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arXiv:2610.01100

Gate Dispersive Charge Detection in Ge/SiGe Quantum Dots

Mukhlasur Rahman Tanvir, Ebrahim Sajadi, Menno Veldhorst, Giordano Scappucci, Joe Salfi

Published Oct 1, 2026• 0 citations
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Abstract

Spin qubits are an attractive platform for scalable quantum computation, and recently, laterally gated quantum dots (QDs) in Ge heterostructures have emerged as a leading implementation. Radio-frequency (RF) single-hole transistor (SHT) spin readout and integrated superconducting resonator charge readout have demonstrated in Ge QDs, but their wiring overhead and on-chip size can pose challenges, while gate dispersive (GD) readout, which requires less on-chip overhead, has yet to be demonstrated for Ge QDs. Here we demonstrate compact, low-overhead RF-GD charge readout of a Ge double QD (DQD), both directly, and assisted by a proximal single-hole box (SHB). We resolve QD loading and interdot transitions with both techniques. We obtain unity signal-to-noise ratio (SNR) in 2.7 millisecond and 135 microsecond integration time, for gate-dispersive detection of the DQD and SHB-DQD systems, respectively. We find that the SHB enhances the interdot signal-to-noise ratio (SNR) of our 540 MHz circuit by a factor of 20 for a DQD coupling $t_c \approx 5$ GHz. Power and temperature dependent measurements verify that SNR is limited by dielectric loss. We argue that with achievable improvements in dielectric loss and amplification, high-fidelity (99\%) readout in less than 1 microsecond should be possible, assuming ideal spin-to-charge conversion, which has seen rapid recent progress. These results establish a simple and compact gate-based readout approach for direct and SHB-enhanced readout of Ge heterostructure QDs.

