⚛️ quantum physics

Probing wetting and solidification dynamics of neon thin films with a high- $T_c$ superconducting circuit

This study utilizes a high- $T_c$ superconducting resonator to characterize the complex wetting, drainage, and oscillation dynamics of liquid neon films near the triple point, revealing that while hermetic packaging enables the investigation of non-equilibrium fluid behaviors and low microwave loss, it fails to provide the precise thickness control necessary for growing solid neon films for quantum applications.

Original authors: Brennan Dizdar, Yueheng Shi, David I. Schuster, Christopher S. Lang

Published 2026-10-20

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CC BY 4.0

Original authors: Brennan Dizdar, Yueheng Shi, David I. Schuster, Christopher S. Lang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build better quantum computers, scientists are constantly searching for materials that can hold delicate quantum information without losing it to the surrounding world. One promising avenue involves trapping electrons on the surface of frozen gases, specifically neon. Because solid neon is incredibly pure and lacks the magnetic noise found in many other materials, electrons hovering above it can maintain their quantum state for surprisingly long periods. However, to use these electrons as reliable bits of information, researchers need to grow the neon film with extreme precision. The challenge is that the conditions required to freeze neon are much warmer than the temperatures where standard quantum circuits usually operate, making it difficult to watch the film form in real time. To solve this, scientists have turned to a special type of superconducting material that remains active at these higher temperatures, allowing them to peer directly into the freezing process.

A team of researchers at the University of Chicago and Stanford University has now used this approach to watch how neon films behave as they form and freeze. They built a tiny, sensitive sensor made from a high-temperature superconductor called YBCO, which stays superconducting even when neon is a liquid. This sensor was placed inside a sealed container where they could introduce neon gas and cool it down. By measuring how the sensor's electrical vibrations changed, the team could track the thickness of the neon layer as it grew from a gas into a liquid and finally into a solid. This setup allowed them to observe the process in real time, something that was previously impossible with standard equipment that stops working at these temperatures.

As they filled the container with neon gas and cooled it to just above its freezing point, the researchers watched the liquid film spread across the sensor. Initially, the neon formed a very thin, uniform layer that settled at a thickness of about 20 nanometers. Once this thin layer was full, adding more neon caused the film to thicken significantly, but the behavior became unpredictable. In some cases, the liquid level would fluctuate wildly, rising and falling in rhythmic waves even without any new gas being added. This instability suggests that thick liquid films in this environment are extremely sensitive to tiny changes in heat and evaporation. The team also discovered that they could control the thickness of the liquid film by applying a small electric voltage to the sensor, which pulled the liquid closer and made the film thicker. This ability to tune the film in place is a powerful tool for studying how these materials behave.

The most critical part of the experiment involved watching what happened when the liquid neon froze into a solid. The researchers cooled the system through the freezing point and measured the thickness of the resulting solid film. They found that the solid film was almost always much thinner than the liquid it came from, often shrinking by a factor of ten or more. More importantly, they discovered that they could not predict the final thickness of the solid film based on how thick the liquid layer was before it froze. Whether they started with a thin liquid layer or a thick one, the resulting solid film varied widely in thickness, showing no strong connection to the starting conditions. This lack of control suggests that the freezing process is chaotic and non-uniform, which poses a significant hurdle for creating the precise, uniform films needed for high-performance quantum devices.

Despite the challenges in controlling the film's thickness, the researchers found a surprising benefit to having neon on the sensor. As the neon film grew, the quality of the sensor's electrical signal actually improved, becoming sharper and more stable. This improvement happened for both liquid and solid neon. The team investigated whether this was simply because the neon was blocking some other source of electrical loss, but their calculations showed that this was not the case. Instead, the presence of the neon film seems to interact with the superconductor in a way that reduces energy loss, possibly by affecting the behavior of tiny electrical charges within the material. This finding suggests that coating superconducting circuits with condensed noble gases could actually help them perform better, rather than just being a passive layer.

The study concludes that while hermetically sealed containers are excellent for observing the complex and often chaotic dynamics of neon films, they may not be the best method for growing solid neon with a precisely controlled thickness. The freezing process appears to be a non-equilibrium event where the final result is largely unpredictable. For the future of electron-based quantum computers, this means researchers may need to look for alternative ways to grow these films, perhaps by depositing the gas directly onto a cold surface rather than freezing a liquid. Such methods might offer the precise control needed to create the smooth, uniform surfaces required for the next generation of quantum technology.

Technical Summary: Probing Wetting and Solidification Dynamics of Neon Thin Films with a High- $T_c$ Superconducting Circuit

Problem Statement
The integration of condensed noble gases with superconducting circuits offers a pathway to hybrid quantum architectures, particularly utilizing electrons on solid neon as high-coherence charge qubits. While solid neon supports pristine two-dimensional electron gases immune to the decoherence channels (ripplons) that limit electrons on liquid helium, the precise nature of the electron wavefunction and the impact of interface conditions on coherence remain incompletely understood. A critical prerequisite for advancing these systems is the ability to grow and characterize solid neon films with controlled thickness. However, existing methods for characterizing neon film growth face significant hurdles: the triple point of neon ( $T_t \approx 24.56$ K) is too high for conventional low- $T_c$ superconducting resonators, which lose their superconducting state before the liquid phase can be studied. Consequently, the dynamics of liquid neon films and the non-equilibrium freezing processes that yield solid films are poorly controlled and difficult to monitor in situ.

