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arXiv:2306.01307

Low-crosstalk optical addressing system for atomic qubits based on multiple objectives and acousto-optic deflectors

Yi-Long Chen, Ruirui Li, Ran He, Shu-Qian Chen, Qi, Wen-Hao, Jin‐Ming Cui, Yun‐Feng Huang, Chuan‐Feng Li, Guang‐Can Guo

Published Jun 2, 2023• 2 citations
arXivHTMLPDF

Abstract

Large-scale programmable trapped ion hardware, featuring high gate fidelity and long coherence times, is promising for realizing a practical fault-tolerant quantum computer (FTQC). However, individual addressing (IA) methods, which are important for implementing programmable gates in near-term quantum devices, can lead to undesired errors between the target ions and neighboring ions. In this work, we present a low-crosstalk optical addressing system based on multiple optical objectives and acousto-optic deflectors (AODs) with a symmetrical configuration. Two counter-propagating Raman operation beams are both tightly focused, generating an overlapping spot with a waist radius of approximately $1~\upmu\mathrm{m}$, to address the target ion. As a result, IA crosstalk, characterized by Rabi rate crosstalk on the spectator ion, is measured to be $1.19(5)\times10^{-3}$, with the two ions separated by approximately 5.5~$\upmu\mathrm{m}$. This low-crosstalk optical addressing system holds promise for high-fidelity entangling operations, and the symmetrically-configured AODs in our method can be readily extended to two dimensions to address a two-dimensional ion crystal.

