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arXiv:1907.08216 · Physical Review Applied

Measurements of Capacitive Coupling Within a Quadruple-Quantum-Dot Array

Samuel F. Neyens, E.R. MacQuarrie, J.P. Dodson, J. Corrigan, Nathan Holman, Brandur Thorgrimsson, M. Palma, Thomas McJunkin, L.F. Edge, Mark G. Friesen, S.N. Coppersmith, M.A. Eriksson

Published Dec 23, 2019• 27 citations
arXivHTMLPDF

Abstract

Understanding the interactions that couple gate-defined quantum-dot qubits is an important step to using such devices in quantum computing. For double-quantum-dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can yield coherent coupling between neighboring qubits. Here researchers reveal the tunability of this capacitive-coupling energy with applied gate voltages in a quadruple-quantum-dot array, tuning the coupling energy from 15 to 32 GHz. Modeling the system as a network of charge nodes joined by capacitors, the authors demonstrate how the capacitive-coupling energy between pairs of double dots depends on the various capacitances in the network.

DOI
10.1103/physrevapplied.12.064049

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Understanding the interactions that couple gate-defined quantum-dot qubits is an important step to using such devices in quantum computing. For double-quantum-dot qubits with an effective charge dipole moment, a capacitive dipole-dipole interaction can yield coherent coupling between neighboring qubits. Here researchers reveal the tunability of this capacitive-coupling energy with applied gate voltages in a quadruple-quantum-dot array, tuning the coupling energy from 15 to 32 GHz. Modeling the system as a network of charge nodes joined by capacitors, the authors demonstrate how the capacitive-coupling energy between pairs of double dots depends on the various capacitances in the network.
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Strong coupling between two qubits is one of the main requirements for high fidelity two-qubit logic operations. Here we experimentally investigate the capacitive coupling between two double quantum dots. A pair of open slot confinement gates is used to enhance the coupling. We find that the coupling energy J can be conveniently tuned in a broad range. Through numerical simulations, we study the effect of J on two-qubit operations. The analysis shows that our experimentally obtained J is adequate to achieve high fidelity two-qubit entanglement and logic gates.
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We investigate a method for entangling two singlet-triplet qubits in adjacent double quantum dots via capacitive interactions. In contrast to prior work, here we focus on a regime with strong interactions between the qubits. The interplay of the interaction energy and simultaneous large detunings for both double dots gives rise to the ``double charge resonant'' regime, in which the unpolarized (1111) and fully polarized (0202) four-electron states in the absence of interqubit tunneling are near degeneracy, while being energetically well separated from the partially polarized (0211 and 1102) states. A rapid controlled-phase gate may be realized by combining time evolution in this regime in the presence of intraqubit tunneling and the interqubit Coulomb interaction with refocusing $\ensuremath{\pi}$ pulses that swap the singly occupied singlet and triplet states of the two qubits via, e.g., magnetic gradients. We calculate the fidelity of this entangling gate, incorporating models for two types of noise---charge fluctuations in the single-qubit detunings and charge relaxation within the low-energy subspace via electron-phonon interaction---and identify parameter regimes that optimize the fidelity. The rates of phonon-induced decay for pairs of GaAs or Si double quantum dots vary with the sizes of the dipolar and quadrupolar contributions and are several orders of magnitude smaller for Si, leading to high theoretical gate fidelities for coupled singlet-triplet qubits in Si dots. We also consider the dependence of the capacitive coupling on the relative orientation of the double dots and find that a linear geometry provides the fastest potential gate.