Strong Coupling of Microwave Photon to Electron on Helium Achieves MHz Interaction Strength

Electrons trapped on the surface of superfluid helium represent a promising platform for building future quantum computers, yet directly observing and controlling individual electrons within this system has proved remarkably difficult. Now, a team led by G. Koolstra, E. O. Glen, and N. R. Beysengulov, alongside colleagues including H. Byeon, K. E. Castoria, and M. Sammon, reports achieving a crucial breakthrough. They demonstrate, for the first time, a strong interaction between microwave light and the motion of a single electron on helium, exceeding limitations imposed by energy loss and instability. This achievement unlocks new possibilities for exploring light-matter interactions at the single-electron level and represents a significant step towards harnessing electrons on helium for quantum information processing.

A circuit quantum electrodynamic (cQED) device, comprising a quantum dot and a high-impedance superconducting resonator, demonstrates strong coupling between the resonator’s microwave field and the motional quantum state of an electron. The team observed a coupling strength of 118MHz, exceeding both the electron’s rate of motion decay and the resonator’s energy loss. These experiments open new avenues for investigating light-matter interaction at the single electron level, and represent a key step towards the measurement and control of electrons on helium-based spin qubits. Electrons are trapped above the surface of superfluid helium in this innovative approach.

Electron-Photon Coupling in Helium-Based Quantum Circuit

Scientists engineered a hybrid circuit quantum electrodynamic (cQED) device to investigate the interaction between single electrons and microwave photons, a crucial step towards scalable quantum computing. The device integrates a quantum dot, formed on the surface of superfluid helium, with a high-impedance coplanar microwave resonator fabricated from titanium nitride. This design allows for strong coupling between the electron’s motion and the resonator’s microwave field. The resonator, operating at 7. 162GHz with a bandwidth of 23MHz, functions as a sensitive detector of electron presence and motion.

Researchers meticulously controlled the number of electrons within the quantum dot using precisely patterned electrodes to apply electrostatic voltages, transporting electrons into the dot from an on-chip reservoir and adjusting the trapping potential to load and unload electrons one at a time. The team achieved a compact quantum dot with dimensions of 1. 4×1. 4 μm², and implemented a high-kinetic inductance resonator with an impedance of 3. 8 kΩ, boosting the electron-photon interaction rate to 110MHz.

This precise control is facilitated by a smooth, defect-free helium surface and a deep electrostatic trap, preventing unwanted electron tunneling. To characterize the system, scientists loaded single electrons into the dot and measured the resulting frequency shift of the resonator as they tuned the electron’s motional frequency, observing substantial frequency shifts exceeding several resonator bandwidths when the electron motion matched the resonator frequency. Finite element method simulations confirmed these observations and validated the device’s design, demonstrating repeatable loading and unloading of electrons and showcasing their precision and the potential for manipulating individual electrons on helium.

Electron-Resonator Coupling and Rabi Splitting Observation

The research successfully demonstrates strong coupling between a single electron and a superconducting resonator, leading to the observation of vacuum Rabi splitting, a crucial step towards building more complex quantum circuits. Simulations modeled the electron’s confinement potential and frequency, refining the parameters to match the experimental results and demonstrating the accuracy of the modeling. The experiments addressed the challenges of stabilizing the system and minimizing noise, employing active vibration cancellation to reduce disturbances and investigating the impact of noise on the electron’s coherence. Precise control of the voltages applied to the quantum dot is crucial for tuning the electron’s frequency and achieving resonance with the resonator. The dephasing rate, a measure of how quickly the electron loses its quantum properties, was measured and analyzed to understand the factors limiting its coherence. The emergence of the vacuum Rabi splitting became more prominent at lower temperatures and more negative voltage offsets.

Strong Electron-Photon Coupling on Helium Surfaces

Scientists have, for the first time, demonstrated strong coupling between the motion of an electron confined to the surface of superfluid helium and microwave photons within a high-impedance resonator. This achievement represents a significant step towards utilizing electrons on helium for quantum computing, as it establishes a crucial link between the electron’s quantum state and electromagnetic fields. The observed coupling strength exceeds both the electron’s rate of motion decay and the resonator’s energy loss, paving the way for precise manipulation and measurement of these electrons. This research opens new avenues for investigating light-matter interactions at the single-electron level and brings the realization of helium-based spin qubits closer to reality.

Importantly, the demonstrated strong coupling is comparable to that achieved in semiconductor quantum dots, suggesting that established circuit quantum electrodynamics techniques can be adapted for controlling electron spins on helium. While the experiments revealed a power-law dependence of decoherence on temperature, potentially linked to helium surface ripples or fluctuating charges, the team successfully achieved the necessary conditions for robust quantum interactions. Future work focuses on refining resonator designs and extending these techniques to control and read out electron spin states, ultimately aiming to build scalable quantum devices based on this unique platform.