Watching Neon Grow: Real-Time Control for Better Qubits

Author: Denis Avetisyan

Researchers have developed a method for monitoring neon film growth in real-time, paving the way for more reliable electron-on-neon qubits.

The study demonstrates how varying the initial gas quantity during film deposition-specifically, 0.008 mol versus 0.003 mol-dictates the resulting material’s phase trajectory, transitioning either through a liquid intermediate phase or directly to solid deposition at a cooling rate of 0.07 K/min, as evidenced by shifts in resonator frequency and inferred film thickness measurements.

Real-time microwave resonator measurements reveal stochastic film thicknesses and demonstrate microwave driving power as a key control parameter for consistent, thin-film growth.

Controlling solid neon film growth-critical for realizing electron-on-neon qubits-remains a significant challenge due to inherent stochasticity. This work, ‘Real-time Monitoring of Neon Film Growth for Electron-on-Neon Qubits’, details a novel technique employing high-temperature superconducting resonators to monitor neon film deposition in real time. We demonstrate that film thickness varies widely but can be consistently reduced to below 100 nm by controlling microwave driving power-a crucial step towards deterministic qubit fabrication. Could this real-time monitoring and control method unlock new pathways for designing and optimizing hybrid quantum systems beyond electron-on-neon qubits?

Trapped Electrons: Building Quantum Stability in Solid Neon

The pursuit of stable and scalable quantum computers hinges on the development of robust qubits – the quantum equivalent of classical bits. While various physical systems are being explored, trapping electrons within solid materials presents a particularly compelling path. Unlike qubits based on fleeting photons or delicate superconducting circuits, trapped electrons offer the potential for long coherence times – the duration for which quantum information can be reliably stored – due to their relative isolation from disruptive environmental noise. This approach leverages the electron’s intrinsic spin as the qubit, and by carefully engineering the solid host, researchers aim to control and manipulate these spins with high precision. The inherent stability and potential for miniaturization offered by solid-state qubits represent a significant step towards realizing practical and powerful quantum information processing.

Solid neon emerges as a particularly compelling platform for hosting qubits due to a unique combination of properties. Its chemical inertness minimizes disruptive interactions with trapped electrons, preserving the delicate quantum states essential for information processing. Unlike many other solid hosts, neon’s atomic structure allows for the creation of highly localized trapping sites, potentially leading to exceptionally high qubit densities-a critical factor in scaling up quantum computers. This increased density, coupled with neon’s simplicity, promises a pathway towards building compact and powerful quantum information processors, offering a significant advantage over materials plagued by complex interactions and lower qubit packing efficiencies. The material’s straightforward structure also facilitates precise control and characterization of the electron’s quantum behavior, paving the way for reliable qubit manipulation and readout.

The realization of stable and manipulable qubits using trapped electrons in solid neon hinges on exquisitely controlling the host material’s environment. Minute imperfections – even a single impurity atom – can disrupt the delicate balance needed to confine an electron and define its quantum state. Researchers are discovering that maintaining ultralow temperatures, often just fractions of a degree above absolute zero, is crucial to minimize thermal vibrations that would otherwise scatter the trapped electron and destroy quantum information. Furthermore, precise control over the neon’s crystalline structure – ensuring a highly ordered lattice – is paramount, as structural defects introduce localized energy levels that can act as unwanted trapping sites or sources of decoherence. Effectively, the neon matrix must be engineered to be exceptionally pure and structurally perfect to serve as a reliable quantum host, demanding advanced material science techniques and a deep understanding of solid-state physics.

A YBCO resonator monitors neon deposition by tracking shifts in resonant frequency, allowing for real-time thickness measurements validated by finite element modeling and conformal growth assumptions.

Precise Film Creation: A Foundation for Quantum Control

Solid neon films are produced via two primary techniques: quench condensation and liquid phase growth. Quench condensation involves rapidly cooling neon gas onto a cryogenic substrate, facilitating direct condensation from the gaseous phase. Liquid phase growth utilizes a reservoir of liquid neon, allowing for a controlled deposition process as the liquid phase transitions to solid. Both methods are employed to create films exhibiting high crystalline quality, crucial for subsequent characterization of dielectric properties and minimizing scattering effects in experimental results. Precise control over deposition parameters, such as gas pressure and substrate temperature, is maintained to optimize film uniformity and density.

The experimental setup utilizes a hermetically-sealed sample cell to maintain a controlled gaseous environment during film deposition and measurement. This cell is thermally coupled to a variable-temperature cryostat, allowing for precise temperature control from approximately 10 K to 300 K. A mass flow controller regulates the introduction of neon gas into the cell, enabling control over gas pressure and deposition rate. This combination of features minimizes contamination and ensures reproducible film growth conditions, critical for accurate characterization of the resulting solid neon films.

Film thickness and dielectric properties were monitored in situ using a YBCO (Yttrium Barium Copper Oxide) resonator. This resonator achieved a quality factor ($Q$) of 6200 at a temperature of 5 K, indicating a narrow bandwidth and high sensitivity. The resonant frequency of the YBCO resonator, directly correlated to the film’s dielectric characteristics, was measured at 2.230 GHz during the film creation process. This real-time monitoring allowed for precise control and characterization of the solid neon films as they were being deposited.

