An Entanglement-Enhanced Microscope

Among the applications of optical phase measurement, the differential interference contrast microscope is widely used for the evaluation of opaque materials or biological tissues. However, the signal to noise ratio for a given light intensity is limited by the standard quantum limit (SQL), which is critical for the measurements where the probe light intensity is limited to avoid damaging the sample. The SQL can only be beaten by using {\it N} quantum correlated particles, with an improvement factor of $\sqrt{N}$. Here we report the first demonstration of an entanglement-enhanced microscope, which is a confocal-type differential interference contrast microscope where an entangled photon pair ({\it N}=2) source is used for illumination. An image of a Q shape carved in relief on the glass surface is obtained with better visibility than with a classical light source. The signal to noise ratio is 1.35$\pm$0.12 times better than that limited by the SQL.

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