Researchers used quantum states of light to enhance the sensitivities of plasmonic sensors. Results of the experiment could lead to enhanced ultratrace label-free plasmonic sensing for a range of applications. Plasmonic sensors have been shown to operate at the shot-noise limit (SNL) — but quantum resources can be used to enhance the sensitivity of a device beyond this limit. By interfacing the noise floor of the sensor with quantum states of light that exhibit reduced noise properties, the noise floor of the sensor can be reduced below the classical SNL. This makes it possible to obtain a quantum-based enhancement of the sensitivity. A research team from the University of Oklahoma, in collaboration with Oak Ridge National Laboratory (ORNL) researchers, utilized entangled twin beams to probe a plasmonic sensor that is used to measure changes in the refractive index. Researchers demonstrated a 56 percent quantum enhancement in the sensitivity of the sensor, compared with the corresponding classical configuration. When compared with an optimal single coherent-state configuration, researchers observed a 24 percent enhancement in sensitivity. Schematic of the experimental setup. One of the twin beams (probe) generated with a four-wave mixing (FWM) process is sent through a plasmonic sensor inside a chamber, while the other one (conjugate) acts as a reference for intensity difference noise measurements. The setup is used to detect small changes in the refractive index of air with a sensitivity below the shot-noise limit. Inset: double-Λ energy level scheme on which the FWM process is based. BD: beam dump; SA: spectrum analyzer; FG: function generator; Rb: rubidium. Courtesy of The Optical Society and Alberto M. Marino. The team measured sensitivities of nearly five orders of magnitude higher than previous implementations of quantum-enhanced plasmonic sensors. “Quantum resources can enhance the sensitivity of a device beyond the classical shot-noise limit and, as a result, revolutionize the field of metrology through the development of quantum enhanced sensors," said professor Alberto M. Marino. "In particular, plasmonic sensors offer a unique opportunity to enhance real-life devices.” Quantum-enhanced plasmonic sensors could make it possible to detect smaller changes in the surrounding index of refraction than existing plasmonic sensors. The results of this experiment could pave the way for further improvements in sensing limits for high-precision biomedical and biochemical detection schemes. The research was published in Optica, a publication of OSA, The Optical Society (doi:10.1364/OPTICA.5.000628).