arXiv subjects

MengKe Feng

Publications and source records attributed to MengKe Feng.

At least 19 recordsLinked to original sources

arXiv2026

Machine Learning for Charge State Characterization of Isolated Double Quantum Dots

Scaling semiconductor quantum dot arrays toward fault-tolerant quantum computing requires efficient tuneup of spin qubits, a process that depends on the analysis of charge stability maps (CSMs) and remains largely manual. While machine learning has been widely applied to CSM analysis in reservoir-coupled devices, automated tuning in the increasingly important isolated-mode regime has received limited attention. In isolated-mode CSMs, charge transitions appear as near-vertical lines, making them well suited to compact, task-specific models. We present two convolutional neural networks with fewer than one million parameters, trained on CSMs collected from 32 silicon metal-oxide-semiconductor (SiMOS) double-quantum-dot devices measured at approximately 1 K using an automated cryogenic probing system. Sixteen devices were used for training and sixteen were held out to evaluate cross-device generalization against hand-labeled ground truth. CSMClassifier identifies charge instability and sensor artifacts, achieving 94% macro-averaged accuracy across three quality classes on 2,407 held-out images. ChargeLineNet localizes charge-transition lines and determines electron occupancy, achieving 95.3% exact line-count accuracy on 1,131 held-out images. Combined into a single pipeline, the models correctly determine electron occupancy for 93.8% of clean held-out images. Pre-training on synthetic images substantially improves label efficiency. Fine-tuning the pre-trained model on limited experimental data maintains over 90% accuracy, whereas training from scratch degrades significantly under the same conditions. Together, the two models occupy only 6.5 MB and process images in less than 60 ms on standard laboratory hardware, demonstrating a practical path toward scalable, automated characterization and tuneup of quantum-dot devices.

cond-mat.mes-hall↗

arXiv2026

Dispersive Readout of a SiMOS Quantum Dot Using a Flip-Chip Integrated Microwave Resonator

Heterogeneous integration provides a promising route to combine semiconductor quantum dot devices and superconducting microwave circuits, while allowing each component to be fabricated using an optimized process flow. Here, we demonstrate a flip-chip integrated platform for dispersive readout of silicon metal-oxide semiconductor (SiMOS) quantum dot devices. A SiMOS double quantum dot chip is bonded to a superconducting aluminum resonator chip using indium bump interconnects to enable microwave coupling to the quantum dot gate. We show that the developed flip-chip process is compatible with cryogenic operation of both the SiMOS device and the superconducting resonator, and demonstrate resonator-based detection of charge transitions in the quantum dot system. The readout signal-to-noise ratio follows a dependence of $\sqrt{t}$ with the integration time, reaching SNR = 1 at an integration time of approximately 0.3 ms. These results establish flip-chip bonding as a viable integration approach for SiMOS quantum dot devices operating at both dc and microwave frequencies, with potential applications for resonator-based techniques such as spin-photon coupling.

cond-mat.mes-hall↗

arXiv2026

Optimal operating temperature for industry-compatible silicon spin quantum computing: colder is not necessarily better

Silicon spin qubits are a leading candidate for large-scale quantum computing owing to their compatibility with semiconductor manufacturing. However, scaling to useful fault-tolerant processors will likely generate thermal loads that exceed the cooling power available at millikelvin temperatures. Raising the operating temperature eases cooling requirements but reduces gate fidelity, increasing the overhead of quantum error correction. Identifying the operating temperature that minimizes total power consumption is therefore a key challenge for commercially viable quantum computers. Here, we use gate set tomography to benchmark two-qubit silicon chips fabricated in both industrial and academic environments over a range of temperatures. Elevated temperatures substantially shorten coherence times and increase gate and state-preparation-and-measurement infidelities. Based on these measurements, we develop a general power model for silicon quantum computers that combines cryogenic cooling requirements with error-correction overheads. We show that a finite optimal operating temperature exists and is strongly influenced by a crossover temperature near 1 K in current devices, above which gate fidelity degrades rapidly. These results connect device-level fidelity limitations to system-level power requirements, providing design guidelines for large-scale silicon quantum computers.

quant-ph↗

arXiv2026

Multi-Qubit Entanglement of Unit Cell Pairs in SiMOS

Spin qubits in silicon-MOS (SiMOS) quantum dots have recently demonstrated compatibility with existing industry standard CMOS fabrication techniques. These devices have routinely achieved single- and two-qubit gate fidelities above 99% and demonstrated highly entangled two-qubit Bell states in isolated double quantum dot (DQD) unit cells, however coupling between unit cells has remained challenging. In this work, we present a two unit cell, four-qubit SiMOS processor with universal controllability and fully parallelised state initialisation and readout. We use this processor to generate maximally entangled three-qubit states, including the Greenberger-Horne-Zeilinger (GHZ) state, and certify multipartite entanglement through violation of the classical Mermin-witness bound. By using a fully symmetric dynamically decoupled gate sequence to create our entangled states, we are able to preserve the lifetime of the entanglement beyond $T_2^*$, to a time limited instead by $T_2^\textrm{Hahn}$. These demonstrations pave a road to the scalable operation of larger SiMOS processors, and achieving high purity, long-lived multi-qubit entangled states in them.

