Dispersive Microwave Sensing for Quantum Computing with Floating Electrons

TL;DR

The work addresses scalable qubit readout for floating-electron qubits on cryogenic substrates by implementing dispersive microwave sensing that leverages both charge and spin degrees of freedom. It demonstrates a high-Q LC readout for electrons on liquid helium enabling quantum-capacitance detection of Rydberg transitions via frequency-modulated RF reflectometry, achieving a capacitance sensitivity of $0.34~\text{aF}/\sqrt{\text{Hz}}$. In parallel, it shows NbTiN nanowire resonators remain high-Q after neon/electron deposition, with theory indicating strong spin–photon coupling and high gate fidelities for neon-based spin qubits. Finally, it introduces a millikelvin-integrated tunnel-diode oscillator as a compact cryogenic microwave source, achieving ~140 MHz operation at ~1 μW, suitable for scalable, low-noise qubit readout. Together these results establish a practical pathway toward large-scale FEB-based quantum processors, combining dispersive readout, spin–photon interfaces, and low-power cryogenic electronics for qubit control and measurement.

Abstract

In this dissertation, resonator-based readout techniques were developed for floating electrons as qubits on cryogenic substrates, using two platforms: electrons on liquid helium and electrons on solid neon. In addition, a cryogenic microwave source was developed to enable low-noise measurement for qubit readout.

Figures (53)

• Figure 1: Bloch sphere representation of a qubit. A pure state $\ket{\psi}$ is represented as a point on the Bloch sphere with coordinate $(r_x,r_y,r_z)$.
• Figure 2: Schematic representation of the image charge effect for an electron hovering above a cryogenic substrate with dielectric constant $\varepsilon_s$.
• Figure 3: Numerically solved eigenenergies (colored dotted lines), wavefunctions $\psi_n$ (colored solid lines), first-excited transition frequencies $f_{12}$, and average electron position $\langle z \rangle_n$ (values in the legend) for electrons confined above liquid helium and solid neon. The calculations correspond to the Rydberg ground state ($n=1$) and the first-excited Rydberg state ($n=2$). The solid, dashed, and dot-dashed gray curves represent the potential energy of an electron on helium and neon under perpendicular electric fields of $E_\perp = 0$ and $E_\perp = 15$ kV/m, respectively. For clarity, helium potentials are not shown below $-800$ GHz. The figure is taken from jennings2024quantum.
• Figure 4: Inverse mobility vs. temperature for FEs with electron density $n_e = 3.2 \times 10^8\ \mathrm{cm}^{-2}$ on the surface of liquid $^4\mathrm{He}$. The dashed curve represents the theoretical result of the single-electron approximation considering both ripplon and helium gas atom scattering. The dotted curve represents ripplon scattering only; the solid curve is the many-electron theory prediction, where the electron distribution function is shaped by electron-electron interactions; the open circles are the experimental data mehrotra1984density. The figure is taken from monarkha2013two.
• Figure 5: Pinned Wigner crystal on solid neon. $\mu(T)/\mu(4.2\,\mathrm{K})$ versus $T^{-1}$ for several datasets (arrows mark the crossover between high- and low-$T$ regimes). The decrease of conductivity at low $T$ is interpreted as pinning of the Wigner crystal by the random surface potential, so the pinned crystal contributes little to electronic conduction. The figure is taken from kajita1985wigner.