arXiv subjects

D. Wan

Publications and source records attributed to D. Wan.

10 recordsLinked to original sources

arXiv2026

Ultra-high Q-factor superconducting tantalum resonators on 300 mm Si wafers

Superconducting resonators are central to superconducting quantum information technologies and essential for bosonic qubit architectures, where long-lived storage modes enable hardware-efficient error correction. Achieving ultra-high quality factors in scalable planar circuits is challenging because multiple dissipation channels contribute to the total loss. Here we report planar $\alpha$-Ta resonators fabricated on 300 mm ultra-high-resistivity ($>10$ k$\Omega$ cm) intrinsic silicon using industrial processes, achieving median internal Q factors exceeding 40 million and maxima above 60 million. Energy-participation-ratio analysis identifies a dominant participation-controlled interface loss mechanism and places conservative upper bounds on substrate-associated dissipation. For the best-performing substrate, the inferred substrate loss tangent is below $1.0 \times 10^{-8}$, establishing industrial MCZ silicon among the lowest-loss substrate platforms reported for superconducting resonators. At the same time, the exceptionally low losses show no clear correlation with commonly cited silicon substrate metrics such as room-temperature resistivity or impurity concentrations. More broadly, these studies establish industrial 300 mm processing, careful interface engineering, and 300 mm MCZ silicon substrates as a promising platform for resonator-heavy superconducting quantum architectures with ultra-high quality factors.

quant-ph↗

arXiv2025

Radio frequency single electron transmission spectroscopy of a semiconductor Si/SiGe quantum dot

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.

cond-mat.mes-hall↗

arXiv2025

Reversing Hydrogen-Related Loss in $\alpha$-Ta Thin Films for Quantum Device Fabrication

$\alpha$-Tantalum ($\alpha$-Ta) is an emerging material for superconducting qubit fabrication due to the low microwave loss of its stable native oxide. However, hydrogen absorption during fabrication, particularly when removing the native oxide, can degrade performance by increasing microwave loss. In this work, we demonstrate that hydrogen can enter $\alpha$-Ta thin films when exposed to 10 vol% hydrofluoric acid for 3 minutes or longer, leading to an increase in power-independent ohmic loss in high-Q resonators at millikelvin temperatures. Reduced resonator performance is likely caused by the formation of non-superconducting tantalum hydride (TaH$_x$) precipitates. We further show that annealing at 500{\deg}C in ultra-high vacuum (10$^{-8}$ Torr) for one hour fully removes hydrogen and restores the resonators' intrinsic quality factors to ~4 million at the single-photon level. These findings identify a previously unreported loss mechanism in $\alpha$-Ta and offer a pathway to reverse hydrogen-induced degradation in quantum devices based on Ta and, by extension also Nb, enabling more robust fabrication processes for superconducting qubits.

quant-ph↗

arXiv2025

Holes in silicon are heavier than expected: transport properties of extremely high mobility electrons and holes in silicon MOSFETs

The quality of the silicon-oxide interface plays a crucial role in fabricating reproducible silicon spin qubits. In this work we characterize interface quality by performing mobility measurements on silicon Hall bars. We find a peak electron mobility of nearly $40,000\,\text{cm}^2/\text{Vs}$ in a device with a $21\,\text{nm}$ oxide layer, and a peak hole mobility of about $2,000\,\text{cm}^2/\text{Vs}$ in a device with $8\,\text{nm}$ oxide, the latter being the highest recorded mobility for a p-type silicon MOSFET. Despite the high device quality, we note an order-of-magnitude difference in mobility between electrons and holes. By studying additional n-type and p-type devices with identical oxides, and fitting to transport theory, we show that this mobility discrepancy is due to valence band nonparabolicity. The nonparabolicity endows holes with a density-dependent transverse effective mass ranging from $0.6m_0$ to $0.7m_0$, significantly larger than the usually quoted bend-edge mass of $0.22m_0$. Finally, we perform magnetotransport measurements to extract momentum and quantum scattering lifetimes.

