arXiv subjects

Mark Friesen

Publications and source records attributed to Mark Friesen.

At least 19 recordsLinked to original sources

arXiv2026

Depth-Resolved Lattice Distortions in a Silicon-Germanium Qubit Host

Semiconductor qubits, promising for quantum computation, inherit properties from their host lattice. Quantum dot spins, occupying the local lowest energy states in the conduction band, necessarily couple to structural disorder and interfaces. While silicon-based systems promise low noise alongside industrially compatible manufacturing, the growth of SiGe---a leading platform---unavoidably introduces lattice dislocations, inhomogeneous strain, and crosshatch patterns, expected to cause fluctuations between devices, qubit failure, and subsequently higher operational overhead. Through X-ray nano-structural mapping of an Intel Si/SiGe chip, we reveal, with 30$~$nm lateral and 200$~$nm functional depth resolution, how extended lattice defects introduced during growth propagate through the heterostructure, creating permanently distorted lattice planes and strain. We correlate these at the $\approx1~\mu$m scale of a quantum dot device and calculate the impact on qubit energy spectra. We observe crosshatch fine structure and find that substrate miscut and growth correlate with the final crosshatch pattern.

cond-mat.mes-hall↗

arXiv2026

Confinement drives valley splitting above 4K in buried silicon quantum wells

Controlling the energy scales of a quantum system is essential for defining robust qubits. In silicon spin qubits, the nearly degenerate conduction-band valleys create a leakage channel from the single-spin computational basis, posing a challenge to scaling and to shuttling-based architectures. Here, we measure the relevant energy scales of single-electron spin qubits in buried silicon quantum wells co-designed for low disorder and high valley splitting. Across a linear array of four quantum dots with an average orbital energy of 2.4(2) meV, we report an average single-electron valley splitting of 0.40(6) meV and an average two-electron singlet-triplet splitting of 0.24(7) meV. In three dots, we observe a strong correlation between valley splitting and orbital energy, with an average linear coefficient of $\approx 0.22$ (meV/meV), demonstrating that electrostatic confinement can increase the valley splitting by several hundred microelectronvolts. In contrast, the remaining dot exhibits the highest valley splitting of 0.76(2) meV and low correlation, suggesting excellent characteristics for spin-qubit operation. Our findings demonstrate that strong confinement can be exploited in buried quantum wells to effectively enhance the valley splitting, thereby establishing a viable path toward the realization of shuttling and sparse-occupation-based architectures in low-disorder heterostructures.

cond-mat.mes-hall↗

arXiv2026

Complete measurement of tunnel- and valley-coupling parameters in a silicon double quantum dot

Tunneling is essential in the initialization, measurement, and control of quantum dot qubits. In silicon, such tunneling connects not only the qubit states but also valley minima in the conduction band on opposite sides of the Brillouin zone, with large consequences for the quantum dot behavior. Here we present a full characterization of the intravalley and intervalley tunnel couplings, including their complex phases -- the valley phases. These phases are shown to control measurable parameters, including the ratios of the gaps at anticrossings between quantum states of a double quantum dot. The valley phases themselves evolve as a function of the quantum dot gate voltages and depend on the underlying atomic structure of the quantum well. Knowledge of the valley phases completes the picture and fills a key gap in our understanding of sample-wide variations of valley couplings and the physical parameters that depend on them, including spin-orbit coupling, valley-orbit mixing, and Land\'e $g$-factors.

quant-ph↗

arXiv2026

Individually tunable Si/SiGe quantum dot operating voltages via gate-biased illumination

Semiconductor quantum dot qubits often require very different voltages on each gate to bring them to a correct operating point. Here, we present a method by which one can controllably and repeatably alter the nanoscale trapped charge distribution at an oxide-semiconductor interface. We demonstrate this method on a Si/SiGe quantum dot device, and we find that the operating voltages can be controlled and made much more uniform. The method relies on illumination with near-infrared light in the presence of applied gate voltages, and it enables the tuning of the device operating point on a gate-by-gate basis. We present an explanation of the underlying physics using self-consistent Schr\"odinger-Poisson simulations. As an application of this method, we tune a triple quantum dot to have uniform and small operating voltages in the (1,1,1) charge configuration. Importantly, we show that shifting the operating voltages in this way does not change the measured charge noise.

