RIKEN Achieves 10× Faster Control of Neon-Based Qubits

Researchers at RIKEN have achieved Rabi frequencies up to 76 MHz, ten times larger than in previous studies, with a qubit built from a single electron residing on solid neon. This development expands the possibilities for this emerging quantum computing platform. The team successfully coupled this electron to a superconducting NbTiN nanowire resonator designed to be compatible with magnetic fields, a crucial step toward realizing spin-qubits. Demonstrating microwave readout and control, the researchers report achieving performance faster than previous methods utilizing electrons on solid neon. The team highlights the pristine environment that makes this approach promising for long-coherence quantum bits. This advance suggests that practical spin-qubit demonstrations using neon are within reach, although challenges remain in electron trapping; the electron was not trapped at the originally intended position due to solid neon surface roughness.

Electrons on Solid Neon as a Quantum Platform

A single electron held stationary on solid neon exhibits Rabi frequencies up to 76 MHz, ten times larger than in previous studies, a result that supports the viability of this unusual material as a platform for quantum computation. The team details their findings in a recent publication, outlining a pathway toward realizing spin-qubits, a promising type of quantum bit, using this novel architecture. The choice of solid neon as a base material is notable; typically a gas, its solidified form offers a uniquely pristine environment for electron manipulation, minimizing interference from material defects. The researchers emphasize the benefit of this isolation. A key component of their design is the use of a magnetic-field-compatible NbTiN nanowire resonator, a crucial step for enabling spin-qubit functionality. This resonator, they explain, “remains relatively robust in magnetic fields,” allowing for the manipulation of electron spin without signal degradation.

Despite challenges in achieving deterministic electron trapping due to solid neon surface roughness, the team was able to characterize the electron’s position by analyzing its differential coupling to nearby electrodes. Although the electron was not trapped at the originally intended location, the researchers estimate that spin-qubit fidelities may be achievable in this platform, suggesting a promising future for solid neon as a foundation for quantum technologies.

Charge Qubit Realization with NbTiN Nanowire Resonators

Researchers are increasingly focused on harnessing the unique properties of electrons suspended above solid surfaces for quantum computation, building on decades of work with electrons on liquid helium. Recent efforts have shifted toward solid neon as a platform, driven by the potential for extended coherence times; however, realizing practical qubits demands precise control and coupling mechanisms. This design incorporates a resonator specifically engineered to function reliably even in the presence of magnetic fields, a critical requirement for future spin-qubit implementations. The team details that the NbTiN nanowire resonator “remains relatively robust in magnetic fields,” enabling the exploration of spin-based qubits.

Microwave Readout and Coherent Control at 76 MHz

Jun Wang and colleagues at the RIKEN Center for Quantum Computing have demonstrated a significant leap in controlling single-electron qubits built on a solid neon platform, achieving Rabi frequencies ten times larger than previously reported methods. The design incorporates a magnetic-field-compatible NbTiN resonator, a deliberate choice to facilitate future spin-qubit implementations. Researchers found that this material “remains relatively robust in magnetic fields,” a key requirement for qubits that rely on manipulating electron spin. Measurements of T1, T2*, and T2 coherence times provide a detailed understanding of the qubit’s stability and potential for maintaining quantum information, paving the way for more complex quantum circuits built on this unusual, yet promising, platform.

Nonlinear Interactions and Qubit Frequency Shifts

The ability to manipulate individual electron spins holds immense promise for building powerful quantum computers, and recent advances are refining the platforms used to house these delicate qubits. This acceleration is not merely incremental; it opens new avenues for performing complex quantum calculations with greater speed and precision. Beyond speed, the team observed a notable shift in the qubit’s frequency when subjected to intense microwave fields. This “downward shift of the qubit transition frequency,” as they describe it, is attributed to an AC Stark shift, a consequence of the increased number of microwave photons interacting within the resonator. Understanding and controlling these nonlinear interactions is critical for designing robust qubits capable of handling complex operations. Although the electron was not trapped at the originally intended position, their estimates indicate that spin-qubit demonstrations remain feasible.