Methodology
To address these challenges, the authors employ a multimode coplanar stripline resonator fabricated from a high- $T_c$ YBCO (Yttrium Barium Copper Oxide) film, which remains superconducting at temperatures well above the neon triple point ( $T_c \approx 88$ K). The device is housed in a hermetic sample cell mounted on a dilution refrigerator, allowing for controlled cooling through the triple point.

The experimental approach relies on the sensitivity of the resonator's dielectric environment to film thickness. As neon condenses onto the chip, it replaces vacuum with a dielectric (liquid $\epsilon_r \approx 1.19$ , solid $\epsilon_r \approx 1.24$ ), increasing the capacitance and lowering the resonance frequency. The authors track two modes: a fundamental $\lambda/4$ differential mode and a first overtone $3\lambda/4$ common mode. The geometry of the stripline sets a "thickness-sensing window" near $1 \, \mu\text{m}$ , where sensitivity ($df/dt$) is maximized. Additionally, the resonator is galvanically connected to a DC voltage source, enabling in-situ electrostatic tuning of the liquid film thickness via polarization forces. Finite-element simulations (Ansys Maxwell) are used to map frequency shifts to effective film thicknesses.

Key Contributions and Results

Characterization of Liquid Film Dynamics:
The study identifies four distinct qualitative regimes of liquid neon dynamics near the triple point:
- Regime I (Uniform Wetting): Gradual, monotonic frequency decrease corresponding to uniform wetting of the surface, saturating at a thickness of approximately 20 nm.
- Regime II (Drainage): Following the saturation of the thin film, subsequent neon doses create thick films that subsequently thin as the liquid redistributes across the cell.
- Regime III (Spontaneous Oscillations): The liquid level becomes unstable, exhibiting periodic thickness oscillations even without new neon insertions. This is attributed to the extreme sensitivity of thick films near saturation and enhanced evaporative responses.
- Regime IV (Saturation): The frequency shift saturates, indicating the resonator mode volumes are completely filled with neon.
  The authors also demonstrate that applying a DC voltage allows for reversible, hysteresis-free tuning of the liquid film thickness, though variance in trials suggests the liquid is not perfectly uniform across the surface.
Solidification Statistics and Thickness Control:
By cooling the system through the triple point from various initial liquid thicknesses, the authors investigate the correlation between liquid and solid film thickness. The results reveal a weak correlation: while thicker liquid films generally yield thicker solid films, the final solid thickness varies by approximately an order of magnitude for a given liquid thickness. Furthermore, the solid films are consistently one to two orders of magnitude thinner than their parent liquid films, indicating a non-equilibrium dewetting process during freezing. The cooling rate (tested at $\sim 1$ K/hr and $\sim 5$ K/hr) showed no significant influence on the distribution of final solid thicknesses.
Impact on Resonator Quality Factor:
Contrary to the expectation that adding a dielectric layer would introduce loss, the authors observe a smooth, two-fold increase in the quality factor ( $Q$ ) of the resonators, reaching values at the level of $10^3$ , as neon film thickness increases. A dielectric loss model attributing the change to the decreasing participation ratio of the lossy CeO $_2$ buffer layer was ruled out via simulation. The authors suggest this improvement may stem from a complex interplay between the neon film and quasiparticle loss mechanisms, noting that YBCO resonators at these temperatures are typically limited by conductor loss.

Significance and Claims
The paper demonstrates that hermetic packaging combined with high- $T_c$ superconducting circuits provides a powerful tool for investigating rich non-equilibrium fluid dynamics of condensed noble gases and their impact on microwave loss. The study highlights that while this method allows for real-time monitoring of film growth, it may not be the optimal modality for growing solid neon films with precisely controlled thickness, given the weak correlation between initial liquid levels and final solid thickness.

The authors conclude that their findings motivate further investigation into electron-on-neon qubits grown via quench-condensation (which offers better thickness control) and the use of cryogenic atomic force microscopy to correlate surface roughness with qubit properties. Ultimately, the ability to grow solid neon films with controllable thickness and low roughness is identified as a potential enabler for high-mobility two-dimensional electron gases for reconfigurable spin qubits, free from the decoherence channels associated with liquid helium.

Technical Summary: Probing Wetting and Solidification Dynamics of Neon Thin Films with a High-TcT_cTc​ Superconducting Circuit

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Technical Summary: Probing Wetting and Solidification Dynamics of Neon Thin Films with a High- $T_c$ Superconducting Circuit