Coupling an Ensemble of Electrons on Superfluid Helium to a Superconducting Circuit

Abstract

The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer. Circuit quantum electrodynamics allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom. Here, we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator. The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath. Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model. The coupling strength of the ensemble to the cavity is found to be ≈1MHz per electron, indicating the feasibility of achieving single electron strong coupling.

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Plain Language Summary

Under appropriate conditions, electrons can be trapped on the surface of superfluid helium and act like a two-dimensional electron gas. The electrons have a complicated relationship with the helium; they are attracted to the surface but cannot stand to be in the liquid. Quantum mechanics resolves this conflict by allowing the electrons to levitate above the surface of the liquid. Because the electrons are essentially in vacuum with only a weak coupling to the helium (itself a nearly perfect liquid), these electrons are exciting candidates for quantum bits and other basic quantum physics experiments. Here, electrons are trapped within a superconducting resonator, and their dynamics are observed at high speeds.

At a temperature of 25 thousandths of a degree above absolute zero, electrons are held in place above the superfluid helium using a bias voltage that creates a parabolic trapping potential above a superconducting resonator. The electrons alter the frequency of the resonator, which allows their presence to be detected. Using these techniques, the properties of the electrons (e.g., their vibrational modes) and the superfluid helium film can be measured with high precision in a nondestructive manner; the electrons can be held and studied for days at a time. These experiments represent an important step in utilizing trapped electrons as part of a quantum processor.

Since an electron’s spin can be used as a form of quantum memory, we expect that our results will motivate additional studies leading toward the development of a viable quantum computer.