Electron Positioning Challenges on Neon Surfaces

Conventional wisdom suggests solid surfaces offer predictable electron placement, yet achieving deterministic trapping proves remarkably difficult when working with solid neon. Surface roughness at the microscopic level introduces uncontrolled electrostatic disorder, a significant hurdle in building reliable quantum devices. Researchers at RIKEN Center for Quantum Computing and collaborating institutions have directly confronted this challenge, characterizing electron positioning based on its differential coupling to nearby electrodes. Although not trapped at the originally intended location, the team successfully inferred the electron’s likely position through careful analysis of these coupling variations. This approach is detailed in their recent publication, which notes that “surface roughness can introduce uncontrolled electrostatic disorder, preventing deterministic positioning of electrons and making device behavior sensitive to the local trapping site.” Despite this imperfect placement, the researchers demonstrated coherent control of a single-electron charge qubit, achieving Rabi frequencies up to 76 MHz, ten times larger than in previous studies. The team’s theoretical modeling suggests that spin-qubit fidelities may be achievable.

Differential Coupling Analysis for Electron Location

A single electron’s location on solid neon has been pinpointed by analyzing its unique ‘coupling’ to nearby electrodes, a technique crucial for advancing quantum computing architectures. This innovative approach addresses a key challenge in solid-neon qubit platforms, where surface roughness can prevent deterministic electron positioning. Although not trapped at the originally intended position, their estimates indicate that spin-qubit demonstrations remain feasible. Their work establishes NbTiN nanowire resonators on solid neon as a viable platform for future spin qubit realization, even without perfectly positioned electrons.

Measured Coherence Times: T1, T2*, and T2

Beyond establishing coherent control, quantifying the longevity of quantum information within the neon-hosted qubit was paramount. Researchers meticulously measured key coherence parameters, T1, the spin-lattice relaxation time; T2*, the dephasing time influenced by various noise sources; and T2, the coherence time reflecting intrinsic quantum limitations. Initial measurements revealed a T2* of 50 μs for charge qubits, a figure already attracting considerable interest within the field. This value demonstrates a substantial improvement over earlier iterations of similar solid-state qubit designs, suggesting the solid neon environment effectively shields the electron from disruptive external influences. Further analysis focused on refining these coherence times. Following a film annealing process, the team observed a longer relaxation time, potentially linked to alterations in the local environment surrounding the electron, including variations in neon film thickness. The precise mechanisms driving this improvement remain under investigation, but it underscores the sensitivity of the system to subtle environmental factors.

Beyond simply realizing a charge qubit, the team focused on optimizing the neon substrate itself, specifically through a post-fabrication annealing process. Following the annealing of the neon film, researchers observed a longer relaxation time, suggesting alterations in the electron’s immediate environment, potentially including shifts in the film’s thickness. This improvement in relaxation time is particularly significant given the challenges inherent in deterministic electron trapping on solid neon.

Feasibility Estimates for Spin-Qubit Fidelities

The potential for scalable quantum computation hinges on achieving high-fidelity qubits, and recent work with electrons on solid neon offers a promising, if unconventional, pathway toward that goal. While deterministic electron placement remains a challenge, influenced by surface roughness that introduces electrostatic disorder, the team’s theoretical modeling suggests significant potential. This calculation incorporates the use of integrated ferromagnets, a crucial step in manipulating electron spin. Rabi frequencies are ten times larger than in previous studies, and their analysis considered the electron trapped at a location deduced from experimental results, rather than the originally intended position, yet still yielded estimates of spin-qubit fidelities may be achievable. These findings solidify the NbTiN nanowire resonator on solid neon as a compelling architecture for future quantum technologies, offering a unique combination of isolation and compatibility with magnetic field control.