Solidification experiments were conducted with a controlled cooling rate of 0.07 K/min. This rate was chosen to facilitate the growth of uniform, solid neon films by minimizing thermal gradients and promoting consistent crystal formation. Maintaining a slow and steady cooling process is critical for preventing defects and ensuring the resulting film possesses the desired structural characteristics for subsequent analysis of its dielectric properties. The chosen cooling rate represents a balance between achieving a reasonable experimental duration and maximizing film quality.

Repeated gas-to-solid deposition trajectories of neon films reveal significant variations in final film thickness-ranging from 10 nm to tens of micrometers-with a weak correlation to initial liquid film thickness (r=0.6), despite consistent liquid condensation onset at 25.5 K.

Refining Film Properties: From Roughness to Morphology

Substrate roughness plays a critical role in the behavior of solid neon films by introducing localized potential fluctuations that affect electron dynamics. Increased roughness leads to a greater density of trapping sites for electrons within the film, reducing electron coherence times. These surface imperfections create spatially varying electric fields that disrupt the free propagation of electrons, promoting localization and decreasing the mean free path. The extent of this influence is directly related to the scale and distribution of the roughness features; larger and more frequent irregularities result in more pronounced electron trapping and a greater reduction in coherence, impacting the film’s overall electronic properties and suitability for applications requiring extended electron coherence.

Annealing techniques are employed post-deposition to refine quench-condensed solid neon films and reduce the impact of inherent imperfections. These imperfections, stemming from the rapid condensation process, include amorphous regions and structural defects that negatively affect film properties. Annealing involves heating the film to a controlled temperature below the melting point, allowing increased atomic mobility and facilitating the rearrangement of atoms into more stable, crystalline configurations. This process reduces defect density and improves the overall film quality, leading to enhanced electron trapping characteristics and coherence times. The specific annealing parameters, including temperature and duration, are optimized to balance defect reduction with the prevention of film roughening or desorption.

Conformal growth and uniform film thickness are achieved through precise control of deposition parameters, including substrate temperature, gas flow rates, and deposition rate. In-situ monitoring techniques, such as quartz crystal microbalance (QCM) and reflectometry, provide real-time feedback on film thickness and growth rate, allowing for dynamic adjustments to maintain desired film properties. This level of control minimizes variations in film thickness across the substrate and ensures complete coverage of complex surface topologies, critical for applications requiring consistent performance and reliability. The combination of parameter optimization and real-time monitoring enables the creation of films with highly reproducible characteristics.

Data obtained from the analysis of 364 film solidification events was used to establish a correlation between initial liquid film thickness and the resulting solid film thickness. Statistical analysis yielded a Pearson’s correlation coefficient of $r = 0.6$, indicating a moderate positive correlation. This suggests that while the liquid film thickness is not a perfect predictor of the solid film thickness, there is a discernible relationship between the two states, and approximately 36% of the variance in solid film thickness can be explained by the liquid film thickness. The dataset used for this analysis comprised measurements taken during a series of quench-condensation experiments.

Film thickness control below 100 nm was achieved through the application of high microwave driving power during deposition. This technique allows for precise regulation of the condensation process, resulting in films with targeted thicknesses within this sub-100 nm range. The methodology demonstrates repeatable fabrication of ultrathin solid neon films, crucial for applications requiring nanoscale precision in material properties. Further experimentation indicates that microwave power is a key variable in managing film morphology and uniformity at these dimensions.

Resonance frequency shifts reveal that microwave driving power controls film dynamics, inducing rapid changes in solid and bare resonators but a slower relaxation in liquid films accompanied by temperature stabilization, ultimately affecting final film thickness as demonstrated by thickness histograms.

Unveiling the Underlying Physics: Phase Diagrams and Thermal Control

The stability of solid neon films is fundamentally governed by the interplay between temperature and pressure, as precisely mapped by the neon phase diagram. This diagram reveals that solid neon emerges as the stable phase only below a critical temperature of 24.59 K and above a vapor pressure of 0.27 atm. Maintaining these conditions is crucial for film deposition; deviations lead to either gaseous evaporation or a transition to a liquid phase. Researchers carefully control the substrate temperature and neon gas pressure during film growth to remain within the solid stability region, ensuring the formation of a robust and consistent film. Furthermore, understanding this phase behavior allows for precise tuning of film density and morphology, critical factors impacting its suitability for quantum computing applications and other advanced technologies requiring tailored material properties.

The behavior of solid neon films is fundamentally governed by established thermodynamic principles, notably latent heat and the ideal gas law. As neon transitions between solid, liquid, and gaseous phases, energy is either absorbed or released at a constant temperature – this is latent heat, crucial for understanding film growth and stability. The ideal gas law, $PV = nRT$, provides a foundational relationship between pressure ($P$), volume ($V$), number of moles ($n$), gas constant ($R$), and temperature ($T$), allowing for the prediction of neon’s density and behavior under varying conditions. These principles aren’t merely theoretical; they dictate the pressure-temperature parameters necessary for stable film formation and explain phenomena like the film’s response to microwave heating, ultimately influencing its suitability as a substrate for superconducting qubits.

