Electrons floating on condensed noble-gas substrates form a remarkably clean material system that is minimally influenced by the underlying host material, and recent experiments have explored their potential as a qubit platform. In particular, single electrons floating on solid neon thin films have attracted significant interest because they exhibit long coherence times when probed with superconducting circuits at frequencies around 6 GHz. Surprisingly, however, the energy spectrum of these qubits resembles that of a two-level-system defect more than the spectrum expected from the Coulomb potential defined by the bias electrodes. In this thesis, I first review floating-electron systems and the experimental methods used to probe their quantum states. I then show that electron-on-neon qubits with transition frequencies of a few GHz are observed only on neon films deposited on silicon surfaces, and not on neon films deposited on sapphire. Motivated by this observation, I propose a model in which a floating electron on the neon surface interacts with fixed positive charge defects in the native oxide at the silicon surface. Using realistic parameters, this model reproduces the observed voltage-dependent transition frequency and coupling strength of electron-on-neon qubits.
Electrons floating on condensed noble-gas substrates form a remarkably clean material system that is minimally influenced by the underlying host material, and recent experiments have explored their potential as a qubit platform. In particular, single electrons floating on solid neon thin films have attracted significant interest because they exhibit long coherence times when probed with superconducting circuits at frequencies around 6 GHz. Surprisingly, however, the energy spectrum of these qubits resembles that of a two-level-system defect more than the spectrum expected from the Coulomb potential defined by the bias electrodes. In this thesis, I first review floating-electron systems and the experimental methods used to probe their quantum states. I then show that electron-on-neon qubits with transition frequencies of a few GHz are observed only on neon films deposited on silicon surfaces, and not on neon films deposited on sapphire. Motivated by this observation, I propose a model in which a floating electron on the neon surface interacts with fixed positive charge defects in the native oxide at the silicon surface. Using realistic parameters, this model reproduces the observed voltage-dependent transition frequency and coupling strength of electron-on-neon qubits.
Abstract Recent experiments show ∼0.1 ms coherence time for a single electron charge qubit on a solid neon surface. This remarkably long coherence time is believed to result from the intrinsic purity of solid neon as a qubit host. In this paper, we present theoretical studies on the decoherence mechanisms of an electron’s charge (lateral motional) states on solid neon. At the typical experimental temperature of ∼10 mK, the two main decoherence mechanisms are the phonon-induced displacement of neon surface and phonon-induced modulation of neon permittivity (dielectric constant). With a qubit frequency increasing from 1 GHz to 10 GHz, the charge coherence time decreases from about 366 s to 7 ms and from about 864 s to 24 ms, respectively, limited by the two mechanisms above. The calculated coherence times are at least one order longer than the observed ones at ∼6.4 GHz qubit frequency, suggesting plenty of room for experimental improvement.
Electrons floating in vacuum provide a clean platform for quantum information processing owing to their isolation from material defects. In particular, electrons on solid neon have emerged as a promising qubit platform because of their potentially long coherence times. Here, toward spin-qubit realization, we couple a single electron on solid neon to a magnetic-field-compatible superconducting NbTiN nanowire resonator. We realize a charge qubit and demonstrate microwave readout and coherent control, with Rabi frequencies up to 76 MHz, an order of magnitude larger than in previous studies. Under strong driving, we observe a qubit frequency shift from nonlinear interactions with the intense microwave field. Deterministic electron trapping at an intended position remains challenging due to solid neon surface roughness; we characterize the electron's position from its differential coupling to distinct electrodes. Although not trapped at an intended position, our estimates indicate that spin-qubit demonstrations remain feasible.
Electrons trapped above cryogenic substrates such as liquid helium and solid neon form exceptionally clean physical systems, since they reside in vacuum and are largely isolated from environmental noise. A recent breakthrough has been the realization of charge qubits using electrons on solid neon surfaces, with coherence times reaching T2 = 2T1 ≈ 100 µs. In this article, we describe in detail the preparation of cryogenic substrates, electron deposition and the measurement of electron mobility. We further discuss the differences in surface morphology between helium and neon, their impact on mobility and qubit performance, and emphasize the necessity of systematic surface characterization as a means to optimize cryogenic substrate preparation and electron deposition.
Electron-on-neon (eNe) qubits have recently emerged as a compelling platform for quantum computing, which combines the vacuum isolation advantages of trapped-ion qubits with the scalability of superconducting circuits. In this system, electrons are trapped in vacuum above a solid neon film deposited on superconducting microwave resonators, where they exhibit strong coupling to the resonators, coherence times of ~0.1 ms, and single-qubit gate fidelities exceeding 99.97%. A central challenge, however, is the spontaneous binding of electrons to neon surface bumps. These bumps, originating from substrate roughness, vary in size: electrons on bumps of suitable sizes within the resonator can couple to microwave photons and function as qubits, whereas those on unfavorable bumps remain inactive yet contribute to background charge noise. Moreover, both the bump landscape and the sites where electrons bind differ from run to run, leading to variable qubit characteristics that hinder scalability. To address this challenging issue, we present an on-chip magnetic-levitation architecture in which arrays of solid-neon microparticles are suspended above the processor chip to act as electron carriers. This design eliminates substrate effects while retaining strong qubit-resonator coupling and supporting inter-qubit connectivity. Our analysis further shows that the qubit transition frequency can be tuned across the gigahertz range and its anharmonicity can reach ~0.8 GHz by tuning the resonator bias voltage. Together, these features suggest a promising pathway toward robust, reproducible, and scalable eNe-based quantum computing.
In this review, we introduce a developing qubit platform: floating-electron-based qubits. Electrons floating in a vacuum above the surface of liquid helium or solid neon emerge as promising candidates for qubits, especially due to their expected long coherence times. Despite being in the early stages, a variety of recent experiments from different groups have shown substantial potential in this role. We survey a range of theoretical proposals and recent experiments, primarily focusing on the use of the spin state as the qubit state, wherein the spin and charge states are hybridized. Throughout these proposals and experiments, the charge state is coupled to an LC resonator, which facilitates both the control and readout mechanisms for the spin state via an artificially introduced spin–charge coupling.
Electrons floating on a solid neon exhibit long charge coherence times, making them attractive for hybrid quantum systems. When combined with high-quality, high-impedance superconducting resonators and a local magnetic field gradient, this platform enables strong charge--photon and spin--charge coupling-key ingredients for scalable spin qubit architectures. In this work, we demonstrate that NbTiN nanowire resonators maintain high quality factors around 10^5 after depositing solid neon onto the resonators and subsequently loading electrons onto the neon surface, validating their suitability for electrons-on-neon platforms. Building on these experimental results, we theoretically analyze micromagnet designs and coupling strategies that can enable spin-photon interactions in this platform. Our analysis outlines performance targets for next-generation devices, showing that, at the charge sweet spot, spin qubit gate fidelities exceeding 99.99% for single-qubit operations and 99.9% for two-qubit operations are achievable with natural neon.