DOI
10.48550/arxiv.2610.01100

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cond-mat.mes-hallCross-listedarXiv:2609.20775•Sep 18, 2026
Spin qubits are an attractive platform for scalable quantum computation, and recently, laterally gated quantum dots (QDs) in Ge heterostructures have emerged as a leading implementation. Radio-frequency (RF) single-hole transistor (SHT) spin readout and integrated superconducting resonator charge readout have demonstrated in Ge QDs, but their wiring overhead and on-chip size can pose challenges, while gate dispersive (GD) readout, which requires less on-chip overhead, has yet to be demonstrated for Ge QDs. Here we demonstrate compact, low-overhead RF-GD charge readout of a Ge double QD (DQD), both directly, and assisted by a proximal single-hole box (SHB). We resolve QD loading and interdot transitions with both techniques. We obtain unity signal-to-noise ratio (SNR) in 2.7 millisecond and 135 microsecond integration time, for gate-dispersive detection of the DQD and SHB-DQD systems, respectively. We find that the SHB enhances the interdot signal-to-noise ratio (SNR) of our 540 MHz circuit by a factor of 20 for a DQD coupling tc≈5t_c \approx 5tc​≈5 GHz. Power and temperature dependent measurements verify that SNR is limited by dielectric loss. We argue that with achievable improvements in dielectric loss and amplification, high-fidelity (99%) readout in less than 1 microsecond should be possible, assuming ideal spin-to-charge conversion, which has seen rapid recent progress. These results establish a simple and compact gate-based readout approach for direct and SHB-enhanced readout of Ge heterostructure QDs.
Spin qubits in gate-defined semiconductor quantum dots are a promising platform for realising a scalable quantum computer. This platform encodes quantum information into the intrinsic angular momentum (spin) of individual electrons or holes. In particular, hole spins in quantum dots enable the possibility of all-electrical spin control due to their intrinsic spin-orbit coupling. One important criterion of a quantum computer is the ability to reliably read out the state of the qubit. This is done by mapping spin information to the charge degree of freedom, combined with a method for charge detection. Charge detection can be facilitated by coupling a resonator directly to the quantum dot gate or to a charge sensor and measuring the resonator's phase or amplitude response. In this thesis, we describe a set of experiments relating to the implementation of hole spin qubit readout with superconducting resonators. First, we study gate-based charge sensing in a Si fin field-effect transistor (finFET) device using a superconducting NbN nanowire resonator. We find that the resonator amplifies capacitive crosstalk when electrical control pulses are applied to gate electrodes in this device. This occurs whenever a spectral component of qubit control pulses overlaps with the resonance frequency, which we show may occur for both baseband and radio-frequency (RF) drive signals. We quantify this crosstalk in a transport-based method, and attribute its source to the capacitance between the bondpads of neighbouring gate electrodes. In a second experiment, we use an RF charge sensor with the same nanowire technology to read out hole spin qubits formed in the quantum well of a Ge/SiGe heterostructure. A screening layer approach is used to facilitate RF matching due to large RF dissipation found in the heterostructure. We implement a double-latched readout scheme in this system to reduce errors arising from both spin and charge decay during readout. The spin-to-charge conversion process and its dependence on magnetic field orientation is studied. Spin and charge decay processes arising after spin-to-charge conversion are mitigated by choosing an optimal working point for the field orientation, field amplitude and readout pulse times. By doing so, we achieve an average single-shot state-preparation-and-measurement fidelity of 97%, primarily bounded by imperfect state preparation and limited signal-to-noise ratio. Finally, we study coplanar waveguide resonators fabricated from NbN. We investigate the feasibility of integrating this type of resonator onto the same chip as the Si finFET quantum dots. We then develop a novel resonator readout technique by the interference of two microwave sidebands. We demonstrate the insensitivity of this technique to common-mode phase and amplitude noise both theoretically and experimentally. This technique is then used to study resonator noise arising from two-level system defects. These results highlight both the practical constraints and the potential for achieving high readout fidelities using resonator-based hole spin qubit readout. This work establishes a blueprint for mitigating resonator-induced crosstalk and enabling high-fidelity readout in lossy materials by incorporating a screening layer.
Rapid single shot spin readout is a key ingredient for fault tolerant quantum computing with spin qubits. An RF-SET (radio-frequency single electron transistor) is predominantly used as its the readout timescale is far shorter than the spin decoherence time. In this work, we experimentally demonstrate a transmission-based RF-SET using a multi-module semiconductor-superconductor assembly. A monolithically integrated SET placed next to a double quantum dot in a Si/SiGe heterostructure is wire-bonded to a superconducting niobium inductor forming the impedance-transforming network. Compared to RF reflectometry, the proposed set-up is experimentally simpler without the need for directional couplers. Read-out performance is benchmarked by the signal-to-noise (SNR) of a dot-reservoir transition (DRT) and an interdot charge transition (ICT) in the double quantum dot near the SET as a function of RF power and integration time. The minimum integration time for unitary SNR is found to be 100 ns for ICT and 300 ns for DRT. The obtained minimum integration times are comparable to the state of the art in conventional RF reflectometry set-ups. Furthermore, we study the turn-on properties of the RF-SET to investigate capacitive shifts and RF losses. Understanding these effects are crucial for further optimisations of the impedance transforming network as well as the device design to assist RF read-out. This new RF read-out scheme also shows promise for multiplexing spin-qubit readout and further studies on rapid charge dynamics in quantum dots.
Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500-800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1 μs, well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed.
Three key metrics for readout systems in quantum processors are measurement speed, fidelity and footprint. Fast high-fidelity readout enables mid-circuit measurements, a necessary feature for many dynamic algorithms and quantum error correction, while a small footprint facilitates the design of scalable, highly-connected architectures with the associated increase in computing performance. Here, we present two complementary demonstrations of fast high-fidelity single-shot readout of spins in silicon quantum dots using a compact, dispersive charge sensor: a radio-frequency single-electron box. The sensor, despite requiring fewer electrodes than conventional detectors, performs at the state-of-the-art achieving spin read-out fidelity of 99.2% in less than 6 μμμs. We demonstrate that low-loss high-impedance resonators, highly coupled to the sensing dot, in conjunction with Josephson parametric amplification are instrumental in achieving optimal performance. We quantify the benefit of Pauli spin blockade over spin-dependent tunneling to a reservoir, as the spin-to-charge conversion mechanism in these readout schemes. Our results place dispersive charge sensing at the forefront of readout methodologies for scalable semiconductor spin-based quantum processors.
Spins in semiconductor quantum dots are a candidate for cryogenic quantum processors due to their exceptionally long coherence times. One major challenge to scaling quantum dot spin qubits is the dense wiring requirements, making it difficult to envision fabricating large arrays of nearest-neighbor-coupled qubits necessary for error correction. We describe a method to solve this problem by spacing the qubits out using high-impedance superconducting resonators with a 2D grid unit cell area of 0.16 mm20.16~\text{mm}^20.16 mm2 using 3D integration. To prove the viability of this approach, we demonstrate 3D integration of a high-impedance TiN resonator coupled to a double quantum dot in a Si/SiGe heterostructure. Using the resonator as a dispersive gate sensor, we tune the device down to the single electron regime with an SNR = 5.36 limited by the resonator-dot capacitance. Characterization of the dot and resonator systems shows such integration can be done while maintaining low charge noise metrics for the quantum dots and with improved loaded quality factors for the superconducting resonator (QL=2.14×104Q_L = 2.14 \times 10^4QL​=2.14×104), allowing for high-sensitivity charge detection and the potential for high fidelity 2-qubit gates. This work paves the way for 2D quantum dot qubit arrays with cavity mediated interactions.
Hole spin qubits in semiconductor quantum dots (QDs) enable high-fidelity all-electric control, but conventional electric dipole spin resonance (EDSR) can require substantial rf drive power at the low magnetic fields that are favorable for qubit coherence and readout. In planar Ge hole spin qubits, this can reach -27 dBm at the device, posing challenges for scalable architectures due to heating and crosstalk. Here, we demonstrate a flopping-mode (FM) qubit in Ge, where a single spin is delocalized in a double QD, combining first-order protection against charge noise with exceptionally efficient electric driving. By mapping out coherence sweet-spots as a function of magnetic field orientation we achieve T2∗=1.4μsT_2^*= 1.4μ\mathrm{s}T2∗​=1.4μs, T2Hahn=11.5μsT_2^{\mathrm{Hahn}}= 11.5 μ\mathrm{s}T2Hahn​=11.5μs, T2φ,CPMG32=130μsT^{φ, \mathrm{CPMG32}}_2= 130 μ\mathrm{s}T2φ,CPMG32​=130μs, and T1=226μsT_1= 226 μ\mathrm{s}T1​=226μs, and a single-qubit gate fidelity of up to 99.76%\%% for a gate time tXπ=88t_{Xπ} = 88tXπ​=88 ns. Importantly, these results are obtained at a nearly in-plane magnetic field of 5 mT using only -52 dBm drive power at the device. We further find that qubit relaxation in this regime is consistent with a two-photon Orbach process, providing a route for further optimization. Our results demonstrate that FM-EDSR supports ultra-low-power, high-fidelity single-qubit operations, improvements that could benefit scalable hole-spin-based architectures and hybrid spin-photon interfaces.