arXiv subjects

Eoin G. Kelly

Publications and source records attributed to Eoin G. Kelly.

6 recordsLinked to original sources

arXiv2026

Engineering two-qubit gates via anisotropic exchange in germanium spin qubits

Germanium hole spin qubits are a promising and versatile platform for quantum computation and simulation. In this system, strong spin-orbit interaction (SOI) renders the single-qubit $g$-tensor anisotropic and electrically tunable, enabling operational sweet spots with reduced noise sensitivity. SOI also transforms the isotropic two-qubit exchange coupling into an anisotropic tensor whose geometry is inherited from the single-qubit $g$-tensors and spin-flip tunnelling. Here, using two hole spin qubits in a strained-germanium quantum well and full vector control of the magnetic field, we map this exchange tensor, separate it into longitudinal and transverse components, and show that they govern controlled-phase and SWAP-like dynamics, respectively. We find that the longitudinal exchange can be tuned via the magnetic field orientation from a conventional positive value, through zero, to an effectively negative one, as measured by inverted exchange-split spin transitions. The magnetic field direction thus provides continuous control over the interaction Hamiltonian: at a point of purely transverse exchange, we engineer a single-pulse baseband iSWAP, unattainable under isotropic exchange. Linking $g$-tensor geometry to exchange anisotropy establishes native Hamiltonian engineering, enabling spin-based quantum simulation and gate sets selected by the global field orientation alone.

cond-mat.mes-hall↗

arXiv2026

A dressed singlet-triplet qubit in germanium

In semiconductor hole spin qubits, low magnetic field ($B$) operation extends the coherence time ($T_\mathrm{2}^*$) but proportionally reduces the gate speed. In contrast, singlet-triplet (ST) qubits are primarily controlled by the exchange interaction ($J$) and can thus maintain high gate speeds even at low $B$. However, a large $J$ introduces a significant charge component to the qubit, rendering ST qubits more vulnerable to charge noise when driven. Here, we demonstrate a highly coherent ST hole spin qubit in germanium, operating at both low $B$ and low $J$. By modulating $J$, we achieve resonant driving of the ST qubit, obtaining an average gate fidelity of $99.68\%$ and a coherence time of $T_\mathrm{2}^*=1.9\,μ$s. Moreover, by applying the resonant drive continuously, we realize a dressed ST qubit with a tenfold increase in coherence time ($T_\mathrm{2ρ}^*=20.3\,μ$s). Frequency modulation of the driving signal enables universal control, with an average gate fidelity of $99.63\%$. Our results demonstrate the potential for extending coherence times while preserving high-fidelity control of germanium-based ST qubits, paving the way for more efficient operations in semiconductor-based quantum processors.

cond-mat.mes-hall↗

arXiv2025

Spatial uniformity of g-tensor and spin-orbit interaction in germanium hole spin qubits

Holes in Ge/SiGe heterostructures are now a leading platform for semiconductor spin qubits, thanks to the high confinement quality, two-dimensional arrays, high tunability, and larger gate structure dimensions. One limiting factor for the operation of large arrays of qubits is the considerable variation in qubit frequencies or properties resulting from the strongly anisotropic $g$-tensor. We study the $g$-tensors of six and seven qubits in an array with a Y geometry across two devices. We report a mean distribution of the tilts of the $g$-tensor's out-of-plane principal axis of around $1.1 °$, where nearby quantum dots are more likely to have a similar tilt. Independently of this tilt, and unlike simple theoretical predictions, we find a strong in-plane $g$-tensor anisotropy with strong correlations between neighboring quantum dots. Additionally, in one device where the principal axes of all g-tensors are aligned along the [100] crystal direction, we extract the spin-flip tunneling vector from adjacent dot pairs and find a pattern that is consistent with a uniform Dresselhaus-like spin-orbit field. The Y arrangement of the gate layout and quantum dots allows us to rule out local factors like electrostatic confinement shape or local strain as the origin of the preferential direction. Our results reveal long-range correlations in the spin-orbit interaction and $g$-tensors that were not previously predicted or observed, and could prove critical to reliably understand $g$-tensors in germanium quantum dots.

cond-mat.mes-hall↗

arXiv2025

Resonant two-qubit gates for fermionic simulations with spin qubits

In gate-defined semiconductor spin qubits, the highly tunable Heisenberg exchange interaction is leveraged to implement fermionic two-qubit gates such as CZ and SWAP. However, the broader family of fermionic simulation (fSim) gates remains unexplored, and has the potential to enhance the performance of near-term quantum simulation algorithms. Here, we demonstrate a method to implement the fSim gate set in spin qubits using a single pulse combining baseband and resonant exchange drives. This approach minimizes gate duration and drive amplitude, mitigating decoherence and crosstalk. We validate its effectiveness by realizing a resonant iSWAP gate between two hole spins in germanium, achieving a fidelity of 93.8(5)% extracted with interleaved randomized benchmarking. Quantum process tomography confirms accurate gate calibration and identifies qubit decoherence as the dominant error source. Our results establish a practical route toward a versatile and efficient two-qubit gate set for spin-based quantum processors.

cond-mat.mes-hall↗

arXiv2024

All-electrical operation of a spin qubit coupled to a high-Q resonator

Building a practical quantum processor involves integrating millions of physical qubits along with the necessary components for individual qubit manipulation and readout. Arrays of gated silicon spins offer a promising route toward achieving this goal. Optimized radio frequency resonators with high internal quality factor are based on superconducting inductors and enable fast spin readout. All-electrical spin control and gate-dispersive readout remove the need for additional device components and simplify scaling. However, superconducting high-Q tank circuits are susceptible to crosstalk induced ringup from electrical qubit control pulses, which causes fluctuations of the quantum dot potential and is suspected to degrade qubit performance. Here, we report on the coherent and all-electrical control of a hole spin qubit at 1.5K, integrated into a silicon fin field-effect transistor and connected to a niobium nitride nanowire inductor gate-sensor. Our experiments show that qubit control pulses with their broad range of higher harmonics ring up the tank when the control pulse spectrum overlaps with the tank resonance. This can cause a reduction of the readout visibility if the tank ringing amplitude exceeds the excited state splitting of the quantum dot, lifting Pauli spin blockade and thus leading to state preparation and measurement errors. We demonstrate how to circumvent these effects by engineering control pulses around the tank resonances. Importantly, we find that the ringup does not limit the spin coherence time, indicating that efficient high-Q resonators in gate-sensing are compatible with all-electrical spin control.

cond-mat.mes-hall↗

arXiv2023

Capacitive crosstalk in gate-based dispersive sensing of spin qubits

In gate-based dispersive sensing, the response of a resonator attached to a quantum dot gate is detected by a reflected radio-frequency signal. This enables fast readout of spin qubits and tune up of arrays of quantum dots, but comes at the expense of increased susceptibility to crosstalk, as the resonator can amplify spurious signals and induce fluctuations in the quantum dot potential. We attach tank circuits with superconducting NbN inductors and internal quality factors $Q_{\mathrm{i}}$>1000 to the interdot barrier gate of silicon double quantum dot devices. Measuring the interdot transition in transport, we quantify radio-frequency crosstalk that results in a ring-up of the resonator when neighbouring plunger gates are driven with frequency components matching the resonator frequency. This effect complicates qubit operation and scales with the loaded quality factor of the resonator, the mutual capacitance between device gate electrodes, and with the inverse of the parasitic capacitance to ground. Setting qubit frequencies below the resonator frequency is expected to substantially suppress this type of crosstalk.

cond-mat.mes-hall↗