Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems.Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems.

Jan 28, 20200 citations

Coherence Protection for Mobile Spin Qubits in Silicon

Jan A. Krzywda, Yuta Matsumoto, Maxim De Smet +6 more

Mobile spin qubit architectures promise flexible connectivity for efficient quantum error correction and relaxed device layout constraints, but their viability rests on preserving spin coherence during transport. While shuttling transforms spatial disorder into time-dependent noise, its net impact on spin coherence remains an open question. Here we demonstrate systematic noise mitigation during spin shuttling in a linear $^{28}$ Si/SiGe quantum dot device. First, by passively reducing magnetic field gradients, we minimize charge-noise coupling to the spin and double the spatially averaged dephasing time $T_2^*(x_n)$ from $4.4$ to $8.5\,μ\text{s}$ . Next, we exploit motional narrowing by periodically shuttling the qubit, achieving a further enhancement in coherence time up to $T_{2}^{*,sh} = 11.5\,μ\text{s}$ . Finally, we incorporate dynamical decoupling techniques while periodically shuttling over distances exceeding $200\,\text{nm}$ , reaching $T_\text{2}^{H,sh}= 32\,μ\text{s}$ . For the same setup, we demonstrate that dressed-state shuttling provides robust protection against low-frequency noise with a decay time $T_R^{\text{sh}} = 21\,μ\text{s}$ , without the overhead of pulsed control and allowing protection during one-way spin transport. By preserving coherence over timescales exceeding typical gate and readout operations, the demonstrated strategies establish mobile spin qubits as a viable solution for scalable silicon quantum processors.Mobile spin qubit architectures promise flexible connectivity for efficient quantum error correction and relaxed device layout constraints, but their viability rests on preserving spin coherence during transport. While shuttling transforms spatial disorder into time-dependent noise, its net impact on spin coherence remains an open question. Here we demonstrate systematic noise mitigation during spin shuttling in a linear $^{28}$Si/SiGe quantum dot device. First, by passively reducing magnetic field gradients, we minimize charge-noise coupling to the spin and double the spatially averaged dephasing time $T_2^*(x_n)$ from $4.4$ to $8.5\,μ\text{s}$. Next, we exploit motional narrowing by periodically shuttling the qubit, achieving a further enhancement in coherence time up to $T_{2}^{*,sh} = 11.5\,μ\text{s}$. Finally, we incorporate dynamical decoupling techniques while periodically shuttling over distances exceeding $200\,\text{nm}$, reaching $T_\text{2}^{H,sh}= 32\,μ\text{s}$. For the same setup, we demonstrate that dressed-state shuttling provides robust protection against low-frequency noise with a decay time $T_R^{\text{sh}} = 21\,μ\text{s}$, without the overhead of pulsed control and allowing protection during one-way spin transport. By preserving coherence over timescales exceeding typical gate and readout operations, the demonstrated strategies establish mobile spin qubits as a viable solution for scalable silicon quantum processors.

cond-mat.mes-hallCross-listedarXiv:2602.09179Feb 11, 2026

Optimizing RF Readout for Silicon Spin Qubits in an Access Array

John Michniewicz, Andrea Ruffino, Yatao Peng +5 more

Efficient and scalable qubit readout is a critical requirement for realizing quantum computing. Radio frequency (RF) readout emerges as a promising candidate due to its high speed and multiplexing capabilities. In this study, we employ a 2-D access array to significantly reduce the number of control lines needed per qubit for readout. We demonstrate frequency-domain multiplexing (FDM) and simultaneous readout at two distinct frequencies in a semiconductor quantum device. Optimizations in circuit design, device proximity, power levels, and RF readout frequency enhanced the signal-to-noise ratio (SNR) by an order of magnitude. Despite these advancements, challenges such as bandwidth overlap persist, which can impact the scalability of FDM. Our findings highlight both the potential and the limitations of RF readout in scalable quantum computing systems.Efficient and scalable qubit readout is a critical requirement for realizing quantum computing. Radio frequency (RF) readout emerges as a promising candidate due to its high speed and multiplexing capabilities. In this study, we employ a 2-D access array to significantly reduce the number of control lines needed per qubit for readout. We demonstrate frequency-domain multiplexing (FDM) and simultaneous readout at two distinct frequencies in a semiconductor quantum device. Optimizations in circuit design, device proximity, power levels, and RF readout frequency enhanced the signal-to-noise ratio (SNR) by an order of magnitude. Despite these advancements, challenges such as bandwidth overlap persist, which can impact the scalability of FDM. Our findings highlight both the potential and the limitations of RF readout in scalable quantum computing systems.

IEEE Transactions on Instrumentation and MeasurementJan 1, 20251 citation

^{28}

Si/SiGe quantum dot device. First, by passively reducing magnetic field gradients, we minimize charge-noise coupling to the spin and double the spatially averaged dephasing time

T_2^*(x_n)

from

4.4

8.5\,μ\text{s}

. Next, we exploit motional narrowing by periodically shuttling the qubit, achieving a further enhancement in coherence time up to

T_{2}^{*,sh} = 11.5\,μ\text{s}

. Finally, we incorporate dynamical decoupling techniques while periodically shuttling over distances exceeding

200\,\text{nm}

, reaching

T_\text{2}^{H,sh}= 32\,μ\text{s}

. For the same setup, we demonstrate that dressed-state shuttling provides robust protection against low-frequency noise with a decay time

T_R^{\text{sh}} = 21\,μ\text{s}

, without the overhead of pulsed control and allowing protection during one-way spin transport. By preserving coherence over timescales exceeding typical gate and readout operations, the demonstrated strategies establish mobile spin qubits as a viable solution for scalable silicon quantum processors.