Direct measurement of the energy spectrum of a quantum dot qubit | J. Reily et al. | ResearchPod

Transcript: Direct measurement of the energy spectrum of a quantum dot qubit

Alex: Welcome to another episode of ResearchPod. Today we're diving into some careful work on quantum dots. Sam, what is this paper about?

Sam: This is a study from researchers at the University of Wisconsin-Madison, titled "Direct measurement of the energy spectrum of a quantum dot qubit," led by J. Reily and colleagues. They tackle a key challenge in building quantum computers using semiconductor quantum dots. These qubits are tiny electron traps acting as quantum bits. The problem is that they often fail quietly because unknown higher-energy states cause electrons to leak out during operations, due to unexpected mixing between states.

Alex: So the core problem is mapping out all those hidden energy levels across different tunings, to spot where those leaks happen?

Sam: Yes, exactly. Picture two adjacent traps for electrons, a double quantum dot. You tune them with gate voltages to control where the electrons sit. Detuning tilts the energy balance between the left dot and the right one—like tipping a seesaw. The average voltage is along what's called the delta axis. Older methods pulse one gate at a time and only catch low-energy states in V-shaped patterns on sensors. They miss many spots where states hybridize. The paper introduces delta-axis spectroscopy, or DAXS. It uses simple square-wave pulses along the average voltage direction while sweeping detuning. This reveals the entire energy spectrum at every point.

Alex: Right—like needing a full road map instead of just major highways, to avoid surprise detours.

Sam: DAXS lights up hybridized states across wide energies. That lets them fit a full 15-level model of energies and tunnel jumps between dots. Think of tunnel jumps like electrons slipping between rooms through a doorway—the strength of that doorway sets how much states mix and repel, causing anticrossings. Those are the spots where energy levels push apart and blend, like two cars swerving to avoid crashing when merging lanes.

Alex: Huh. So it shows the mixing that causes those silent failures.

Sam: That's the notable advance. It extracts base energies in each dot and tunnel couplings from the data. The fits match experiments well and distinguish true dot states from reservoir noise—loose electrons nearby that aren't part of the qubit.

Alex: How do they turn those DAXS patterns into numbers for the model? And how do they tell reservoir peaks apart from real dot states?

Sam: They use a two-step process. First, they draw guide curves by hand over each visible energy path in the map, then smooth the data with a local filter—like polishing a bumpy road without flattening the hills. For each vertical slice, they fit bell-shaped curves to lock in peak centers. Those centers trace each energy level's path across detuning. Then they tweak the model's predicted paths to match exactly—like sliding puzzle pieces until everything aligns. The model is a grid of numbers for base energies and jump strengths between states.

Direct measurement of the energy spectrum of a quantum dot qubit

J. Reily, Daniel J. King, Jonathan C. Marcks, M.A. Wolfe, Piotr Marciniec, E.S. Joseph, Tyler J. Kovach, Brighton X. Coe, Mark Friesen, Benjamin D. Woods, M.A. Eriksson

Mar 31, 2026·6:14·arxiv·Original Paper

Delta-axis spectroscopy (DAXS)Double quantum dot (DQD)Hubbard-like HamiltonianTunnel couplingEnergy spectrumDouble Quantum Dot (DQD)

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6 min

Briefing

Abstract

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.

Direct measurement of the energy spectrum of a quantum dot qubit: Research Paper Podcast

Direct measurement of the energy spectrum of a quantum dot qubit

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