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Applied Physics Reviews

Implementation of an advanced dressing protocol for global qubit control in silicon

Ingvild Hansen, Amanda E. Seedhouse, K. W. Chan, Fay E. Hudson, Kohei M. Itoh, Arne Laucht, André Saraiva, Chih Hwan Yang, Andrew S. Dzurak

Published Sep 1, 2022• 33 citations
Publisher

Abstract

Quantum computing based on solid state spins allows for densely packed arrays of quantum bits. However, the operation of large-scale quantum processors requires a shift in paradigm toward global control solutions. Here, we report a proof-of-principle demonstration of the SMART (sinusoidally modulated, always rotating, and tailored) qubit protocol. We resonantly drive a two-level system and add a tailored modulation to the dressing field to increase robustness to frequency detuning noise and microwave amplitude fluctuations. We measure a coherence time of 2 ms, corresponding to two orders of magnitude improvement compared to a bare spin, and an average Clifford gate fidelity exceeding 99%, despite the relatively long qubit gate times. We stress that the potential of this work lies in the scalability of the protocol and the relaxation of the engineering constraints for a large-scale quantum processor. This work shows that future scalable spin qubit arrays could be operated using global microwave control and local gate addressability, while increasing robustness to relevant experimental inhomogeneities.

DOI
10.1063/5.0096467

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Quantum computing based on solid state spins allows for densely packed arrays of quantum bits. However, the operation of large-scale quantum processors requires a shift in paradigm toward global control solutions. Here, we report a proof-of-principle demonstration of the SMART (sinusoidally modulated, always rotating, and tailored) qubit protocol. We resonantly drive a two-level system and add a tailored modulation to the dressing field to increase robustness to frequency detuning noise and microwave amplitude fluctuations. We measure a coherence time of 2 ms, corresponding to two orders of magnitude improvement compared to a bare spin, and an average Clifford gate fidelity exceeding 99%, despite the relatively long qubit gate times. We stress that the potential of this work lies in the scalability of the protocol and the relaxation of the engineering constraints for a large-scale quantum processor. This work shows that future scalable spin qubit arrays could be operated using global microwave control and local gate addressability, while increasing robustness to relevant experimental inhomogeneities.
Global control strategies for arrays of qubits are a promising pathway to scalable quantum computing. A continuous-wave global field provides decoupling of the qubits from background noise. However, this approach is limited by variability in the parameters of individual qubits in the array. Here we show that by modulating a global field simultaneously applied to the entire array, we are able to encode qubits that are less sensitive to the statistical scatter in qubit resonance frequency and microwave amplitude fluctuations, which are problems expected in a large-scale system. We name this approach the SMART (sinusoidally modulated, always rotating, and tailored) qubit protocol. We show that there exist optimal modulation conditions for qubits in a global field that robustly provide improved coherence times. We discuss in further detail the example of spins in silicon quantum dots, in which universal one- and two-qubit control is achieved electrically by controlling the spin-orbit coupling of individual qubits and the exchange coupling between spins in neighboring dots. This paper provides a high-fidelity qubit operation scheme in a global field, significantly improving the prospects for scalability of spin-based quantum computer architectures.
Scalable quantum computation demands high-fidelity two-qubit gates. However, decoherence and control errors are inevitable, which can decrease the quality of implemented quantum operations. We propose a robust iswap gate protocol for semiconductor spin qubits, which is a promising platform for scalable quantum computing. Our scheme uses only local microwave drives on conventional exchange-coupled spin qubits. This approach simultaneously addresses two critical challenges to semiconductor quantum computing: It suppresses low-frequency noise via continuous dynamical decoupling, and it circumvents the control difficulties associated with the ac modulation of the exchange interaction. We further develop a composite pulse sequence to remove drive-strength constraints and a dynamically corrected method to provide first-order immunity to microwave amplitude errors. Numerical simulations confirm that our scheme can achieve fidelity above the fault-tolerance threshold under current experimental conditions, offering a building block for practical quantum processors.
