The observation of a continuous spatial photon echo in a magnetically compensated supersonic beam of samarium atoms is reported. Due to the magnetic field gradient used to compensate the Doppler shifts of the diverging supersonic beam, the echo signal exhibits novel features. In the experiments, a supersonic atomic beam traverses two continuous pump regions. These regions play the roles of the usual /2 and pulses in temporal rephasing phenomena as seen in the atom frame. Hence inhomogeneous dephasing of the macroscopic polarization during the traversal time between the two pump beams is reversed after the second pump region. The system employs a magnetically compensated supersonic beam to achieve a long interaction path length in the medium with a narrow Doppler bandwidth. This permits a large optical polarization to be obtained. The echo field generated by the atomic medium 0.5 mm downstream from the pump region is visible with the naked eye. A typical echo signal is shown. The presence of a magnetic field gradient in addition to residual Doppler broadening leads to a quadratic time dependence for the phase of the macroscopic polarization in the atom frame. This leads to unexpected changes in the echo decay curve versus position along the atomic beam axis.
