Quantum-Enhanced Fiber Bragg Grating Sensors in the Presence of Background Noise*

Optical quantum metrology leverages the reduced noise properties of quantum states of light, such as two-mode squeezed states (twin beams), to enhance the sensitivity of measurements and devices beyond the shot noise limit (SNL). Here, we focus on fiber Bragg grating sensors (FBGs), as they are highly robust and have resonance wavelengths that can be tuned for specific applications by controlling the grating separation during manufacturing. FBGs are used for sensing applications that require an accurate measure of temperature, strain, or vibrations, as they can detect small perturbations to the fiber core's refractive index. When narrow-band light interacts with an FBG, with a wavelength close to its resonance, a strain modulation on the FBG due to the physical quantity being measured gets transduced to the light as an intensity modulation signal on top of a background noise caused by any ambient temperature fluctuations and vibrations. We show that it is possible to obtain a quantum enhancement when probing these devices with twin beams, with the quantum enhancement characterized by comparing the signal-to-noise ratio when probing the FBGs with twin beams to that when probing with coherent states. To this end, we generate fiber coupled twin beams with a large degree of quantum correlations and interface them with specially designed FBGs that have a resonance near the wavelength of our twin beams. To enhance the signal-to-noise ratio and reduce the background noise, we have developed a system in which the twin beams are coupled to two separate FBGs to implement a differential detection scheme. To test the common noise rejection capabilities of the system, we show that when both FBGs are perturbed with a common modulation, which emulate noise, it cancels out as a result of the differential detection. However, when a signal perturbs mainly one of the FBGs, in addition to the common modulation, the common modulation cancels while the signal remains. Cancellation of excess background noise is critical to achieve a quantum enhancement as it will dominate over any quantum noise reduction that can be achieved by the use of twin beams.