3,302 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
Control of dephasing in spin qubits during coherent transport in silicon
One of the key pathways towards scalability of spin-based quantum computing
systems lies in achieving long-range interactions between electrons and
increasing their inter-connectivity. Coherent spin transport is one of the most
promising strategies to achieve this architectural advantage. Experimental
results have previously demonstrated high fidelity transportation of spin
qubits between two quantum dots in silicon and identified possible sources of
error. In this theoretical study, we investigate these errors and analyze the
impact of tunnel coupling, magnetic field and spin-orbit effects on the spin
transfer process. The interplay between these effects gives rise to double dot
configurations that include regimes of enhanced decoherence that should be
avoided for quantum information processing. These conclusions permit us to
extrapolate previous experimental conclusions and rationalize the future design
of large scale quantum processors.Comment: 18 pages, 9 figure
Improved Single-Shot Qubit Readout Using Twin RF-SET Charge Correlations
High fidelity qubit readout is critical in order to obtain the thresholds
needed to implement quantum error correction protocols and achieve
fault-tolerant quantum computing. Large-scale silicon qubit devices will have
densely-packed arrays of quantum dots with multiple charge sensors that are, on
average, farther away from the quantum dots, entailing a reduction in readout
fidelities. Here, we present a readout technique that enhances the readout
fidelity in a linear SiMOS 4-dot array by amplifying correlations between a
pair of single-electron transistors, known as a twin SET. By recording and
subsequently correlating the twin SET traces as we modulate the dot detuning
across a charge transition, we demonstrate a reduction in the charge readout
infidelity by over one order of magnitude compared to traditional readout
methods. We also study the spin-to-charge conversion errors introduced by the
modulation technique, and conclude that faster modulation frequencies avoid
relaxation-induced errors without introducing significant spin flip errors,
favouring the use of the technique at short integration times. This method not
only allows for faster and higher fidelity qubit measurements, but it also
enhances the signal corresponding to charge transitions that take place farther
away from the sensors, enabling a way to circumvent the reduction in readout
fidelities in large arrays of qubits
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
Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV
The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8 TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum
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