DOI
10.48550/arxiv.2306.01307

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Large-scale programmable trapped ion hardware, featuring high gate fidelity and long coherence times, is promising for realizing a practical fault-tolerant quantum computer (FTQC). However, individual addressing (IA) methods, which are important for implementing programmable gates in near-term quantum devices, can lead to undesired errors between the target ions and neighboring ions. In this work, we present a low-crosstalk optical addressing system based on multiple optical objectives and acousto-optic deflectors (AODs) with a symmetrical configuration. Two counter-propagating Raman operation beams are both tightly focused, generating an overlapping spot with a waist radius of approximately 1 \upmum1~\upmu\mathrm{m}1 \upmum, to address the target ion. As a result, IA crosstalk, characterized by Rabi rate crosstalk on the spectator ion, is measured to be 1.19(5)×10−31.19(5)\times10^{-3}1.19(5)×10−3, with the two ions separated by approximately 5.5~\upmum\upmu\mathrm{m}\upmum. This low-crosstalk optical addressing system holds promise for high-fidelity entangling operations, and the symmetrically-configured AODs in our method can be readily extended to two dimensions to address a two-dimensional ion crystal.
Individual addressing with low crosstalk is crucial for high-fidelity quantum gates in trapped-ion systems, but technical challenges in reducing ion crosstalk have thwarted progress. This study uses symmetrically configured acousto-optic deflectors to achieve a Rabi-rate crosstalk as low as $1.19(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Additionally, such deflectors offer flexible manipulation and two-dimensional addressing, making them a promising platform for scalable quantum circuits. This result could significantly advance the practical application of fault-tolerant quantum computing.
We present a compact design for a beam-steering system based on acousto-optic-deflectors (AODs) used as an individual addressing system for trapped-ion quantum computing. The design targets to minimize the optomechanical degrees of freedom and the optical beam paths to improve optical stability, and we successfully implemented a solution with a compact footprint of less than 1 square foot. The system characterization results show that we achieve clean Gaussian beams at 355nm wavelength with a beam steering range of $\sim$50 times the beam diameter, and an intensity crosstalk of <9×10−4< 9 \times 10^{-4}<9×10−4 at all neighboring ions in a five-ion chain. Based on these capabilities, we experimentally demonstrate individual addressing of a 30-ion chain. We estimate the beam switching time of the AOD to be $\sim$240 ns. The compact system design is expected to provide high optical stability, providing the potential for high-fidelity trapped-ion quantum computing with long ion chains.
Two-dimensional (2D) ion crystals may represent a promising path to scale up qubit numbers for ion trap quantum information processing. However, to realize universal quantum computing in this system, individually addressed high-fidelity two-qubit entangling gates still remain challenging due to the inevitable micromotion of ions in a 2D crystal as well as the technical difficulty in 2D addressing. Here we demonstrate two-qubit entangling gates between any ion pairs in a 2D crystal of four ions. We use symmetrically placed crossed acousto-optic deflectors (AODs) to drive Raman transitions and achieve an addressing crosstalk error below 0.1%. We design and demonstrate a gate sequence by alternatingly addressing two target ions, making it compatible with any single-ion addressing techniques without crosstalk from multiple addressing beams. We further examine the gate performance versus the micromotion amplitude of the ions and show that its effect can be compensated by a recalibration of the laser intensity without degrading the gate fidelity. Our work paves the way for ion trap quantum computing with hundreds to thousands of qubits on a 2D ion crystal. Quantum gates in 2D ion crystals are more challenging than in 1D. Here, the authors use their 2D ion trap platform and acousto-optical deflectors to demonstrate a 2-qubit gate that can stand the ion micromotion in such configuration.
We present an experimental realization of coherent crosstalk cancellation in a trapped ion cryogenic system. The qubit register consists of a linear string of calcium ions which are individually addressed using tightly focused laser beams oriented perpendicular to the crystal axis. This individual control is provided by a multi-core photonic crystal fiber that allow for cancellation pulses to be applied to neighbouring ion sites to destructively interfere with crosstalk from addressing pulses on target qubits. With this technique we find > 104fold reduction in intensity crosstalk.In order for this novel method of crosstalk elimination to be stable, phase coherence must be preserved between the cancellation and addressing beams. We further develop the appropriate calibration methods and procedures to mitigate phase drifts between these different optical paths. This includes a method to minimize the effects of variable duty cycles of individual pulses that may cause for differential phase noise.In addition, we extended this work by including the results a laser written waveguide array used to provide single-ion addressing capability, which is found to produce intensity crosstalk on the order of 10−4. This device may be used in combination with the approach of optical cancellation developed for crosstalk compensation to result in negligible crosstalk errors between neighbouring qubit. As a consequence, such a system may allow for quantum error correction and fault-tolerant circuits to be viable as the system scales up to larger numbers of qubits.
Scalable classical controllers are a key component of future fault-tolerant quantum computers. Neutral atom quantum computers leverage commercially available optoelectronic devices for generating large-scale tweezer arrays and performing parallel readout, but implementing massively parallel, locally addressed gate operations is an open challenge. In this work, we demonstrate an optical modulator system based on off-the-shelf components, which can generate a two-dimensional array of over 10,000 focused spots with uniform frequency and amplitude, and switching them on and off individually in arbitrary configurations at rates of up to 43 kHz. Through careful control of aberrations, the modulator achieves an extinction ratio of 46 dB, and nearest-neighbor crosstalk of −44dB with a beam spacing of 4.6 waists. The underlying components can operate at wavelengths from the UV to the NIR, and sustain high laser intensities. This approach is suitable for local addressing of gates with low cross-talk error rates in any optically addressed qubit platform, including neutral atoms, trapped ions, or solid-state atomic defects.
Physical qubits in experimental quantum information processors are inevitably exposed to different sources of noise and imperfections, which lead to errors that typically accumulate hindering our ability to perform long computations reliably. Progress towards scalable and robust quantum computation relies on exploiting quantum error correction (QEC) to actively battle these undesired effects. In this work, we present a comprehensive study of crosstalk errors in a quantum-computing architecture based on a single string of ions confined by a radio-frequency trap, and manipulated by individually-addressed laser beams. This type of errors affects spectator qubits that, ideally, should remain unaltered during the application of single- and two-qubit quantum gates addressed at a different set of active qubits. We microscopically model crosstalk errors from first principles and present a detailed study showing the importance of using a coherent vs incoherent error modelling and, moreover, discuss strategies to actively suppress this crosstalk at the gate level. Finally, we study the impact of residual crosstalk errors on the performance of fault-tolerant QEC numerically, identifying the experimental target values that need to be achieved in near-term trapped-ion experiments to reach the break-even point for beneficial QEC with low-distance topological codes.