Speaker: Albert Schliesser, Niels Bohr Institute.
We use phononic engineering to realize mechanical resonators and waveguides that maintain quantum coherence as long as 100 milliseconds. With these systems we experimentally explore key concepts of quantum measurement, such as quantum backaction, the standard quantum limit (SQL), and the reduction of the quantum state by measurement. We also demonstrate techniques that have allowed us, for the first time, to break the SQL for displacement and force measurements, and to verify a past quantum state of the mechanical system (quantum retrodiction).
Furthermore, we harness the coupling to optical and microwave resonators to control the mechanical quantum state—either through measurement-based control and feedback, or using coherent control, in particular sideband cooling. A combination of both even allows preparing a room-temperature mechanical system close to its quantum ground state. We also analyze and demonstrate a protocol for pulsed control, with which we write, store and retrieve optical signals in the mechanical resonator, as required for an optomechanical quantum memory.
Finally we discuss a range of potential applications of highly coherent mechanical systems, in particular for sensing forces down to and below the attoNewton level. This can be combined with spatial scanning, to yield an ultrasensitive variant of an atomic force microscope, a proof-of-principle implementation of which has already been used to image individual viruses and nanoparticles. Magnetic functionalization could then allow detecting individual electron spins in a sample, or even map nuclear spin density in three dimensions for nano-scale magnetic resonance imaging. More near-term applications lie in extremely wide-range pressure gauges, and detection of single molecules through their mass and infrared signature.