Above the mirror-like surface of superfluid helium cooled to 7 millikelvin, a single electron dances in perfect waltz with a microwave photon. For the first time, this dance has been captured in the laboratory of EeroQ Corporation: a particle hovering 10 nanometers above the liquid surface exchanges energy with the field of a superconducting resonator so quickly that their coupling becomes strong.
The electron is held above the helium thanks to its image charge: it is repelled from the surface but cannot penetrate the liquid due to a barrier of about 1 eV. The depth of the potential well for the electron at the dot is roughly 1000 GHz, which is hundreds of times larger than the energy of its motion. To make the particle and the photon dance in unison, the engineers built a quantum dot of size 1.4×1.4 μm² and a titanium nitride resonator with high kinetic impedance (Z = 3.8 kΩ). Such a resonator enhances the coupling to the electron motion by a factor of 20 compared to previous designs.
Previously, electrons on helium remained just a beautiful idea: their spins promised long-lived quantum memory, but no one could couple the particle strongly enough to a microwave field for readout and control. Now this barrier has been overcome. Voltage control on the electrodes allowed tuning the electron motion frequency from 5.5 to 10 GHz and reducing decoherence by applying a negative bias. The energy relaxation time T1 reached 0.76 μs and was limited by Purcell emission into the resonator at small detunings. The decoherence Γ2 increased with temperature following a power law $T^0$.5, indicating coupling to low-frequency excitations of the helium surface or fluctuating charges.
The next step is to introduce a magnetic field gradient to couple the electron's spin state to its motion and, through it, to the resonator. Then the dance will gain a third participant: spin, motion, and photon will whirl in a unified rhythm, opening the way to reading out spin qubits in microseconds and possibly to two-qubit operations. The architecture can be scaled to arrays of qubits on a common helium film. Thus, quantum computing with electrons on superfluid helium is no longer an exotic idea but a realistic platform.
The electron is held above the helium thanks to its image charge: it is repelled from the surface but cannot penetrate the liquid due to a barrier of about 1 eV. The depth of the potential well for the electron at the dot is roughly 1000 GHz, which is hundreds of times larger than the energy of its motion.