7 research outputs found
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 for the matching decoder and as
low as 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