Motion detection of a micromechanical resonator embedded in a d.c. SQUID

Superconducting quantum interference devices (SQUIDs) are the most sensitive detectors of magnetic flux(1) and are also used as quantum two-level systems (qubits)(2). Recent proposals have explored a novel class of devices that incorporate micromechanical resonators into SQUIDs to achieve controlled entanglement of the resonator ground state and a qubit(3) as well as permitting cooling and squeezing of the resonator modes and enabling quantum-limited position detection(4-10). In spite of these intriguing possibilities, no experimental realization of an on-chip, coupled mechanical-resonator-SQUID system has yet been achieved. Here, we demonstrate sensitive detection of the position of a 2 MHz flexural resonator that is embedded into the loop of ad.c. SQUID. We measure the resonator's thermal motion at millikelvin temperatures, achieving an amplifier-limited displacement sensitivity of 10 fm Hz(-1/2) and a position resolution that is 36 times the quantum limit.