75 research outputs found
Path integral simulation of exchange interactions in CMOS spin qubits
The boom of semiconductor quantum computing platforms created a demand for
computer-aided design and fabrication of quantum devices. Path integral Monte
Carlo (PIMC) can have an important role in this effort because it intrinsically
integrates strong quantum correlations that often appear in these
multi-electron systems. In this paper we present a PIMC algorithm that
estimates exchange interactions of three-dimensional electrically defined
quantum dots. We apply this model to silicon metal-oxide-semiconductor (MOS)
devices and we benchmark our method against well-tested full configuration
interaction (FCI) simulations. As an application, we study the impact of a
single charge trap on two exchanging dots, opening the possibility of using
this code to test the tolerance to disorder of CMOS devices. This algorithm
provides an accurate description of this system, setting up an initial step to
integrate PIMC algorithms into development of semiconductor quantum computers.Comment: 10 pages , 5 figure
Impact of electrostatic crosstalk on spin qubits in dense CMOS quantum dot arrays
Quantum processors based on integrated nanoscale silicon spin qubits are a
promising platform for highly scalable quantum computation. Current CMOS spin
qubit processors consist of dense gate arrays to define the quantum dots,
making them susceptible to crosstalk from capacitive coupling between a dot and
its neighbouring gates. Small but sizeable spin-orbit interactions can transfer
this electrostatic crosstalk to the spin g-factors, creating a dependence of
the Larmor frequency on the electric field created by gate electrodes
positioned even tens of nanometers apart. By studying the Stark shift from tens
of spin qubits measured in nine different CMOS devices, we developed a
theoretical frawework that explains how electric fields couple to the spin of
the electrons in increasingly complex arrays, including those electric
fluctuations that limit qubit dephasing times . The results will aid in
the design of robust strategies to scale CMOS quantum technology.Comment: 9 pages, 4 figure
Bounds to electron spin qubit variability for scalable CMOS architectures
Spins of electrons in CMOS quantum dots combine exquisite quantum properties
and scalable fabrication. In the age of quantum technology, however, the
metrics that crowned Si/SiO2 as the microelectronics standard need to be
reassessed with respect to their impact upon qubit performance. We chart the
spin qubit variability due to the unavoidable atomic-scale roughness of the
Si/SiO interface, compiling experiments in 12 devices, and developing
theoretical tools to analyse these results. Atomistic tight binding and path
integral Monte Carlo methods are adapted for describing fluctuations in devices
with millions of atoms by directly analysing their wavefunctions and electron
paths instead of their energy spectra. We correlate the effect of roughness
with the variability in qubit position, deformation, valley splitting, valley
phase, spin-orbit coupling and exchange coupling. These variabilities are found
to be bounded and lie within the tolerances for scalable architectures for
quantum computing as long as robust control methods are incorporated.Comment: 20 pages, 8 figure
Consistency of high-fidelity two-qubit operations in silicon
The consistency of entangling operations between qubits is essential for the
performance of multi-qubit systems, and is a crucial factor in achieving
fault-tolerant quantum processors. Solid-state platforms are particularly
exposed to inconsistency due to the materials-induced variability of
performance between qubits and the instability of gate fidelities over time.
Here we quantify this consistency for spin qubits, tying it to its physical
origins, while demonstrating sustained and repeatable operation of two-qubit
gates with fidelities above 99% in the technologically important silicon
metal-oxide-semiconductor (SiMOS) quantum dot platform. We undertake a detailed
study of the stability of these operations by analysing errors and fidelities
in multiple devices through numerous trials and extended periods of operation.
Adopting three different characterisation methods, we measure entangling gate
fidelities ranging from 96.8% to 99.8%. Our analysis tools also identify
physical causes of qubit degradation and offer ways to maintain performance
within tolerance. Furthermore, we investigate the impact of qubit design,
feedback systems, and robust gates on implementing scalable, high-fidelity
control strategies. These results highlight both the capabilities and
challenges for the scaling up of spin-based qubits into full-scale quantum
processors
'To live and die [for] Dixie': Irish civilians and the Confederate States of America
Around 20,000 Irishmen served in the Confederate army in the Civil War. As a result, they left behind, in various Southern towns and cities, large numbers of friends, family, and community leaders. As with native-born Confederates, Irish civilian support was crucial to Irish participation in the Confederate military effort. Also, Irish civilians served in various supporting roles: in factories and hospitals, on railroads and diplomatic missions, and as boosters for the cause. They also, however, suffered in bombardments, sieges, and the blockade. Usually poorer than their native neighbours, they could not afford to become 'refugees' and move away from the centres of conflict. This essay, based on research from manuscript collections, contemporary newspapers, British Consular records, and Federal military records, will examine the role of Irish civilians in the Confederacy, and assess the role this activity had on their integration into Southern communities. It will also look at Irish civilians in the defeat of the Confederacy, particularly when they came under Union occupation. Initial research shows that Irish civilians were not as upset as other whites in the South about Union victory. They welcomed a return to normalcy, and often 'collaborated' with Union authorities. Also, Irish desertion rates in the Confederate army were particularly high, and I will attempt to gauge whether Irish civilians played a role in this. All of the research in this paper will thus be put in the context of the Drew Gilpin Faust/Gary Gallagher debate on the influence of the Confederate homefront on military performance. By studying the Irish civilian experience one can assess how strong the Confederate national experiment was. Was it a nation without a nationalism
High-fidelity operation and algorithmic initialisation of spin qubits above one kelvin
The encoding of qubits in semiconductor spin carriers has been recognised as
a promising approach to a commercial quantum computer that can be
lithographically produced and integrated at scale. However, the operation of
the large number of qubits required for advantageous quantum applications will
produce a thermal load exceeding the available cooling power of cryostats at
millikelvin temperatures. As the scale-up accelerates, it becomes imperative to
establish fault-tolerant operation above 1 kelvin, where the cooling power is
orders of magnitude higher. Here, we tune up and operate spin qubits in silicon
above 1 kelvin, with fidelities in the range required for fault-tolerant
operation at such temperatures. We design an algorithmic initialisation
protocol to prepare a pure two-qubit state even when the thermal energy is
substantially above the qubit energies, and incorporate high-fidelity
radio-frequency readout to achieve an initialisation fidelity of 99.34 per
cent. Importantly, we demonstrate a single-qubit Clifford gate fidelity of
99.85 per cent, and a two-qubit gate fidelity of 98.92 per cent. These advances
overcome the fundamental limitation that the thermal energy must be well below
the qubit energies for high-fidelity operation to be possible, surmounting a
major obstacle in the pathway to scalable and fault-tolerant quantum
computation
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