A quantum-enabled Rydberg atom electrometer
  There is no fundamental limit to the precision of a classical measurement. The position of a meter's needle can be determined with an arbitrarily small uncertainty. In the quantum realm, however, fundamental quantum fluctuations due to the Heisenberg principle limit the measurement precision. The simplest measurement procedures, involving semi-classical states of the meter, lead to a fluctuation-limited imprecision at the standard quantum limit. By engineering the quantum state of the meter system, the measurement imprecision can be reduced down to the fundamental Heisenberg Limit (HL). Quantum-enabled metrology techniques are thus in high demand and the focus of an intense activity. We report here a quantum-enabled measurement of an electric field based on this approach. We cast Rydberg atoms in Schr\"odinger cat states, superpositions of atomic levels with radically different polarizabilites. We use a quantum interference process to perform a measurement close to the HL, reaching a single-shot sensitivity of 1.2 mV/cm for a 100 ns interaction time, corresponding to 30 $\mu$V/cm/Hz^(1/2) at our 3 kHz repetition rate. This highly sensitive, non-invasive space- and time-resolved field measurement extends the realm of electrometric techniques and could have important practical applications. Detection of individual electrons in mesoscopic devices at a ~100 $\mu$m distance, with a MegaHertz bandwith is within reach. 
Adrien Facon, Eva-Katharina Dietsche, Dorian Grosso, Serge Haroche,   Jean-Michel Raimond, Michel Brune and S\'ebastien Gleyzes
quant-ph