Formation of Strain-Induced Quantum Dots in Gated Semiconductor Nanostructures

Elastic strain changes the energies of the conduction band in a semiconductor, which will affect transport through a semiconductor nanostructure. We show that the typical strains in a semiconductor nanostructure from metal gates are large enough to create strain-induced quantum dots (QDs). We simulate a commonly used QD device architecture, metal gates on bulk silicon, and show the formation of strain-induced QDs. The strain-induced QD can be eliminated by replacing the metal gates with poly-silicon gates. Thus strain can be as important as electrostatics to QD device operation operation.Comment: 5 pages, 3 figures, plus supplementary informatio

Electrostatic and Strain-Induced Quantum Dots in Silicon Nanostructures

Quantum dots (QDs) are nanometer scale regions that can trap charges. In this dissertation I describe my work on understanding the reproducibility of silicon QDs, and why unintentional QDs are so common. I studied both the reproducibility and predictability of gate capacitances to intentional QDs. I found that, in our devices, electrostatic QDs have gate capacitances that are reproducible to within 10% and predictable using a capacitance simulator to within 20%. I describe a technique that uses the gate capacitances to determine the locations of the unintentional QDs in a nanowire with a precision of a few nanometers. I do this by comparing the measured gate capacitances to simulated gate capacitances. I suggest that strain from the gates or contacts may be the cause of many of the observed unintentional QDs. Strain can cause QDs because it changes the band structure, thus changing the energy of the conduction band and the valence band. I discuss the effects of strain in three common device architectures: a mesa-etched nanowire with poly-silicon gates, metal-gated bulk silicon, and chemically grown nanowires with metal contacts. Because strain can affect these very different architectures, I suggest that the strain in a QD can be as important as the electrostatics to understanding how a device works

Simulating Capacitances to Silicon Quantum Dots: Breakdown of the Parallel Plate Capacitor Model

Many electrical applications of quantum dots rely on capacitively coupled gates; therefore, to make reliable devices we need those gate capacitances to be predictable and reproducible. We demonstrate in silicon nanowire quantum dots that gate capacitances are reproducible to within 10% for nominally identical devices. We demonstrate the experimentally that gate capacitances scale with device dimensions. We also demonstrate that a capacitance simulator can be used to predict measured gate capacitances to within 20%. A simple parallel plate capacitor model can be used to predict how the capacitances change with device dimensions; however, the parallel plate capacitor model fails for the smallest devices because the capacitances are dominated by fringing fields. We show how the capacitances due to fringing fields can be quickly estimated.Comment: 4 pages, 3 figures, to be published in IEEE Trans. Nan

Determining the Location and Cause of Unintentional Quantum Dots in a Nanowire

We determine the locations of unintentional quantum dots (U-QDs) in a silicon nanowire with a precision of a few nanometers by comparing the capacitances to multiple gates with a capacitance simulation. Because we observe U-QDs in the same location of the wire in multiple devices, their cause is likely to be an unintended consequence of the fabrication, not random atomic-scale defects as is typically assumed. The locations of the U-QDs appear consistent with conduction band modulation from strain from the oxide and the gates. This allows us to suggest methods to reduce the frequency of U-QDs

TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation

Superconducting qubit lifetimes must be both long and stable to provide an adequate foundation for quantum computing. This stability is imperiled by two-level systems (TLSs), currently a dominant loss mechanism, which exhibit slow spectral dynamics that destabilize qubit lifetimes on hour timescales. Stability is also threatened at millisecond timescales, where ionizing radiation has recently been found to cause bursts of correlated multi-qubit decays, complicating quantum error correction. Here we study both ionizing radiation and TLS dynamics on a 27-qubit processor, repurposing the standard transmon qubits as sensors of both radiation impacts and TLS dynamics. Unlike prior literature, we observe resilience of the qubit lifetimes to the transient quasiparticles generated by the impact of radiation. However, we also observe a new interaction between these two processes, "TLS scrambling," in which a radiation impact causes multiple TLSs to jump in frequency, which we suggest is due to the same charge rearrangement sensed by qubits near a radiation impact. As TLS scrambling brings TLSs out of or in to resonance with the qubit, the lifetime of the qubit increases or decreases. Our findings thus identify radiation as a new contribution to fluctuations in qubit lifetimes, with implications for efforts to characterize and improve device stabilityComment: 14 pages, 10 figure

Matching and maximum likelihood decoding of a multi-round subsystem quantum error correction experiment

Quantum error correction offers a promising path for performing quantum computations with low errors. Although a fully fault-tolerant execution of a quantum algorithm remains unrealized, recent experimental developments, along with improvements in control electronics, are enabling increasingly advanced demonstrations of the necessary operations for applying quantum error correction. Here, we perform quantum error correction on superconducting qubits connected in a heavy-hexagon lattice. The full processor can encode a logical qubit with distance three and perform several rounds of fault-tolerant syndrome measurements that allow the correction of any single fault in the circuitry. Furthermore, by using dynamic circuits and classical computation as part of our syndrome extraction protocols, we can exploit real-time feedback to reduce the impact of energy relaxation error in the syndrome and flag qubits. We show that the logical error varies depending on the use of a perfect matching decoder compared to a maximum likelihood decoder. We observe a logical error per syndrome measurement round as low as $\sim0.04$ for the matching decoder and as low as $\sim0.03$ for the maximum likelihood decoder. Our results suggest that more significant improvements to decoders are likely on the horizon as quantum hardware has reached a new stage of development towards fully fault-tolerant operations.Comment: 15 pages, 6 figures, 5 table