enhance the valley splitting rely on sharp interfaces or modifications in the quantum well barriers that can be difficult to grow. Here, we propose and demonstrate a new heterostructure, the "Wiggle Well", whose key feature is Ge concentration oscillations inside the quantum well. Experimentally, we show that placing Ge in the quantum well does not significantly impact our ability to form and manipulate single-electron quantum dots. We further observe large and widely tunable valley splittings, from 54 to 239 μeV. Tight-binding calculations, and the tunability of the valley splitting, indicate that these results can mainly be attributed to random concentration fluctuations that are amplified by the presence of Ge alloy in the heterostructure, as opposed to a deterministic enhancement due to the concentration oscillations. Quantitative predictions for several other heterostructures point to the Wiggle Well as a robust method for reliably enhancing the valley splitting in future qubit devices., (© 2022. The Author(s).)

The current practice of manually tuning quantum dots (QDs) for qubit operation is a relatively time-consuming procedure that is inherently impractical for scaling up and applications. In this work, we report on the in situ implementation of a recently proposed autotuning protocol that combines machine learning (ML) with an optimization routine to navigate the parameter space. In particular, we show that a ML algorithm trained using exclusively simulated data to quantitatively classify the state of a double-QD device can be used to replace human heuristics in the tuning of gate voltages in real devices. We demonstrate active feedback of a functional double-dot device operated at millikelvin temperatures and discuss success rates as a function of the initial conditions and the device performance. Modifications to the training network, fitness function, and optimizer are discussed as a path toward further improvement in the success rate when starting both near and far detuned from the target double-dot range.

Thermal management efforts in nanoscale devices must consider both the thermal properties of the constituent materials and the interfaces connecting them. It is currently unclear whether alloy/alloy semiconductor superlattices such as InAlAs/InGaAs have lower thermal conductivities than their constituent alloys. We report measurements of the crossplane thermal resistivity of InAlAs/InGaAs superlattices at room temperature, showing that the superlattice resistivities are larger by a factor of 1.2-1.6 than that of the constituent bulk materials, depending on the strain state and composition. We show that the additional resistance present in these superlattices can be tuned by a factor of 2.5 by altering the lattice mismatch and thereby the phonon-mode mismatch at the interfaces, a principle that is commonly assumed for superlattices but has not been experimentally verified without adding new elements to the layers. We find that the additional resistance in superlattices does not increase significantly when the layer thickness is decreased from 4 to 2 nm. We also report measurements of 250-1000 nm thick films of undoped InGaAs and InAlAs lattice-matched to InP substrates, for there is no published thermal conductivity value for the latter, and we find it to be 2.24 ± 0.09 at 22 °C, which is ∼2.7 times smaller than the widely used estimates.

We theoretically demonstrate that screw dislocation (SD), a 1D topological defect widely present in semiconductors, exhibits ubiquitously a new form of spin-orbit coupling (SOC) effect. Differing from the widely known conventional 2D Rashba-Dresselhaus (RD) SOC effect that typically exists at surfaces or interfaces, the deep-level nature of SD-SOC states in semiconductors readily makes it an ideal SOC. Remarkably, the spin texture of 1D SD-SOC, pertaining to the inherent symmetry of SD, exhibits a significantly higher degree of spin coherency than the 2D RD-SOC. Moreover, the 1D SD-SOC can be tuned by ionicity in compound semiconductors to ideally suppress spin relaxation, as demonstrated by comparative first-principles calculations of SDs in Si/Ge, GaAs, and SiC. Our findings therefore open a new door to manipulating spin transport in semiconductors by taking advantage of an otherwise detrimental topological defect.

Now that it is possible to achieve measurement and control fidelities for individual quantum bits (qubits) above the threshold for fault tolerance, attention is moving towards the difficult task of scaling up the number of physical qubits to the large numbers that are needed for fault-tolerant quantum computing. In this context, quantum-dot-based spin qubits could have substantial advantages over other types of qubit owing to their potential for all-electrical operation and ability to be integrated at high density onto an industrial platform. Initialization, readout and single- and two-qubit gates have been demonstrated in various quantum-dot-based qubit representations. However, as seen with small-scale demonstrations of quantum computers using other types of qubit, combining these elements leads to challenges related to qubit crosstalk, state leakage, calibration and control hardware. Here we overcome these challenges by using carefully designed control techniques to demonstrate a programmable two-qubit quantum processor in a silicon device that can perform the Deutsch-Josza algorithm and the Grover search algorithm-canonical examples of quantum algorithms that outperform their classical analogues. We characterize the entanglement in our processor by using quantum-state tomography of Bell states, measuring state fidelities of 85-89 per cent and concurrences of 73-82 per cent. These results pave the way for larger-scale quantum computers that use spins confined to quantum dots.