cond-mat.mes-hall↗

arXiv2025

A trilinear quantum dot architecture for semiconductor spin qubits

Semiconductor quantum dot spin qubits hold significant potential for scaling to millions of qubits for practical quantum computing applications, as their structure highly resembles the structure of conventional transistors. Since classical semiconductor manufacturing technology has reached an unprecedented level of maturity, reliably mass-producing CMOS chips with hundreds of billions of components, conventional wisdom dictates that leveraging CMOS technologies for quantum dot qubits can result in upscaled quantum processors with thousands or even millions of interconnected qubits. However, the interconnect requirements for quantum circuits are very different from those for classical circuits, where for each qubit individual control and readout wiring could be needed. Although significant developments have been demonstrated on small scale systems, qubit numbers remain limited, to a large extent due to the lack of scalable qubit interconnect schemes. Here, we present a trilinear quantum dot array that is simple in physical layout while allowing individual wiring to each quantum dot. By means of electron shuttling, the trilinear architecture provides qubit connectivity that is equivalent to or even surpasses that of 2D square lattice. Assuming the current qubit fidelities of small-scale devices can be extrapolated to large-scale arrays, medium-length shuttling arrays on the order of tens of microns would allow million-scale qubit systems, while maintaining manageable overheads. We also present a scalable control scheme, where the qubit chip is 3D-integrated with a low-power switch-based cryoCMOS circuit for parallel qubit operation with limited control inputs. As our trilinear quantum dot array is fully compatible with existing semiconductor technologies, this qubit architecture represents one possible framework for future research and development of large-scale spin qubit systems.

quant-ph↗

arXiv2023

Argon milling induced decoherence mechanisms in superconducting quantum circuits

The fabrication of superconducting circuits requires multiple deposition, etch and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence limiting step, using niobium and aluminum superconducting resonators as a proxy for surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality-factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide hosting larger two-level system defects. However, a short dry etch can fully recover the argon milling induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.

quant-ph↗

arXiv2022

Manufacturing high-Q superconducting {\alpha}-tantalum resonators on silicon wafers

The performance of state-of-the-art superconducting quantum devices is currently limited by microwave dielectric losses at different surfaces and interfaces. {\alpha}-tantalum is a superconductor that has proven effective in reducing dielectric loss and improving device performance due to its thin low-loss oxide. However, without the use of a seed layer, this tantalum phase has so far only been realised on sapphire substrates, which is incompatible with advanced processing in industry-scale fabrication facilities. Here, we demonstrate the fabrication of high-quality factor {\alpha}-tantalum resonators directly on silicon wafers over a variety of metal deposition conditions and perform a comprehensive material and electrical characterization study. By comparing experiments with simulated resonator loss, we demonstrate that two-level-system loss is dominated by surface oxide contributions and not the substrate-metal interface. Our study paves the way to large scale manufacturing of low-loss superconducting circuits and to materials-driven advancements in superconducting circuit performance.

quant-ph↗

arXiv2022

Path toward manufacturable superconducting qubits with relaxation times exceeding 0.1 ms

As the superconducting qubit platform matures towards ever-larger scales in the race towards a practical quantum computer, limitations due to qubit inhomogeneity through lack of process control become apparent. To benefit from the advanced process control in industry-scale CMOS fabrication facilities, different processing methods will be required. In particular, the double-angle evaporation and lift-off techniques used for current, state-of-the art superconducting qubits are generally incompatible with modern day manufacturable processes. Here, we demonstrate a fully CMOS compatible qubit fabrication method, and show results from overlap Josephson junction devices with long coherence and relaxation times, on par with the state-of-the-art. We experimentally verify that Argon milling - the critical step during junction fabrication - and a subtractive etch process nevertheless result in qubits with average qubit energy relaxation times T1 reaching 70 $\mu$s, with maximum values exceeding 100 $\mu$s. Furthermore, we show that our results are still limited by surface losses and not, crucially, by junction losses. The presented fabrication process therefore heralds an important milestone towards a manufacturable 300 mm CMOS process for high-coherence superconducting qubits and has the potential to advance the scaling of superconducting device architectures.

quant-ph↗

arXiv2020

Investigation of microwave loss induced by oxide regrowth in high-Q Nb resonators

The coherence of state-of-the-art superconducting qubit devices is predominantly limited by two-level-system defects, found primarily at amorphous interface layers. Reducing microwave loss from these interfaces by proper surface treatments is key to push the device performance forward. Here, we study niobium resonators after removing the native oxides with a hydrofluoric acid etch. We investigate the reappearance of microwave losses introduced by surface oxides that grow after exposure to the ambient environment. We find that losses in quantum devices are reduced by an order of magnitude, with internal Q-factors reaching up to 7 $\cdot$ 10$^6$ in the single photon regime, when devices are exposed to ambient conditions for 16 min. Furthermore, we observe that Nb2O5 is the only surface oxide that grows significantly within the first 200 hours, following the extended Cabrera-Mott growth model. In this time, microwave losses scale linearly with the Nb$_2$O$_5$ thickness, with an extracted loss tangent tan$\delta$ = 9.9 $\cdot$ 10$^{-3}$. Our findings are of particular interest for devices spanning from superconducting qubits, quantum-limited amplifiers, microwave kinetic inductance detectors to single photon detectors.

physics.app-ph↗

arXiv2020

Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.

cond-mat.mtrl-sci↗