cond-mat.mes-hall↗

arXiv2026

High-fidelity EDSR in Si/SiGe Wiggle Wells

Si/SiGe quantum wells that incorporate Ge concentration oscillations, known as long-period Wiggle Wells, have been shown to enhance the Dresselhaus spin-orbit coupling of conduction-band electrons. Such intrinsic spin-orbit coupling is desirable when performing spin-qubit gate operations based on electric dipole spin resonance (EDSR) because it eliminates the need for external micromagnets. However, random-alloy disorder plays a key role in this materials system by spatially randomizing the valley splitting and the valley phase $\phi_{s,s}$, and it has not been fully accounted for in recent EDSR analyses. Here, we show that alloy disorder affects EDSR in two main ways. First, the Rabi frequency $\Omega$ acquires a dependence on the valley phase, given by $\cos\phi_{s,s}$, which causes spatial randomization of $\Omega$. Despite this variability, we show that fast EDSR can be achieved at most locations across a given sample. Second, a new Rabi driving mechanism emerges, enabled by valley dipoles and the hybridization of ground and excited valley states, which arise from alloy disorder and EDSR driving, respectively. This mechanism is dominant in regions of low valley splitting. Alloy disorder can therefore strengthen EDSR, but it can also cause gradients in $\Omega$ that lead to dephasing in the rotating frame. We explore this problem by first locating "sweet spots," where EDSR is relatively insensitive to electric-field fluctuations. We then show that high-fidelity Rabi oscillations can be achieved in the presence of realistic charge noise. These results suggest that Wiggle Wells are a promising platform for high-quality, micromagnet-free gate operations.

cond-mat.mes-hall↗

arXiv2026

Using a spin-triplet encoding to enhance shuttling fidelities in Si/SiGe quantum wells

Spatial variations of the valley splitting in a quantum well present a key challenge for conveyor-mode shuttling of electron spins in Si/SiGe, giving rise to Landau-Zener-like excitations that cause leakage outside the qubit subspace. Here, we propose an unconventional two-electron qubit encoding, based on valley-singlet states, that is largely immune to Landau-Zener leakage processes. In contrast to single-electron spins, the shuttling fidelity actually improves for small valley splittings, in this case. We show that high fidelities can be achieved without applying any special procedures, such as fine-tuning of the shuttling path.

cond-mat.mes-hall↗

arXiv2026

Large quantum dot energy level shifts in anomalous photon-assisted tunneling

Orbital energy splittings are important quantum dot parameters for the operation of hole spin qubits. They are known to depend on the lateral confinement of the quantum dots. However, when changing top, plunger gate voltages, which are the typical control parameter for qubit applications, such energy splitting changes are typically negligible, both as measured in experiment and as assumed in effective theories. Here, we study the singlet-triplet (ST) splittings, which depend on the orbital splittings, of a double quantum dot (DQD) in a Ge/SiGe heterostructure using photon-assisted tunneling (PAT) and pulsed-gate spectroscopy. We find that the ST splittings have a surprising, strong dependence on the top gate voltages, leading to anomalous PAT measurements. We combine data from both measurements in a model that well describes the linear gate-voltage dependence of the ST splittings. Finally, we show that the ST splittings of the two dots exhibit similar linear gate-voltage dependences when the device is retuned such that their ratio is significantly different.