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.

We report the fabrication and characterization of a gate-defined double quantum dot formed in a Si/SiGe nanomembrane. In the past, all gate-defined quantum dots in Si/SiGe heterostructures were formed on top of strain-graded virtual substrates. The strain grading process necessarily introduces misfit dislocations into a heterostructure, and these defects introduce lateral strain inhomogeneities, mosaic tilt, and threading dislocations. The use of a SiGe nanomembrane as the virtual substrate enables the strain relaxation to be entirely elastic, eliminating the need for misfit dislocations. However, in this approach the formation of the heterostructure is more complicated, involving two separate epitaxial growth procedures separated by a wet-transfer process that results in a buried non-epitaxial interface 625 nm from the quantum dot. We demonstrate that in spite of this buried interface in close proximity to the device, a double quantum dot can be formed that is controllable enough to enable tuning of the inter-dot tunnel coupling, the identification of spin states, and the measurement of a singlet-to-triplet transition as a function of an applied magnetic field.

The operation of solid-state qubits often relies on single-shot readout using a nanoelectronic charge sensor, and the detection of events in a noisy sensor signal is crucial for high fidelity readout of such qubits. The most common detection scheme, comparing the signal to a threshold value, is accurate at low noise levels but is not robust to low-frequency noise and signal drift. We describe an alternative method for identifying charge sensor events using wavelet edge detection. The technique is convenient to use and we show that, with realistic signals and a single tunable parameter, wavelet detection can outperform thresholding and is significantly more tolerant to 1/f and low-frequency noise.

The qubit is the fundamental building block of a quantum computer. We fabricate a qubit in a silicon double-quantum dot with an integrated micromagnet in which the qubit basis states are the singlet state and the spin-zero triplet state of two electrons. Because of the micromagnet, the magnetic field difference ΔB between the two sides of the double dot is large enough to enable the achievement of coherent rotation of the qubit's Bloch vector around two different axes of the Bloch sphere. By measuring the decay of the quantum oscillations, the inhomogeneous spin coherence time T2* is determined. By measuring T2* at many different values of the exchange coupling J and at two different values of ΔB, we provide evidence that the micromagnet does not limit decoherence, with the dominant limits on T2* arising from charge noise and from coupling to nuclear spins.

Poly(dimethylsiloxane), PDMS, a versatile elastomer, is the polymer of choice for microfluidic systems. It is inexpensive, relatively easy to pattern, and permeable to oxygen. Unmodified PDMS is highly hydrophobic. It is typically exposed to an oxygen plasma to reduce this hydrophobicity. Unfortunately, the PDMS surface soon returns to its original hydrophobic state. We present two alternative plasma treatments that yield long-term modification of the wetting properties of a PDMS surface. An oxygen plasma pretreatment followed by exposure to a SiCl4 plasma and an oxygen-CCl4 mixture plasma both cause a permanent reduction in the hydrophobicity of the PDMS surface. We investigate the properties of the plasma-treated surfaces with X-ray photoelectron spectroscopy (XPS) and contact angle measurements. We propose that the plasma treated PDMS surface is a dynamic mosaic of high- and low-contact-angle functionalities. The SiCl4 and CCl4 plasmas attach polar groups that block coverage of the surface by low-molecular-weight groups that exist in PDMS. We describe an application that benefits from these new plasma treatments, the use of a PDMS stencil to form dense arrays of DNA on a surface.

We demonstrate single-shot readout of a silicon quantum dot spin qubit, and we measure the spin relaxation time T1. We show that the rate of spin loading can be tuned by an order of magnitude by changing the amplitude of a pulsed-gate voltage, and the fraction of spin-up electrons loaded can also be controlled. This tunability arises because electron spins can be loaded through an orbital excited state. Using a theory that includes excited states of the dot and energy-dependent tunneling, we find that a global fit to the loading rate and spin-up fraction is in good agreement with the data.

While many processes have been developed to modify the surface of glass and other oxides for biomolecule attachment, they rely primarily upon wet chemistry and are costly and time-consuming. We describe a process that uses a cold plasma and a subsequent in vacuo vapor-phase reaction to terminate a variety of oxide surfaces with epoxide chemical groups. These epoxide groups can react with amine-containing biomolecules, such as proteins and modified oligonucleotides, to form strong covalent linkages between the biomolecules and the treated surface. The use of a plasma activation step followed by an in vacuo vapor-phase reaction allows for the precise control of surface functional groups, rather than the mixture of functionalities normally produced. By maintaining the samples under vacuum throughout the process, adsorption of contaminants is effectively eliminated. This process modifies a range of different oxide surfaces, is fast, consumes a minimal amount of reagents, and produces attachment densities for bound biomolecules that are comparable to or better than commercially available substrates.