The application of microwave power to the neon film introduces a critical thermal component that must be carefully managed. This energy isn’t distributed uniformly; instead, it creates localized heating within the film due to the film’s inherent microwave absorption properties. This non-uniform temperature distribution can significantly affect the performance of any quantum information stored within the film, specifically impacting qubit coherence times – the duration for which quantum information remains stable. Furthermore, the increased energy within the film can induce Kerr nonlinearity, a phenomenon where the refractive index of the material changes with the intensity of the microwave field. This nonlinearity can shift the resonant frequency of the superconducting coplanar waveguide resonator used to probe the film, complicating data analysis and potentially introducing errors if not properly accounted for. Therefore, a precise understanding and mitigation of these thermal and nonlinear effects are crucial for reliable qubit operation and accurate characterization of the neon film’s properties.

Measurements revealed a characteristic frequency relaxation time constant ($\tau$) of 120 ± 3 seconds for liquid neon under observation. This finding indicates the timescale over which the neon film responds to changes in microwave frequency, providing insight into its dynamic properties and energy dissipation mechanisms. The relatively long relaxation time suggests a substantial thermal inertia within the liquid neon, potentially linked to its low thermal conductivity and the limited rate of energy transfer within the confined film. Understanding this temporal behavior is crucial for optimizing experimental parameters and minimizing decoherence effects in quantum devices utilizing neon films, as any fluctuations occurring on timescales comparable to $\tau$ can significantly impact qubit performance.

To characterize the neon film’s dielectric properties and loss tangent, researchers employed a sophisticated lumped-element circuit model integrated with a coplanar waveguide (CPW) resonator. This approach treats the resonator as an electrical circuit composed of inductors and capacitors, allowing for precise analysis of its resonant frequency and quality factor. By fitting the model’s predictions to experimental measurements of the resonator’s scattering parameters-specifically, the $S_{21}$ and $S_{11}$ parameters-critical film properties could be extracted. The model effectively decouples the contributions of the neon film from the underlying substrate, providing an accurate assessment of the film’s behavior at microwave frequencies and enabling optimization of its performance in superconducting qubit applications. This method proves particularly useful in determining the film’s effective permittivity and loss, parameters essential for predicting qubit coherence and designing robust quantum circuits.

The experimental setup utilizes a 3 K cryostat to precisely control neon deposition and monitor cooling trajectories, as demonstrated by phase transitions observed in the temperature rate of change and illustrated on the neon phase diagram.

The pursuit of controlled film growth, as demonstrated in this research concerning neon films for qubits, echoes a fundamental principle of responsible innovation. This work highlights the delicate interplay between control parameters-like microwave driving power-and stochastic outcomes, revealing the necessity of real-time monitoring to achieve desired film thicknesses. As Louis de Broglie once stated, “It is in the interplay between waves and particles that the true nature of reality is revealed.” This resonance finds a parallel in the study; understanding the wave-like behavior of microwave energy is crucial for controlling the particle-like deposition of neon atoms, ultimately shaping the foundation for robust quantum computation. The research underscores that progress in quantum technologies, like any advancement, demands not only technical prowess but also a commitment to understanding and mitigating inherent uncertainties.

Where Do We Go From Here?

The ability to monitor neon film growth in real-time, while a technical achievement, merely illuminates the inherent stochasticity of what is attempted. The precision gained through microwave resonator feedback does not eliminate randomness; it offers a means of navigating it, of selecting for outcomes that approximate a desired state. This is a crucial distinction. The research subtly underscores that control is not dominion, but rather informed selection within a probabilistic landscape. The focus now shifts from simply growing thinner films to understanding the fundamental limits of uniformity and the implications of that inherent variation for qubit coherence.

Furthermore, the identified role of microwave driving power as a control parameter demands deeper investigation. Is this merely a technique for achieving thinner films, or does it fundamentally alter the material properties of the neon itself? The line between fabrication technique and materials science blurs, and a more holistic understanding of the neon film’s quantum behavior is required. Data itself is neutral, but models reflect human bias, and a reliance on resonator signal alone risks obscuring more subtle, but equally important, phenomena.

Ultimately, this work highlights a recurring theme in quantum engineering: the relentless pursuit of control over inherently unpredictable systems. Tools without values are weapons, and the value here must be not simply faster or smaller qubits, but reliable ones. The challenge now lies in translating this real-time monitoring capability into a predictive framework, anticipating and mitigating sources of decoherence before they manifest, and accepting that perfect uniformity is a theoretical ideal, not a practical goal.

Original article: https://arxiv.org/pdf/2511.20765.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

Trapped Electrons: Building Quantum Stability in Solid Neon

Precise Film Creation: A Foundation for Quantum Control

Refining Film Properties: From Roughness to Morphology

Unveiling the Underlying Physics: Phase Diagrams and Thermal Control

Where Do We Go From Here?

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