cond-mat.mes-hall↗

arXiv2026

Direct measurement of the energy spectrum of a quantum dot qubit

The mapping between gate voltages applied to a double quantum dot, and the parameters of a Hubbard-like Hamiltonian, is of utmost importance for understanding and operating spin qubits. State-of-the-art techniques for measuring Hamiltonian parameters (e.g., detuning axis pulsed spectroscopy, DAPS) provide details about energy levels; however, tunnel coupling estimates typically reveal only a small portion of the full Hamiltonian. Here, we demonstrate a Hamiltonian-agnostic technique for measuring the double dot energy spectrum over a wide energy range, at every value of the detuning, called delta-axis spectroscopy (DAXS). We apply the DAXS method to obtain the energy spectrum of a Si/SiGe double quantum dot and use this data to extract the diagonal and off-diagonal couplings of a 15-level Hubbard-like Hamiltonian, demonstrating very good agreement with the experimental measurements.

quant-ph↗

arXiv2026

FAlCon: A unified framework for algorithmic control of quantum dot devices

As spin-based quantum systems scale, their setup and control complexity increase sharply. In semiconductor quantum dot (QD) experiments, device-to-device variability, heterogeneous control-electronics stacks, and differing operational modalities make it difficult to reuse characterization, calibration, and control logic across laboratories. We present FAlCon, an open-source software ecosystem for portable, automated characterization and tuning measurement workflows. FAlCon provides (i) a lightweight domain-specific language for expressing state-based tuning logic in a hardware-agnostic form; (ii) specialized transmittable libraries of physics-informed QD data structures (``tuning vernacula''); and (iii) extensible libraries of shared measurement protocols enabling an interoperable lab-agnostic measurement stack. By separating algorithm intent from instrument realization, while preserving traceability and supporting typed scripting, FAlCon enables researchers and engineers to exchange, adapt, and deploy characterization and autotuning routines across heterogeneous QD setups. The framework supports all users, ranging from end users running prebuilt algorithms with custom initial conditions to developers extending instrumentation support and contributing new tuning strategies. Although the present release targets QD experiments, other qubit modalities and scientific experiments could reuse FAlCon's modular abstractions by providing new tuning data types and instrument control templates.

quant-ph↗

arXiv2025

A scanning probe microscopy approach for identifying defects in aluminum oxide

The coherence of quantum dot qubits fabricated in semiconductors is often limited by charge noise from defects in gate dielectrics, which are material- and process-dependent. Characterizing these defects is an important step towards reducing their impact and improving qubit coherence. The identification of individual defects requires atomic-scale spatial resolution, however, and sufficient spectral sensitivity to determine their electronic structure. Electrostatic force microscopy (EFM) provides highly resolved maps of the surface potential of dielectrics, and importantly, is also sensitive to single-electron charging processes that reflect the spectral structure of underlying defects. In this work, we use cryogenic EFM to characterize aluminum oxide grown by atomic layer deposition (ALD) on bulk silicon. These measurements reveal defects close to the surface that exchange electrons with the EFM tip as they transition through different charge states. Detailed electrostatic modeling opens the door to powerful techniques for mapping tip-backgate charging voltages onto defect transition energies, allowing defects such as aluminum vacancies, and carbon, oxygen, or hydrogen impurities to be identified, by comparing to density functional theory (DFT). These results point towards EFM as a powerful tool for exploring defect structures in solid-state qubits.

