Steady-State Microwave Mode Cooling with a Diamond N-V Ensemble

A fundamental result of quantum mechanics is that the fluctuations of a bosonic field are given by its temperature T. An electromagnetic mode with frequency w in the microwave band has a significant thermal photon occupation at room temperature according to the Bose-Einstein distribution n over bar = kBT/hco. The room-temperature thermal state of a (27r x 3)-GHz mode, for example, is characterized by a mean photon number n over bar & SIM; 2000 and variance An2 n over bar 2. This thermal variance sets the measurement noise floor in applications ranging from wireless communications to positioning, navigation, and timing to mag-netic resonance imaging. We overcome this barrier in continuously cooling a (27r x 2.87)-GHz cavity mode by coupling it to an ensemble of optically spin-polarized nitrogen-vacancy (N -V) centers in a room-temperature diamond. The N -V spins are pumped into a low entropy state via a green laser and act as a heat sink to the microwave mode through their collective interaction with microwave photons. Using a simple detection circuit, we report a peak noise reduction of -2.3 & PLUSMN; 0.1 dB and minimum cavity mode temperature of 150 & PLUSMN; 5 K. We also present a linearized model to identify the important features of the cooling, and demonstrate its validity through magnetically tuned, spectrally resolved measure-ments. The realization of efficient mode cooling at ambient temperature opens the door to applications in precision measurement and communication, with the potential to scale towards fundamental quantum limits.