Phonon number sensitive electromechanics

TL;DR

This paper demonstrates a method to detect, manipulate, and cool a micro-mechanical oscillator using a superconducting qubit's non-linear properties, enabling quantum non-demolition measurements and non-classical state preparation.

Contribution

It introduces a technique to directly measure and control phonon number states in a mechanical oscillator via a superconducting qubit's non-linearity, advancing quantum electromechanics.

Findings

Qubit frequency shifts by 0.52 MHz per phonon

Achieved phonon number distribution measurement

Increased ground state population to 0.48

Abstract

We use the strong intrinsic non-linearity of a microwave superconducting qubit with a 4 GHz transition frequency to directly detect and control the energy of a micro-mechanical oscillator vibrating at 25 MHz. The qubit and the oscillator are coupled electrostatically at a rate of approximately $2\pi\times$22 MHz. In this far off-resonant regime, the qubit frequency is shifted by 0.52 MHz per oscillator phonon, or about 14 % of the 3.7 MHz qubit linewidth. The qubit behaves as a vibrational energy detector and from its lineshape we extract the phonon number distribution of the oscillator. We manipulate this distribution by driving number state sensitive sideband transitions and creating profoundly non-thermal states. Finally, by driving the lower frequency sideband transition, we cool the oscillator and increase its ground state population up to 0.48$\pm$0.13, close to a factor of 8 above its value at thermal equilibrium. These results demonstrate a new class of electromechanics experiments that are a promising strategy for quantum non-demolition measurements and non-classical state preparation.