Store γ rays by Moving, and Retrieve by Reversing

Imagine a collection of nuclear targets where each of them moves with different integral multiple of certain velocity (see the top panel of the above figure), a comb structure will show up in the γ-ray absorption spectrum (energy domain). When input γ rays are covered by above nuclear comb, we (together with Profs. Marlan Scully, and Olga Kocharovskaya from IQSE TAMU) find that one can use nuclei as a γ-ray memory, namely, a time delay between input and output can be essentially controlled. How could this be possible? Here is the trick. Just simultaneously drive each target in reverse! According to our calculation, a mirror image about the reverting instant, i.e., the γ-ray echo, will happen (see the bottom panel). Please check our work for more details in Phys. Rev. Lett. 123, 250504 (2019).