Towards standing-wave Mølmer-Sørensen gates on a quadrupole transition

We present progress towards a laser-driven Mølmer-Sørensen (MS) gate implemented using a novel beam configuration that nulls the carrier coupling, hence allowing for a significant increase in gate speed [1]. 

We couple to the λ=674 nm quadrupole qubit transition, 5S_1/2 → 4D_5/2, in ⁸⁸Sr⁺ with a free-space optical lattice formed by two counter-propagating beams. By controlling the phase of the optical lattice, ions can be placed at the antinodes of the standing wave where the coupling to the carrier is strongly suppressed while the motional coupling is maximised [2]. The two counter-propagating beams form an optical interferometer whose arm length is stabilised by feedback to an AOM. The lattice phase seen by the ion trapped about 50 cm away from the closing point of the interferometer is used to compensate for any changes in the optical path length between this point and the ion position. We demonstrate coherent operations with the lattice for one and two ions and present progress towards implementing the gate interaction on two ions.

[1]  A. M. Steane et al., New J. Phys., 16, p.053049 (2014)

[2]  A. Mundt et al., Phys. Rev. Lett., 89, p.103001 (2002)