quant-ph↗

arXiv2025

Mid-circuit logic executed in the qubit layer of a quantum processor

Practical quantum computers need to continuously exchange data between classical and quantum subsystems during a computation. Mid-circuit measurements of a qubits state are transferred to the classical electronics layer, and their outcome can inform feedforward operations that close the loop back to the quantum layer. These operations are crucial for fault-tolerant quantum computers, but the quantum-classical loop must be completed before the qubits decohere, presenting a substantial engineering challenge for full-scale systems comprising millions of qubits. Here we perform the first mid-circuit measurements in a system of silicon spin qubits, and show that feedforward operations can be performed without needing to route information to the classical layer. This in-layer approach leverages a backaction-driven control technique that has previously been considered a source of error. We benchmark our in-layer strategy, together with the standard FPGA-enabled approach, and analyse the performance of both methods using gate set tomography. Our results provide the first step towards moving resource-intensive classical processing into the quantum layer, an advance that could solve key engineering challenges, and drastically reduce the power budget of future quantum computers.

quant-ph↗

arXiv2025

Eight-Qubit Operation of a 300 mm SiMOS Foundry-Fabricated Device

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 $T_2^*$ up to 41(2)$~\mu$s and Hahn-echo coherence times $T_2^{\mathrm{Hahn}}$ 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.

quant-ph↗

arXiv2025

Interplay of Zeeman Splitting and Tunnel Coupling in Coherent Spin Qubit Shuttling

Spin shuttling offers a promising approach for developing scalable silicon-based quantum processors by addressing the connectivity limitations of quantum dots. In this work, we demonstrate high-fidelity bucket-brigade spin shuttling in a silicon MOS device, utilizing Pauli-spin-blockade readout. We achieve an average shuttling fidelity of \SI{99.8}{\percent}. The residual shuttling error is highly sensitive to the ratio between interdot tunnel coupling and Zeeman splitting, with tuning of these parameters enabling up to a 20-fold variation in error rate. An appropriate four-level Hamiltonian model supports our findings. These results provide valuable insights for optimizing high-performance spin-shuttling systems in future quantum architectures.

cond-mat.mes-hall↗

arXiv2025

Scalable quantum current source on commercial CMOS process technology

Many quantum technologies require a precise electrical current standard that can only be achieved with expensive cryogenics, or through the secondary standards, such as resistance or voltage. Silicon-based charge pumps could provide such a standard in an inherently scalable way, through their compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication methods. However, coherent quantized charge transfer has so far been demonstrated only in nanoscale devices that are custom-fabricated in academic cleanrooms or research technology foundries. Here, we show that a CMOS device manufactured with commercial 22-nm process node can be used to define a quantum current standard in the International System of Units (SI). We measure an accuracy of (1.2 +/- 0.1)E-3 A/A at 50 MHz with reference to SI voltage and resistance standards in a pumped helium system. We then propose a practical monolithic CMOS chip that incorporates one million parallel connected charge pumps along with on-chip control electronics. This chip could be operated as a table-top primary standard that can be easily integrated with CMOS electronics, generating quantum currents of up to microampere levels.

physics.app-ph↗

arXiv2025

Enhancement of Electric Drive in Silicon Quantum Dots with Electric Quadrupole Spin Resonance

Quantum computation with electron spin qubits requires coherent and efficient manipulation of these spins, typically accomplished through the application of alternating magnetic or electric fields for electron spin resonance (ESR). In particular, electrical driving allows us to apply localized fields on the electrons, which benefits scale-up architectures. However, we have found that Electric Dipole Spin Resonance (EDSR) is insufficient for modeling the Rabi behavior in recent experimental studies. Therefore, we propose that the electron spin is being driven by a new method of electric spin qubit control which generalizes the spin dynamics by taking into account a quadrupolar contribution of the quantum dot: electric quadrupole spin resonance (EQSR). In this work, we explore the electric quadrupole driving of a quantum dot in silicon, specifically examining the cases of 5 and 13 electron occupancies.

cond-mat.mes-hall↗

arXiv2024

A 300 mm foundry silicon spin qubit unit cell exceeding 99% fidelity in all operations

