Mechanical Purcell Filter for Microwave Quantum Machines

Measuring the state of a superconducting qubit introduces a loss channel which can enhance spontaneous emission through the Purcell effect. This can be mitigated by implementing a Purcell filter which strongly suppresses signal propagation at the qubit frequency. If the filter is well-matched at the readout cavity frequency, it will protect the qubit from decoherence channels without sacrificing measurement bandwidth. In this talk, we propose and analyze design for a mechanical Purcell filter, composed of an array of nanomechanical resonators in thin-film lithium niobate, whose frequencies are chosen to produce a bandpass response. A modest footprint, steep band edges, and lack of cross-talk make these filters a novel and appealing alternative to electromagnetic versions currently used in microwave quantum machines. We will present a circuit model depiction of this filter, as well as design, fabrication, and characterization results. Our filters achieve over 200 MHz bandwidth at 3.5 GHz, with over 50 dB out-of-band suppression, while occupying less than 0.3 square mm on-chip in a qubit-compatible material platform.