In this talk, I will give an overview of work in our group connected to quantum sensing of radio frequency photons and vibrations. In the first part of the talk, I will describe our work extending circuit QED to radio frequencies using a qubit to observe the quantisation of radio-frequency eletromagnetic fields ("radio frequency photonics"), along with cooling of the RF mode and stabilisation of non-classical quantum states of RF light. In the second part of talk, I will introduce recent work in which we have been using microwave "squid cavities" for sensing vibrations and quantised currents at radio frequencies.
Quantum control over a physical system requires thermal fluctuations and thermal decoherence to be negligible, which becomes more challenging with decreasing natural frequencies of the target system. For microwave circuits, the quantum regime can be reached simply by cooling them to mK temperatures. Radio-frequency (RF) systems in the MHz regime, however, require further cooling or have to be coupled to an auxiliary quantum system with a coupling rate exceeding their thermal decoherence rate. A powerful tool to cool below the thermodynamic bath temperature is sideband-cooling, a technique that originated from the field of trapped ions and cold atoms and that has been applied in cavity optomechanics for groundstate cooling of mechanical motion. Here, we engineer a system of two superconducting LC circuits coupled by a current-mediated photon-pressure interaction and demonstrate sideband-cooling of a hot RF circuit using a microwave cavity and the regime of quantum-coherent coupling between the circuits. Due to a dramatically increased coupling strength, we obtain a large single-photon quantum cooperativity
Cq0∼1 and reduce the residual thermal RF occupancy by 75% through sideband-cooling with less than a single pump photon. For larger pump powers, the photon-pressure coupling rate exceeds the RF thermal decoherence rate by a factor of three and the RF circuit is cooled into the quantum groundstate. Our results demonstrate photon-pressure coupling with a hot radio-frequency circuit in the quantum regime and lay the foundation for radio-frequency quantum photonics.
Phase-stable radio frequency (RF) transfer attracts a growing research interest and attention in recent years, and transferring high stable RF signal to multiple places is greatly needed in many important applications such as frequency synchronization for very long base-line interferometry and connected-element interferometry in radio astronomy. [1]. For example, the Atacama Large Millimeter Array (ALMA) radio astronomy project is consist of an array of up to 64 antennas, and in order to achieve the unprecedented sensitivity and angular resolution, requires the local oscillator reference signal arriving at remote antennas with a maximum path length up to 18 kilometers must be stabilized under one picosecond [2]. The optical fiber link is a promising method for long-distance highly stable frequency distribution, however it suffers from the environment perturbations such as physical vibration and temperature fluctuations, which degrade the phase stability to be delivered [3].