Fabrication of quantum processors in advanced 300 mm wafer-scale complementary metal-oxide-semiconductor (CMOS) foundries provides a unique scaling pathway towards commercially viable quantum computing with potentially millions of qubits on a single chip. Here, we show precise qubit operation of a silicon two-qubit device made in a 300 mm semiconductor processing line. The key metrics including single- and two-qubit control fidelities exceed 99% and state preparation and measurement fidelity exceeds 99.9%, as evidenced by gate set tomography (GST). We report coherence and lifetimes up to $T_\mathrm{2}^{\mathrm{*}} = 30.4$ $\mu$s, $T_\mathrm{2}^{\mathrm{Hahn}} = 803$ $\mu$s, and $T_1 = 6.3$ s. Crucially, the dominant operational errors originate from residual nuclear spin carrying isotopes, solvable with further isotopic purification, rather than charge noise arising from the dielectric environment. Our results answer the longstanding question whether the favourable properties including high-fidelity operation and long coherence times can be preserved when transitioning from a tailored academic to an industrial semiconductor fabrication technology.

cond-mat.mes-hall↗

arXiv2024

Violating Bell's inequality in gate-defined quantum dots

Superior computational power promised by quantum computers utilises the fundamental quantum mechanical principle of entanglement. However, achieving entanglement and verifying that the generated state does not follow the principle of local causality has proven difficult for spin qubits in gate-defined quantum dots, as it requires simultaneously high concurrence values and readout fidelities to break the classical bound imposed by Bell's inequality. Here we employ heralded initialization and calibration via gate set tomography (GST), to reduce all relevant errors and push the fidelities of the full 2-qubit gate set above 99 %, including state preparation and measurement (SPAM). We demonstrate a 97.17 % Bell state fidelity without correcting for readout errors and violate Bell's inequality with a Bell signal of S = 2.731 close to the theoretical maximum of $2\sqrt{2}$. Our measurements exceed the classical limit even at elevated temperatures of 1.1 K or entanglement lifetimes of 100 $\mu s$.

cond-mat.mes-hall↗

arXiv2024

Electronic Correlations in Multielectron Silicon Quantum Dots

Silicon quantum computing has the potential to revolutionize technology with capabilities to solve real-life problems that are computationally complex or even intractable for modern computers [1] by offering sufficient high quality qubits to perform complex error-corrected calculations. Silicon metal-oxide-semiconductor based quantum dots present a promising pathway for realizing practical quantum computers. To improve certain qubit properties, it is a common strategy to incorporate multiple electrons in the same dot in order to form qubits in higher confined orbital states. Theoretical modelling is an essential part of understanding the quantum behaviour of these electrons, providing a basis for validating the physical working of device models as well as providing insights into experimental data. Hartree-Fock theory is an imperative tool for the electronic structure modelling of multi-electron quantum dots due to its ability to simulate a large number of electrons with manageable computation load. However, an efficient calculation of the self-consistent field becomes hard because dot formations in silicon are characterized by strong electron-electron interactions and conduction band valleys, besides the relatively high comparative effective mass, which add to create a behaviour dominated by repulsion between electrons rather than a well established shell structure. In this paper, we present a Hartree-Fock-based method that accounts for these complexities for the modelling of silicon quantum dots. With this method, we first establish the significance of including electron-electron interactions and valley degree of freedom and their implications. We then explore a simple case of anisotropic dots and observe the impact of anisotropy on dot formations.

cond-mat.mes-hall↗

arXiv2023

Entangling gates on degenerate spin qubits dressed by a global field

Coherently dressed spins have shown promising results as building blocks for future quantum computers owing to their resilience to environmental noise and their compatibility with global control fields. This mode of operation allows for more amenable qubit architecture requirements and simplifies signal routing on the chip. However, multi-qubit operations, such as qubit addressability and two-qubit gates, are yet to be demonstrated to establish global control in combination with dressed qubits as a viable path to universal quantum computing. Here we demonstrate simultaneous on-resonance driving of degenerate qubits using a global field while retaining addressability for qubits with equal Larmor frequencies. Furthermore, we implement SWAP oscillations during on-resonance driving, constituting the demonstration of driven two-qubit gates. Significantly, our findings highlight the fragility of entangling gates between superposition states and how dressing can increase the noise robustness. These results represent a crucial milestone towards global control operation with dressed qubits. It also opens a door to interesting spin physics on degenerate spins.

cond-mat.mes-hall↗

arXiv2023

Silicon charge pump operation limit above and below liquid helium temperature

Semiconductor tunable barrier single-electron pumps can produce output current of hundreds of picoamperes at sub ppm precision, approaching the metrological requirement for the direct implementation of the current standard. Here, we operate a silicon metal-oxide-semiconductor electron pump up to a temperature of 14 K to understand the temperature effect on charge pumping accuracy. The uncertainty of the charge pump is tunnel limited below liquid helium temperature, implying lowering the temperature further does not greatly suppress errors. Hence, highly accurate charge pumps could be confidently achieved in a $^4$He cryogenic system, further promoting utilization of the revised quantum current standard across the national measurement institutes and industries worldwide.

