Low Temperature Simulations Framework for Quantum Dots and Point Contacts

Quantum computing is becoming increasingly important due to its potential in solving complex optimization problems such as protein folding, and the ability to model correlated electronic systems. Designing of semiconductor based quantum computers is challenging due to the vast number of parameters that need to be optimized from fabrication to their operation. Simulations of these devices could help with the design process. A computational modeling framework is presented that can model quantum point contacts and quantum dots, which are the building blocks of semiconductor based quantum computers. Care was taken to minimize the number of parameters, and use only those parameters that are connected directly to the devices and materials. The devices for fractional quantum Hall effect based topological quantum computers and electron spin based quantum computers are considered. The simulation results matched experiments, based on which predictions for improved devices are made

Low Temperature Simulations Framework For Quantum Dots And Point Contacts.

Quantum computing is becoming increasingly important due to its potential in solving complex optimization problems such as protein folding, and the ability to model correlated electronic systems. Designing of semiconductor based quantum computers is challenging due to the vast number of parameters that need to be optimized from fabrication to their operation. Simulations of these devices could help with the design process. A computational modeling framework is presented that can model quantum point contacts and quantum dots, which are the building blocks of semiconductor based quantum computers. Care was taken to minimize the number of parameters, and use only those parameters that are connected directly to the devices and materials. The devices for fractional quantum Hall effect based topological quantum computers and electron spin based quantum computers are considered. The simulation results matched experiments, based on which predictions for improved devices are made

Interface-induced spin-orbit interaction in silicon quantum dots and prospects for scalability

We identify the presence of monatomic steps at the Si/SiGe or Si/SiO2 interface as a dominant source of variations in the dephasing time of silicon (Si) quantum dot (QD) spin qubits. First, using atomistic tight-binding calculations we show that the g-factors and their Stark shifts undergo variations due to these steps. We compare our theoretical predictions with experiments on QDs at a Si/SiO2 interface, in which we observe significant differences in Stark shifts between QDs in two different samples. We also experimentally observe variations in the g-factors of one-electron and three-electron spin qubits realized in three neighboring QDs on the same sample, at a level consistent with our calculations. The dephasing times of these qubits also vary, most likely due to their varying sensitivity to charge noise, resulting from different interface conditions. More importantly, from our calculations we show that by employing the anisotropic nature of the spin-orbit interaction (SOI) in a Si QD, we can minimize and control these variations. Ultimately, we predict that the dephasing times of the Si QD spin qubits will be anisotropic and can be improved by at least an order of magnitude, by aligning the external dc magnetic field towards specific crystal directions, given other decoherence mechanisms do not dominate over charge noise.QCD/Veldhorst La

Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS₂ Heterostructures

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene–WS₂ heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene–WS₂ heterojunctions as-grown on sapphire and transferred to SiO₂ with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS₂, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene–WS₂ than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene–WS₂ junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials

RRI-GBT MULTI-BAND RECEIVER: MOTIVATION, DESIGN, AND DEVELOPMENT

We report the design and development of a self-contained multi-band receiver (MBR) system, intended for use with a single large aperture to facilitate sensitive and high time-resolution observations simultaneously in 10 discrete frequency bands sampling a wide spectral span (100-1500 MHz) in a nearly log-periodic fashion. The development of this system was primarily motivated by need for tomographic studies of pulsar polar emission regions. Although the system design is optimized for the primary goal, it is also suited for several other interesting astronomical investigations. The system consists of a dual-polarization multi-band feed (with discrete responses corresponding to the 10 bands pre-selected as relatively radio frequency interference free), a common wide-band radio frequency front-end, and independent back-end receiver chains for the 10 individual sub-bands. The raw voltage time sequences corresponding to 16 MHz bandwidth each for the two linear polarization channels and the 10 bands are recorded at the Nyquist rate simultaneously. We present the preliminary results from the tests and pulsar observations carried out with the Robert C. Byrd Green Bank Telescope using this receiver. The system performance implied by these results and possible improvements are also briefly discussed