arXiv subjects

A. C. Hughes

Publications and source records attributed to A. C. Hughes.

8 recordsLinked to original sources

arXiv2025

Trapped-ion two-qubit gates with >99.99% fidelity without ground-state cooling

We introduce the 'smooth gate', an entangling method for trapped-ion qubits where residual spin-motion entanglement errors are adiabatically eliminated by ramping the gate detuning. We demonstrate electronically controlled two-qubit gates with an estimated error of $8.4(7)\times10^{-5}$ without ground-state cooling. We further show that the error remains $\lesssim 5\times10^{-4}$ for ions with average phonon occupation up to $\bar{n}=9.4(3)$ on the gate mode. These results indicate that trapped-ion quantum computation can achieve high fidelity at temperatures above the Doppler limit, which enables faster and simpler device operation.

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arXiv2025

Subspace Leakage Error Randomized Benchmarking of Mølmer-Sørensen Gates

We demonstrate a new technique that adapts single-qubit randomized benchmarking to two-qubit Mølmer-Sørensen gates. We use the controllable gate phase to generate Cliffords that act on a two-state subspace, enabling benchmarking of two-qubit gates without single-qubit operations. In addition to quantifying the gate infidelity, the protocol provides valuable information about the type of error by distinguishing between those that conserve the two-state subspace and those that result in leakage out of it. We demonstrate the protocol for calibrating and validating all-electronic maximally entangling gates in a trapped-ion quantum computer, achieving a two-qubit gate error of $2.6 (2)\times10^{-4}$.

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arXiv2025

Comparison of trapped-ion entangling gate mechanisms for mixed species

Entangling gates are an essential capability of quantum computers. There are different methods for implementing two-qubit gates, with respective advantages and disadvantages. We investigate the experimentally relevant differences and commonalities of laser-based $σ_z\otimesσ_z$ light-shift and $σ_ϕ\otimesσ_ϕ$ Moelmer-Soerensen gates, highlighting the phases of experimental control fields and their long-term stabilities, in the specific case of mixed-species gates. We implement these gates on qubits with very different magnetic field sensitivities, encoded in $^{43}\mathrm{Ca}^+$ and $^{88}\mathrm{Sr}^+$, achieving fidelities of $99.8\%$ for the $σ_z\otimesσ_z$ and $99.6\%$ for the $σ_ϕ\otimesσ_ϕ$ gate.

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arXiv2024

Scalable, high-fidelity all-electronic control of trapped-ion qubits

The central challenge of quantum computing is implementing high-fidelity quantum gates at scale. However, many existing approaches to qubit control suffer from a scale-performance trade-off, impeding progress towards the creation of useful devices. Here, we present a vision for an electronically controlled trapped-ion quantum computer that alleviates this bottleneck. Our architecture utilizes shared current-carrying traces and local tuning electrodes in a microfabricated chip to perform quantum gates with low noise and crosstalk regardless of device size. To verify our approach, we experimentally demonstrate low-noise site-selective single- and two-qubit gates in a seven-zone ion trap that can control up to 10 qubits. We implement electronic single-qubit gates with 99.99916(7)% fidelity, and demonstrate consistent performance with low crosstalk across the device. We also electronically generate two-qubit maximally entangled states with 99.97(1)% fidelity and long-term stable performance over continuous system operation. These state-of-the-art results validate the path to directly scaling these techniques to large-scale quantum computers based on electronically controlled trapped-ion qubits.

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arXiv2022

Synthesizing a $\hatσ_z$ spin-dependent force for optical, metastable, and ground state trapped-ion qubits

A single bichromatic field near-resonant to a qubit transition is typically used for $\hatσ_x$ or $\hatσ_y$ Mølmer-Sørensen type interactions in trapped ion systems. Using this field configuration, it is also possible to synthesize a $\hatσ_z$ spin-dependent force by merely adjusting the beat-note frequency. Here, we expand on previous work and present a comprehensive theoretical and experimental investigation of this scheme with a laser near-resonant to a quadrupole transition in $^{88}$Sr$^+$. Further, we characterise its robustness to optical phase and qubit frequency offsets, and demonstrate its versatility by entangling optical, metastable, and ground state qubits.

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arXiv2022

Coherent Control of Trapped Ion Qubits with Localized Electric Fields

We present a new method for coherent control of trapped ion qubits in separate interaction regions of a multi-zone trap by simultaneously applying an electric field and a spin-dependent gradient. Both the phase and amplitude of the effective single-qubit rotation depend on the electric field, which can be localised to each zone. We demonstrate this interaction on a single ion using both laser-based and magnetic field gradients in a surface-electrode ion trap, and measure the localisation of the electric field.

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arXiv2020

Benchmarking a high-fidelity mixed-species entangling gate

We implement a two-qubit logic gate between a $^{43}\mathrm{Ca}^+\,$ hyperfine qubit and a $^{88}\mathrm{Sr}^+\,$ Zeeman qubit. For this pair of ion species, the S--P optical transitions are close enough that a single laser of wavelength $402\,\mathrm{nm}$ can be used to drive the gate, but sufficiently well separated to give good spectral isolation and low photon scattering errors. We characterize the gate by full randomized benchmarking, gate set tomography and Bell state analysis. The latter method gives a fidelity of $99.8(1)\%$, comparable to that of the best same-species gates and consistent with known sources of error.

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arXiv2019

Probing Qubit Memory Errors at the Part-per-Million Level

Robust qubit memory is essential for quantum computing, both for near-term devices operating without error correction, and for the long-term goal of a fault-tolerant processor. We directly measure the memory error $ε_m$ for a $^{43}$Ca$^+$ trapped-ion qubit in the small-error regime and find $ε_m<10^{-4}$ for storage times $t\lesssim50\,\mbox{ms}$. This exceeds gate or measurement times by three orders of magnitude. Using randomized benchmarking, at $t=1\,\mbox{ms}$ we measure $ε_m=1.2(7)\times10^{-6}$, around ten times smaller than that extrapolated from the $T_{2}^{\ast}$ time, and limited by instability of the atomic clock reference used to benchmark the qubit.

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