High-resolution noncontact atomic force microscope (AFM) images were successfully taken on the Ge105-(1 x 2) structure formed on the Si105 substrate and revealed all dangling bonds of the surface regardless of their electronic situation, surpassing scanning tunneling microscopy, whose images strongly deviated from the atomic structure by the electronic states involved. An atomically resolved electrostatic potential profile by a Kelvin-probe method with AFM shows potential variations among the dangling bond states, directly observing a charge transfer between them. These results clearly demonstrate that high-resolution noncontact AFM with a Kelvin-probe method is an ideal tool for analysis of atomic structures and electronic properties of surfaces.

The nanomechanical properties of thin silicon films will become increasingly critical in semiconductor devices, particularly in the context of substrates that consist of a silicon film on an insulating layer (known as silicon-on-insulator, or SOI, substrates). Here we use very small germanium crystals as a new type of nanomechanical stressor to demonstrate a surprising mechanical behaviour of the thin layer of silicon in SOI substrates, and to show that there is a large local reduction in the viscosity of the oxide on which the silicon layer rests. These findings have implications for the use of SOI substrates in nanoelectronic devices.

DNA computing on surfaces is where complex combinatorial mixtures of DNA molecules are immobilized on a substrate and subsets are tagged and enzymatically modified (DESTROY) in repeated cycles of the DNA computation. A restriction enzyme has been chosen for the surface DESTROY operation. For the READOUT operation, both cycle sequencing and PCR amplification followed by addressed array hybridization were studied to determine the DNA sequences after the computations.

A multi-base encoding strategy is used in a one word approach to surface-based DNA computation. In this designed DNA model system, a set of 16 oligonucleotides, each a 16mer, is used with the format 5'-FFFFvvvvvvvvFFFF-3' in which 4-8 bits of data are stored in eight central variable ('v') base locations, and the remaining fixed ('F') base locations are used as a word label. The detailed implementations are reported here. In order to achieve perfect discrimination between each oligonucleotide, the efficiency and specificity of hybridization discrimination of the set of 16 oligonucleotides were examined by carrying out the hybridization of each individual fluorescently tagged complement to an array of 16 addressed immobilized oligonucleotides. A series of preliminary hybridization experiments are presented and further studies about hybridization, enzymatic destruction, read out and demonstrations of a SAT problem are forthcoming.

A scalable approach to DNA-based computations is described. Complex combinatorial mixtures of DNA molecules encoding all possible answers to a computational problem are synthesized and attached to the surface of a solid support. This set of molecules is queried in successive MARK (hybridization) and DESTROY (enzymatic digestion) operations. Determination of the sequence of the DNA molecules remaining on the surface after completion of these operations yields the answer to the computational problem. Experimental demonstrations of aspects of the strategy are presented.

A method was developed to examine DNA repair within the intact cell. Ultrasoft x-rays were used to induce DNA double-strand breaks (DSBs) in defined subnuclear volumes of human fibroblasts and DNA repair was visualized at those sites. The DSBs remained in a fixed position during the initial stages of DNA repair, and the DSB repair protein hMre11 migrated to the sites of damage within 30 minutes. In contrast, hRad51, a human RecA homolog, did not localize at sites of DNA damage, a finding consistent with the distinct roles of these proteins in DNA repair.

It is recommended that a very clear distinction be made between draw-sealed and tip-sealed glass ampoules. We would, that is, defend the draw-seal for its proven value and its demonstrated freedom from microscopic channels. Where tip-sealing is still employed, we would suggest its reevaluation. Little more can be said in the absence of an immediate reason for the differences in the behavior of the glass in the course of the two procedures. Others may be able to explain the persistence of the capillary in the tip-seal in physico-chemical terms. As to balloon-seals, the third "variety" in common use, it would seem that their characteristics must derive from the way they are made. Ballooned tip-seals appear to have been shown by Greiff et al. (1975) to suffer the same shortcomings as the simple tip-seal. Where a draw-sealed ampoule is heated at the tip after the completion of the seal, a ballooned end would seem to have to retain the true seal effected in the draw process. It is perhaps sufficient to let the subject rest with a reassurance as to the adequacy of the common draw-seal.