Double pass quantum volume hologram

We propose a new scheme for parallel spatially multimode quantum memory for light. The scheme is based on the propagating in different directions quantum signal wave and strong classical reference wave, like in a classical volume hologram and the previously proposed quantum volume hologram [Phys. Rev. A 81, 020302(R)(2010)]. The medium for the hologram consists of a spatially extended ensemble of cold spin-polarized atoms. In absence of the collective spin rotation during the interaction, two passes of light for both storage and retrieval are required, and therefore the present scheme can be called a double pass quantum volume hologram. The scheme is less sensitive to diffraction and therefore is capable of achieving higher density of storage of spatial modes as compared to the thin quantum hologram of [Phys. Rev. A 77, 020302(R)(2008)], which also requires two passes of light for both storage and retrieval. On the other hand, the present scheme allows to achieve a good memory performance with a lower optical depth of the atomic sample as compared to the quantum volume hologram. A quantum hologram capable of storing entangled images can become an important ingredient in quantum information processing and quantum imaging.