Actively Controlled Trajectory Folding for Microwave Ramsey Interrogation Using a Moving Optical Lattice

Microwave Ramsey interrogation with ultracold atoms is an important tool for coherent manipulation and precision measurement. Its frequency resolution improves with increasing free evolution time. However, in a compact geometry the usable Ramsey time is limited by the available trajectory length of the atomic motion. Here we combine Bloch oscillations in a moving optical lattice with microwave Ramsey interrogation of ultracold ⁸⁷Rb atoms and demonstrate a single-fold trajectory folding scheme. In this sequence, the moving optical lattice is applied between two microwave π/2 pulses and provides upward momentum compensation to the falling ultracold atoms, directing them back to the microwave interaction region for the second Ramsey pulse. Ramsey spectra recorded without and with lattice-assisted trajectory control show that the moving lattice can be incorporated into the Ramsey sequence while maintaining a reproducible Ramsey interference signal. With single-fold trajectory folding, the free evolution time is extended to 151 ms, and a Ramsey linewidth of 3.12 ± 0.07 Hz is obtained, comparable to the linewidth scale set by the inverse of the Ramsey time. These results demonstrate actively controlled trajectory folding for microwave Ramsey interrogation of ultracold atoms and provide a possible route for long coherent evolution in a limited spatial volume.