# Observing and Verifying the Quantum Trajectory of a Mechanical Resonator

## TL;DR

This paper demonstrates real-time observation and verification of the quantum trajectory of a macroscopic mechanical resonator using continuous optical measurement, achieving high-purity states and enabling tests of quantum decoherence.

## Contribution

It introduces a method to track and verify the quantum state of a mechanical resonator in real-time, advancing quantum measurement and control techniques for macroscopic systems.

## Key findings

- Achieved 78% purity in motional quantum states
- Developed a retrodictive measurement protocol for state verification
- Observed state collapse and decoherence dynamics

## Abstract

Continuous weak measurement allows localizing open quantum systems in state space, and tracing out their quantum trajectory as they evolve in time. Efficient quantum measurement schemes have previously enabled recording quantum trajectories of microwave photon and qubit states. We apply these concepts to a macroscopic mechanical resonator, and follow the quantum trajectory of its motional state conditioned on a continuous optical measurement record. Starting with a thermal mixture, we eventually obtain coherent states of 78% purity--comparable to a displaced thermal state of occupation 0.14. We introduce a retrodictive measurement protocol to directly verify state purity along the trajectory, and furthermore observe state collapse and decoherence. This opens the door to measurement-based creation of advanced quantum states, and potential tests of gravitational decoherence models.