A new photon recoil measurement to determine the fine-structure constant α

We aim for measuring the recoil frequency ωr = hk2/MCs ∝ 2π . 4 kHz of a Cs atom that emits/absorbs photon momenta hκ for a new determination of α with an accuracy better than 10-9. This allows us to test the quantum field theoretical calculation of the electron's anomalous magnetic moment, arguably the most accurate prediction in science, with sensitivity to the muon and non-QED terms. We use a differential measurement between two atom interferometers having up to 100 positive and negative recoils, respectively. We use multiphoton Bragg diffractions between different momentum states, leaving the atomic states unaffected. We thus remove systematics such as sensitivity to magnetic fields and the ac Stark effect. The two interferometers are addressed simultaneously to cancel vibrational noise by two counterpropagating Raman laser pulses that consist of two frequencies each. Our setup generates these pulses with an ultra-low phase noise of-138dBc/Hz. We further stabilize the phase of the pulses as seen by the atoms, using a high-speed feedback that locks the phase of each pulse anew. We also present a feedback system that reduces misalignment of the counterpropagating beams to microradian accuracy, based on optical interferometry. (orig.)