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arXiv:1810.05767 · Journal of Applied Physics

Sensitive radiofrequency readout of quantum dots using an ultra-low-noise SQUID amplifier

F. J. Schupp, Florian Vigneau, Y. Wen, A. Mavalankar, J. Griffiths, G. A. C. Jones, I. Farrer, D. A. Ritchie, C. G. Smith, L. C. Camenzind, L. Yu, Dominik M. Zumbühl, G. A. D. Briggs, N. Ares, Edward A. Laird

Published Jun 28, 2020• 28 citations
arXivHTMLPDF

Abstract

Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 600 mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07aF/Hz. The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1μs of integration time.

DOI
10.1063/5.0005886

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Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 600 mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07aF/Hz. The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1μs of integration time.
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Radio-frequency reflectometry allows for fast and sensitive electrical readout of charge and spin qubits hosted in quantum dot devices coupled to resonant circuits. Optimizing readout, however, requires frequency tuning of the resonators and impedance matching. This is difficult to achieve using conventional semiconductor or ferroelectric-based varactors in the detection circuit as their performance degrades significantly in the mK temperature range relevant for solid-state quantum devices. Here we explore a different type of material, strontium titanate, a quantum paraelectric with exceptionally large field-tunable permittivity at low temperatures. Using strontium titanate varactors we demonstrate perfect impedance matching and resonator frequency tuning at 6 mK and characterize the varactors at this temperature in terms of their capacitance tunability, dissipative losses and magnetic field sensitivity. We show that this allows us to optimize the radio-frequency readout signal-to-noise ratio of carbon nanotube quantum dot devices to achieve a charge sensitivity of 4.8 μμμe/Hz1/2^{1/2}1/2 and capacitance sensitivity of 0.04 aF/Hz1/2^{1/2}1/2.
Abstract Radiofrequency reflectometry can provide fast and sensitive electrical read-out of charge and spin qubits in quantum dot devices coupled to resonant circuits. In situ frequency tuning and impedance matching of the resonator circuit using voltage-tunable capacitors (varactors) is needed to optimize read-out sensitivity, but the performance of conventional semiconductor- and ferroelectric-based varactors degrades substantially in the millikelvin temperature range relevant for solid-state quantum devices. Here we show that strontium titanate and potassium tantalate, materials which can exhibit quantum paraelectric behaviour with large field-tunable permittivity at low temperatures, can be used to make varactors with perfect impedance matching and resonator frequency tuning at 6 mK. We characterize the varactors at 6 mK in terms of their capacitance tunability, dissipative losses and magnetic field insensitivity. We use the quantum paraelectric varactors to optimize the radiofrequency read-out of carbon nanotube quantum dot devices, achieving a charge sensitivity of 4.8 μe Hz−1/2 and a capacitance sensitivity of 0.04 aF Hz−1/2.
Spins in semiconductor quantum dots hold great promise as building blocks of quantum processors. Trapping them in SiMOS transistor-like devices eases future industrial scale fabrication. Among the potentially scalable readout solutions, gate-based dispersive radiofrequency reflectometry only requires the already existing transistor gates to readout a quantum dot state, relieving the need for additional elements. In this effort towards scalability, traveling-wave superconducting parametric amplifiers significantly enhance the readout signal-to-noise ratio (SNR) by reducing the noise below typical cryogenic low-noise amplifiers, while offering a broad amplification band, essential to multiplex the readout of multiple resonators. In this work, we demonstrate a 3GHz gate-based reflectometry readout of electron charge states trapped in quantum dots formed in SiMOS multi-gate devices, with SNR enhanced thanks to a Josephson traveling-wave parametric amplifier (JTWPA). The broad, tunable 2GHz amplification bandwidth combined with more than 10dB ON/OFF SNR improvement of the JTWPA enables frequency and time division multiplexed readout of interdot transitions, and noise performance near the quantum limit. In addition, owing to a design without superconducting loops and with a metallic ground plane, the JTWPA is flux insensitive and shows stable performances up to a magnetic field of 1.2T at the quantum dot device, compatible with standard SiMOS spin qubit experiments.
A scalable spin-qubit quantum processor requires fast, high-fidelity readout circuits operating at cryogenic temperatures (CTs). This work analyzes the radio frequency (RF) reflectometry readout mechanism to establish the link between conductance modulation, single-electron transistor (SET) matching network design, reflected signal power, and receiver (RX) noise temperature. Based on this analysis, a cryogenic RX architecture employing a passive switched-capacitor front-end is proposed. The front-end performs RF amplification and downconversion using transistors as switches, avoiding active RF amplification and mitigating shot-noise limitations. The RX is implemented in 40-nm CMOS and operates from 100 MHz to 2.2 GHz. Measurements demonstrate a minimum noise temperature of approximately 15 K at 4.2 K. When integrated with a quantum device, the RX achieves a signal-to-noise ratio (SNR) of 9.8 dB for a 10-µs integration time and enables 99.9% charge-state readout fidelity without external cryogenic amplifiers. This work represents the first demonstration of such fidelity using a cryo-CMOS RX and constitutes an important step toward scalable integration of qubit readout electronics.
We report an rf parametric amplifier that exploits the gate-tunable quantum capacitance of a semiconductor two-dimensional electron gas at cryogenic temperatures. The prototype amplifier exhibits a gain approximately 30 dB up to an input power of − 66 dBm and a noise temperature T N ∼ 2 K at approximately 370 MHz while dissipating approximately a microwatt of power. Operable at tesla-scale magnetic fields and temperatures ranging from millikelvin to a few kelvin, the quantum capacitance parametric amplifier may find utility in the readout circuit of spin qubits. Here, a quantum dot system is used to verify amplifier performance, improving the signal-to-noise ratio of the readout by approximately 9 dB.