An enduring challenge in constructing mechanical-oscillator-based hybrid quantum systems is to ensure engineered coupling to an auxiliary degree of freedom and maintain good mechanical isolation from the environment, that is, low quantum decoherence, consisting of thermal decoherence and dephasing. Here we overcome this challenge by introducing a superconducting-circuit-based optomechanical platform that exhibits low quantum decoherence and has a large optomechanical coupling, which allows us to prepare the quantum ground and squeezed states of motion with high fidelity. We directly measure a thermal decoherence rate of 20.5 Hz (corresponding to T-1 = 7.7 ms) as well as a pure dephasing rate of 0.09 Hz, yielding a 100-fold improvement in the quantum state lifetime compared with prior optomechanical systems. This enables us to reach a motional ground-state occupation of 0.07 quanta (93% fidelity) and realize mechanical squeezing of -2.7 dB below the zero-point fluctuation. Furthermore, we observe the free evolution of the mechanical squeezed state, preserving its non-classical nature over millisecond timescales. Such ultralow quantum decoherence not only increases the fidelity of quantum control and measurement of macroscopic mechanical systems but may also benefit interfacing with qubits, and places the system in a parameter regime suitable for tests of quantum gravity.

Achieving low decoherence is challenging in hybrid quantum systems. A superconducting-circuit-based optomechanical platform realizes millisecond-scale quantum state lifetime, which allows tracking of the free evolution of a squeezed mechanical state.