PhD Defense by Christian Folkersen Bærentsen

Generation of non-classical states in a hybrid spin-optomechanical system

This thesis covers the generation of non-classical states in a hybrid spin-optomechanical system. The macroscopic spin oscillator comprises cesium atoms confined in a hot vapor cell of 330 K, coupling to light through the Faraday effect. The optomechanical system is a highly stressed silicon nitride membrane positioned in an optical cavity within a 4 K cryostat. 

Due to quantum back-action, the parts of the hybrid system are dominated by the interaction with the probing light field. The spin system can be prepared in the highest energy state, effectively creating a negative mass oscillator. The interaction between the sub-systems mediated by the light field generates a quantum back-action evading measurement, suppressing the quantum back-action noise by 4.6 dB. This allows for an entangled link for the hybrid system, estimated by a continuous variable Einstein-Podolsky-Rosen state with a conditional variance V = 0.83 ± 0.03 < 1, below the separability limit. This establishes a new benchmark for the achieved quantum links between hybrid quantum systems. 

In addition, an enhanced coupling to the spin oscillator has been accomplished by improved motional averaging attained by spatially shaping the probe beam into a square tophat, realizing a continuous measurement of light squeezing for two advanced regimes of readout. A measurement slower than the oscillation frequency generates 11.5 dB of squeezing and detects 8.5 dB of squeezing, and a measurement faster than the oscillation frequency detecting 4.7 dB of squeezing spanning more than one order of magnitude below the oscillation frequency, demonstrating a new milestone for the performance of quantum sensors, which enables strong coherent coupling to other material systems. 

The conceived hybrid system opens avenues for teleportation protocols in a spin-optomechanical system and quantum back-action evading measurements. Furthermore, the spin system constitutes a new regime for the performance of quantum oscillators, upholding the spin system's esteemed reputation as a quantum platform.