cond-mat.mes-hall↗

arXiv2023

Methods for transverse and longitudinal spin-photon coupling in silicon quantum dots with intrinsic spin-orbit effect

In a full-scale quantum computer with a fault-tolerant architecture, having scalable, long-range interaction between qubits is expected to be a highly valuable resource. One promising method of achieving this is through the light-matter interaction between spins in semiconductors and photons in superconducting cavities. This paper examines the theory of both transverse and longitudinal spin-photon coupling and their applications in the silicon metal-oxide-semiconductor (SiMOS) platform. We propose a method of coupling which uses the intrinsic spin-orbit interaction arising from orbital degeneracies in SiMOS qubits. Using theoretical analysis and experimental data, we show that the strong coupling regime is achievable in the transverse scheme. We also evaluate the feasibility of a longitudinal coupling driven by an AC modulation on the qubit. These coupling methods eschew the requirement for an external micromagnet, enhancing prospects for scalability and integration into a large-scale quantum computer.

cond-mat.mes-hall↗

arXiv2023

High-fidelity operation and algorithmic initialisation of spin qubits above one kelvin

The encoding of qubits in semiconductor spin carriers has been recognised as a promising approach to a commercial quantum computer that can be lithographically produced and integrated at scale. However, the operation of the large number of qubits required for advantageous quantum applications will produce a thermal load exceeding the available cooling power of cryostats at millikelvin temperatures. As the scale-up accelerates, it becomes imperative to establish fault-tolerant operation above 1 kelvin, where the cooling power is orders of magnitude higher. Here, we tune up and operate spin qubits in silicon above 1 kelvin, with fidelities in the range required for fault-tolerant operation at such temperatures. We design an algorithmic initialisation protocol to prepare a pure two-qubit state even when the thermal energy is substantially above the qubit energies, and incorporate radio-frequency readout to achieve fidelities up to 99.34 per cent for both readout and initialisation. Importantly, we demonstrate a single-qubit Clifford gate fidelity of 99.85 per cent, and a two-qubit gate fidelity of 98.92 per cent. These advances overcome the fundamental limitation that the thermal energy must be well below the qubit energies for high-fidelity operation to be possible, surmounting a major obstacle in the pathway to scalable and fault-tolerant quantum computation.

quant-ph↗

arXiv2023

Improved Single-Shot Qubit Readout Using Twin RF-SET Charge Correlations

High fidelity qubit readout is critical in order to obtain the thresholds needed to implement quantum error correction protocols and achieve fault-tolerant quantum computing. Large-scale silicon qubit devices will have densely-packed arrays of quantum dots with multiple charge sensors that are, on average, farther away from the quantum dots, entailing a reduction in readout fidelities. Here, we present a readout technique that enhances the readout fidelity in a linear SiMOS 4-dot array by amplifying correlations between a pair of single-electron transistors, known as a twin SET. By recording and subsequently correlating the twin SET traces as we modulate the dot detuning across a charge transition, we demonstrate a reduction in the charge readout infidelity by over one order of magnitude compared to traditional readout methods. We also study the spin-to-charge conversion errors introduced by the modulation technique, and conclude that faster modulation frequencies avoid relaxation-induced errors without introducing significant spin flip errors, favouring the use of the technique at short integration times. This method not only allows for faster and higher fidelity qubit measurements, but it also enhances the signal corresponding to charge transitions that take place farther away from the sensors, enabling a way to circumvent the reduction in readout fidelities in large arrays of qubits.

cond-mat.mes-hall↗

arXiv2023

Path integral simulation of exchange interactions in CMOS spin qubits

The boom of semiconductor quantum computing platforms created a demand for computer-aided design and fabrication of quantum devices. Path integral Monte Carlo (PIMC) can have an important role in this effort because it intrinsically integrates strong quantum correlations that often appear in these multi-electron systems. In this paper we present a PIMC algorithm that estimates exchange interactions of three-dimensional electrically defined quantum dots. We apply this model to silicon metal-oxide-semiconductor (MOS) devices and we benchmark our method against well-tested full configuration interaction (FCI) simulations. As an application, we study the impact of a single charge trap on two exchanging dots, opening the possibility of using this code to test the tolerance to disorder of CMOS devices. This algorithm provides an accurate description of this system, setting up an initial step to integrate PIMC algorithms into development of semiconductor quantum computers.

cond-mat.mes-hall↗