arXiv subjects

J. Reily

Publications and source records attributed to J. Reily.

5 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

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

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↗

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

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↗