Strong gate coupling of high-Q nanomechanical resonators.

The detection of mechanical vibrations near the quantum limit is a formidable challenge since the displacement becomes vanishingly small when the number of phonon quanta tends toward zero. An interesting setup for on-chip nanomechanical resonators is that of coupling them to electrical microwave cavities for detection and manipulation. Here we show how to achieve a large cavity coupling energy of up to (2π) 1 MHz/nm for metallic beam resonators at tens of megahertz. We used focused ion beam (FIB) cutting to produce uniform slits down to 10 nm, separating patterned resonators from their gate electrodes, in suspended aluminum films. We measured the thermomechanical vibrations down to a temperature of 25 mK, and we obtained a low number of about 20 phonons at the equilibrium bath temperature. The mechanical properties of Al were excellent after FIB cutting, and we recorded a quality factor of Q ∼ 3 × 10(5) for a 67 MHz resonator at a temperature of 25 mK. Between 0.2 and 2 K we find that the dissipation is linearly proportional to the temperature.

Electronic refrigeration at the quantum limit.

We demonstrate quantum-limited electronic refrigeration of a metallic island in a low-temperature microcircuit. We show that matching the impedance of the circuit enables refrigeration at a distance, of about 50 microm in our case, through superconducting leads with a cooling power determined by the quantum of thermal conductance. In a reference sample with a mismatched circuit this effect is absent. Our results are consistent with the concept of electromagnetic heat transport. We observe and analyze the crossover between electromagnetic and quasiparticle heat flux in a superconductor.