QNDINTERF · Atom interferometry at the Heisenberg limit using an in-cavity Bose-Einstein condensate and quantum non demolition detection

Atom interferometry at the Heisenberg limit using an in-cavity Bose-Einstein condensate and quantum non demolition detection

The aim of the project was to bring together several latest developments in ultracold atom science to realise a new apparatus to test and push further atom interferometry with non-classical states. The project combined the extensive experience of Dr A. Bertoldi in atom interferometry with the broad expertise of a leading European laboratory. During the two year activity the new setup has been implemented, important results have been already achieved, and several more are in the analysis stage or expected soon, even if the project is now officially ended. Until recently, atom interferometry was a single particle effect, where the use of a big number N of uncorrelated atoms gave a classic improvement of the sensitivity scaling like N-1/2. Using suitable quantum mechanical correlations among the atoms, it is possible to reduce the fluctuations in a specific observable - that used for the precision measurement - beyond the shot-noise level, at the expanse of an increase of the noise level in the orthogonal observable. This effect is named spin squeezing, and after its experimental realisation, it is now investigated to enhance the sensitivity in quantum metrology, and precision measurements towards an N-1 scaling. QNDINTERF project is placed in this rapidly developing context, with several groups realising proof-of-concept experiments that demonstrate measurements beyond the atomic-shot-noise limit. The first phase of QNDINTERF consisted in the implementation of the experimental setup, whose core is a macroscopic optical cavity placed in vacuum and used to optically trap rubidium 87 atoms. The cavity injected with resonant radiation provides a strong light field that is exploited to optically trap the neutral atoms at the position of maximal intensity. For the lock of the laser to the resonator we implemented an innovative method, based on serrodyne shifting the laser frequency: the result is a compact and robust setup that could broaden the use of optical cavity to include applications in noisy environments. The optical potential generated by the 1 560 nm seed laser was tomographically characterised by using a novel method that relies on the intensity dependent atomic fluorescence of atoms released from a MOT. The pumping laser can be locked selectively to different transverse cavity modes of the Hermite-Gauss set, which produces a scalable system of potential wells and then multi-dimensional arrays of atomic samples. The research activity shifted to the realisation of experiments with ultracold atoms. We developed a heterodyne measurement system to probe atoms in free space or trapped, as concerning atom number or number difference. With a weak pulsed probe we followed the state evolution of an atomic ensemble undergoing Rabi oscillations or more in general coherent manipulations of the collective spin on the Bloch sphere. To show the potential importance of the method we are implementing an atomic clock which makes use of weak measurements to extend the interrogation time T; the target is to show a scaling of the sensitivity better than the typical T-1/2 for atomic clocks, with potential impact in several technological fields, like global positioning and time keeping. Moreover, the ability to follow continuously the atomic state during spin manipulation will be key to achieve CW atom interferometry. We evaporated the atomic sample in the cavity by ramping down the optical power, so as to achieve condensation in the fundamental cavity mode: this BEC is the first completely obtained in an optical cavity, and making use of the resonator build-up it requires only one tenth of the usual optical power, which opens applications in portable and low consumption setups. We are presently pursuing a multi-well BEC by using higher order cavity modes, whose nodes will be then linked by switching between different resonator modes, or using QND measurement to entangle atoms placed in different potential wells.

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