Abstract Silicon spin qubits offer a promising path to scalable quantum computing due to their compatibility with industrial semiconductor manufacturing. A key requirement for scaling quantum processors is the ability to perform high-fidelity operations in parallel across many qubits. However, simultaneous control remains a major challenge in silicon devices, as fidelities typically degrade under parallel operation. Here we identify microwave-drive-induced a.c.-Stark shifts as the dominant source of this degradation in a five-qubit silicon spin array. We address this by introducing a scalable mitigation protocol based solely on pairwise phase calibrations. Using tailored control pulses on a shared control line, we achieve primitive X and Y gate fidelities well above 99.99% for each qubit individually, with some approaching 99.999%. Furthermore, these fidelities are preserved above 99.99% during simultaneous operation of up to three qubits. During parallel five-qubit operation, fidelities remain above the widely targeted fault-tolerant threshold of 99.9%. By maintaining high-fidelity control during simultaneous operation, shared qubit-control lines can support the development of large-scale semiconductor quantum processors.
Scalable superconducting quantum processors require balancing critical constraints in coherence, control complexity, and spectral crowding. Fixed-frequency architectures suppress flux noise and simplify control via all-microwave operations but remain limited by residual ZZ crosstalk. Here we propose a microwave-activated three-qubit gate protocol for fixed-frequency transmon qubits in the large-detuning regime (∣Δ∣≫g|Δ| \gg g∣Δ∣≫g), leveraging the third-order nonlinear interaction to coherently exchange ∣001⟩↔∣110⟩\ket{001} \leftrightarrow \ket{110}∣001⟩↔∣110⟩ states. By incorporating a phase-compensated optimization protocol, numerical simulations demonstrate a high average gate fidelity exceeding 99.9%99.9\%99.9%. Systematic error analysis identifies static long-range ZZ coupling as the dominant error source in multi-qubit systems, which can be suppressed via operations in the large-detuning regime (∼1\sim 1∼1 GHz). The protocol maintains process fidelities exceeding 98%98\%98% under decoherence, while demonstrating intrinsic robustness to fabrication-induced parameter variations and compatibility with existing all-microwave two-qubit gate architectures. This hardware-efficient strategy advances scalable quantum computing systems by improving coherence properties, reducing spectral congestion, and expanding the experimental toolkit for error-resilient quantum operations in the noisy intermediate-scale quantum era.
Practical quantum computers require a large network of highly coherent qubits, interconnected in a design robust against errors. Donor spins in silicon provide state-of-the-art coherence and quantum gate fidelities, in a platform adapted from industrial semiconductor processing. Here we present a scalable design for a silicon quantum processor that does not require precise donor placement and leaves ample space for the routing of interconnects and readout devices. We introduce the flip-flop qubit, a combination of the electron-nuclear spin states of a phosphorus donor that can be controlled by microwave electric fields. Two-qubit gates exploit a second-order electric dipole-dipole interaction, allowing selective coupling beyond the nearest-neighbor, at separations of hundreds of nanometers, while microwave resonators can extend the entanglement to macroscopic distances. We predict gate fidelities within fault-tolerance thresholds using realistic noise models. This design provides a realizable blueprint for scalable spin-based quantum computers in silicon.Quantum computers will require a large network of coherent qubits, connected in a noise-resilient way. Tosi et al. present a design for a quantum processor based on electron-nuclear spins in silicon, with electrical control and coupling schemes that simplify qubit fabrication and operation.
Silicon spin qubits are a promising platform for quantum computing due to their high coherence, controllability, and CMOS manufacturability, yet scalable implementations have so far been limited to a few qubits. Here, to take a step towards larger qubit systems, we tune and coherently control an eight-dot linear array of silicon spin qubits fabricated in a 300~mm CMOS-compatible foundry process, establishing operational scalability beyond the two-qubit regime. All eight qubits are successfully tuned and characterized as four double-dot pairs, exhibiting Ramsey dephasing times T2∗T_2^*T2∗​ up to 41(2) μ~μ μs and Hahn-echo coherence times T2HahnT_2^{\mathrm{Hahn}}T2Hahn​ up to 1.31(4) ~ ms. Readout of the central four qubits is achieved via a cascaded charge-sensing protocol, enabling high-fidelity measurements of the entire multi-qubit array in a two step process. Additionally, we demonstrate a two-qubit gate operation between adjacent qubits with low phase noise. We show that silicon spin qubit arrays can be scaled to medium-sized arrays of 8 qubits while maintaining system coherence.