  The development of new techniques to improve measurements is crucial for all
sciences. By employing quantum systems as sensors to probe some physical
property of interest allows the application of quantum resources, such as
coherent superpositions and quantum correlations, to increase measurement
precision. Here we experimentally investigate a scheme for quantum target
detection based on linear optical measurment devices, when the object is
immersed in unpolarized background light. By comparing the quantum
(polarization-entangled photon pairs) and the classical (separable polarization
states), we found that the quantum strategy provides us an improvement over the
classical one in our experiment when the signal to noise ratio is greater than
1/40, or about 16dB of noise. This is in constrast to quantum target detection
considering non-linear optical detection schemes, which have shown resilience
to extreme amounts of noise. A theoretical model is developed which shows that,
in this linear-optics context, the quantum strategy suffers from the
contribution of multiple background photons. This effect does not appear in our
classical scheme. By improving the two-photon detection electronics, it should
be possible to achieve a polarization-based quantum advantage for a signal to
noise ratio that is close to 1/400 for current technology.