cond-mat.mes-hall↗

arXiv2025

The effects of alloy disorder on strongly-driven flopping mode qubits in Si/SiGe

In Si quantum dot systems, large magnetic field gradients are needed to implement spin rotations via electric dipole spin resonance (EDSR). By increasing the effective electron dipole, flopping mode qubits can provide faster gates with smaller field gradients. Moreover, operating in the strong-driving limit can reduce their sensitivity to charge noise. However, alloy disorder in Si/SiGe heterostructures randomizes the valley energy splitting and the valley phase difference between dots, enhancing the probably of valley excitations while tunneling between the dots, and opening a leakage channel. In this work, we analyze the performance of flopping mode spin qubits in the presence of charge noise and alloy disorder, and we optimize these qubits for a variety of valley configurations, in both weak and strong charge-noise regimes. When the charge noise is weak, high fidelity qubits can be implemented across a wide range of valley parameters, provided the electronic pulse is fine-tuned for a given valley configuration. When the charge noise is strong, high-fidelity pulses can still be engineered, provided the valley splittings in each dot are relatively large and the valley phase difference is relatively small. We analyze how charge noise-induced fluctuations of the inter-dot detuning, as well as small shifts in other qubit parameters, impact qubit fidelities. We find that strongly driven pulses are less sensitive to detuning fluctuations but more sensitive to small shifts in the valley parameters, which can actually dominate the qubit infidelities in some regimes. Finally, we discuss schemes to tune devices away from poor-performing configurations, enhancing the scalability of flopping-mode-based qubit architectures.

cond-mat.mes-hall↗

arXiv2025

Fast high-fidelity baseband reset of a latched state for quantum dot qubit readout

A common method for reading out the state of a spin qubit is by latching one logical qubit state, either $|1\rangle$ or $|0\rangle$, onto a different, metastable charge state. Such a latched state can provide a superior charge sensing signal for qubit readout, and it can have a lifetime chosen to be long enough that the charge sensed readout can be high fidelity. However, the passive reset out of latched states is inherently long, which is not desirable. In this work, we demonstrate an on-demand, high fidelity (> 99%) re-initialization of a quantum dot qubit out of a latched readout state. The method is simple to apply as it involves a single baseband voltage pulse to a specific region in the quantum dot stability diagram where the relaxation time from the latched state to the ground state is over 50 times faster. We describe the mechanism for the reset process as well as the boundaries for the optimal reset region in the qubit gate voltage space.

cond-mat.mes-hall↗

arXiv2025

Bayesian and geometric analyses of power spectral densities of spin qubits in Si/SiGe quantum dot devices

Charge noise with a power-law spectrum poses a significant challenge to high-fidelity operation of spin qubits in semiconductor devices. Recently, considerable experimental work characterized this noise using qubits as spectrometers. It apparently arises from a collection of two-level fluctuating electric dipole systems (TLS). This suggests using the data to infer the positions, orientations, and other physical characteristics of the TLS. We identify a fundamental difficulty in this program: the inference of the TLS parameters is strongly undetermined, since the quantity of data is not sufficient to fix them uniquely. We describe two approaches to deal with this situation. The first approach is a qualitative method based on analytic calculations and simulations of small model systems that recognizes certain patterns. The second approach, more appropriate for detailed data analysis, is a Bayesian computation that assigns probabilities to candidate dipole configurations. We propose that the Brier score, a measure of confidence in the probabilities, can be used as a quantitative tool to judge the efficacy of experimental noise-measurement setups. Together, the analytical and computational Bayesian methods constrain, but do not fix, the density, the positions, the orientations, and the strengths of the dipole noise sources.

quant-ph↗

arXiv2025

Statistical characterization of valley coupling in Si/SiGe quantum dots via $g$-factor measurements near a valley vortex

The presence of low-energy valley excitations in Si/SiGe heterostructures often causes spin qubits to fail. It is therefore important to develop robust protocols for characterizing the valley coupling. Here, we show that realistically sized samplings of valley energy distributions tend to dramatically overestimate the average valley coupling. But we find that knowledge of the valley phase, in addition to the valley splitting, can significantly improve our estimates. Motivated by this understanding, we propose a novel method to probe the valley phase landscape across the quantum well using simple $g$-factor measurements. An important calibration step in this procedure is to measure $g$ in a loop enclosing a valley vortex, where the valley phase winds by $\pm 2\pi$ around a zero of the valley splitting. This proposal establishes an important new tool for probing spin qubits, and it can be implemented in current experiments.

cond-mat.mes-hall↗

arXiv2025

Engineering Ge profiles in Si/SiGe heterostructures for increased valley splitting

