Quantum phenomena such as entanglement can improve fundamental limits on the sensitivity of a measurement probe. In optical interferometry, a probe consisting of N entangled photons provides up to a $$\sqrt{N}$$
N
enhancement in phase sensitivity compared to a classical probe of the same energy. Here, we employ high-gain parametric down-conversion sources and photon-number-resolving detectors to perform interferometry with heralded quantum probes of sizes up to N = 8 (i.e. measuring up to 16-photon coincidences). Our probes are created by injecting heralded photon-number states into an interferometer, and in principle provide quantum-enhanced phase sensitivity even in the presence of significant optical loss. Our work paves the way toward quantum-enhanced interferometry using large entangled photonic states.
We contend that Microwave Spin Pumping was first predicted and observed -albeit using a different and more sensitive detection mechanism than Inverse Spin Hall Effect -in the 1950's. This discovery was the founding step in the widely used analytical tool that is now known as Dynamic Nuclear Polarisation. Recognising this hitherto unsung connection between 20 th Century Magnetic Resonance and 21 st Century Spintronics not only helps to explain and unify contemporary metallic spin pumping observations: it is also the key to unlocking the immense and very sophisticated toolbox of Magnetic Resonance and placing it at the disposal of the future of Spintronics.
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