Quantum estimation of a two-phase spin rotation

We study the estimation of an infinitesimal rotation of a spin-j system, characterised by two unknown phases, and compare the estimation precision achievable with two different strategies. The first is a standard \u2018joint estimation\u2019 strategy, in which a single probe state is used to estimate both parameters, while the second is a \u2018sequential\u2019 strategy in which the two phases are estimated separately, each on half of the total number of system copies. In the limit of small angles we show that, although the joint estimation approach yields in general a better performance, the two strategies possess the same scaling of the total phase sensitivity with respect to the spin number j, namely ' 1/j. Finally, we discuss a simple estimation strategy based on spin squeezed states and spin measurements, and compare its performance with the ultimate limits to the estimation precision that we have derived above

Multijoule scaling of laser-induced condensation in air

Using 100 TW laser pulses, we demonstrate that laser-induced nanometric particle generation in air increases much faster than the beam-averaged incident intensity. This increase is due to a contribution from the photon bath, which adds up with the previously identified one from the filaments and becomes dominant above 550 GW/cm2. It appears related to ozone formation via multiphotondissociation of the oxygen molecules and demonstrates the critical need for further increasing the laser energy in view of macroscopic effects in laser-induced condensation