Electron spin qubits in Si/SiGe quantum wells are limited by the small and variable energy separation of the conduction band valleys. While sharp quantum well interfaces are pursued to increase the valley splitting energy deterministically, here we explore an alternative approach to enhance the valley splitting on average. We grow increasingly thinner quantum wells with broad interfaces to controllably increase the overlap of the electron wave function with Ge atoms. In these quantum wells, comprehensive quantum Hall measurements of two-dimensional electron gases reveal a linear correlation between valley splitting and disorder. Benchmarked against quantum wells with sharp interfaces, we demonstrate enhanced valley splitting while maintaining a low-disorder potential environment. Simulations using the experimental Ge concentration profiles predict an average valley splitting in quantum dots that matches the enhancement observed in two-dimensional systems. Our results motivate the experimental realization of quantum dot spin qubits in these heterostructures.

cond-mat.mes-hall↗

arXiv2025

Valley Splitting Correlations Across a Silicon Quantum Well Containing Germanium

Quantum dots in SiGe/Si/SiGe heterostructures host coherent electron spin qubits, which are promising for future quantum computers. The silicon quantum well hosts near-degenerate electron valley states, creating a low-lying excited state that is known to reduce spin qubit readout and control fidelity. The valley energy splitting is dominated by the microscopic disorder in the SiGe alloy and at the Si/SiGe interfaces, and while Si devices are compatible with large-scale semiconductor manufacturing, achieving a uniformly large valley splitting energy across a many-qubit device spanning mesoscopic distances is an outstanding challenge. In this work we study valley splitting variations in a 1D quantum dot array, formed in a Si$_{0.972}$Ge$_{0.028}$ quantum well, manufactured by Intel. We observe correlations in valley splitting, at both sub-100nm (single gate) and >1$\mu$m (device) lengthscales, that are consistent with alloy disorder-dominated theory and simulation. Our results develop the mesoscopic understanding of Si/SiGe heterostructures necessary for scalable device design.

cond-mat.mes-hall↗

arXiv2024

g-factor theory of Si/SiGe quantum dots: spin-valley and giant renormalization effects

Understanding the $g$-factor physics of Si/SiGe quantum dots is crucial for realizing high-quality spin qubits. While previous work has explained some aspects of $g$-factor physics in idealized geometries, the results do not extend to general cases and they miss several important features. Here, we construct a theory that gives $g$ in terms of readily computable matrix elements, and can be applied to all Si/SiGe heterostructures of current interest. As a concrete example, which currently has no $g$-factor understanding, we study the so-called Wiggle Well structure, containing Ge concentration oscillations inside the quantum well. Here we find a significant renormalization of the $g$-factor compared to conventional Si/SiGe quantum wells. We also uncover a giant $g$-factor suppression of order $\mathcal{O}(1)$, which arises due to spin-valley coupling, and occurs at locations of low valley splitting. Our work therefore opens up new avenues for $g$-factor engineering in Si/SiGe quantum dots.

cond-mat.mes-hall↗

arXiv2024

Omnidirectional shuttling to avoid valley excitations in Si/SiGe quantum wells

Conveyor-mode shuttling is a key approach for implementing intermediate-range coupling between electron-spin qubits in quantum dots. Initial implementations are encouraging; however, long shuttling trajectories are guaranteed to encounter regions of low conduction-band valley energy splittings, due to the presence of random-alloy disorder in Si/SiGe quantum wells. Here, we theoretically explore two schemes for avoiding valley-state excitations at these valley-splitting minima, by allowing the electrons to detour around them. A multichannel shuttling scheme allows electrons to tunnel between parallel channels, while a two-dimensional (2D) shuttler provides full omnidirectional control. Using simulations, we estimate shuttling fidelities in these two schemes, obtaining a clear preference for the 2D shuttler. Based on such encouraging results, we propose a modular qubit architecture based on 2D shuttling, which enables all-to-all connectivity within qubit plaquettes and high-fidelity communication between different plaquettes.

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