Single-photon emission is a well-explored process. But in recent years interest in two-photon emission has grown. Nature Photonics spoke to Meir Orenstein and Alex Hayat in Israel about their ...

Here we describe the emission of highly collimated, spatially coherent X-rays, at a wavelength of about 1 nanometre and at photon energies extending to 1.3 kiloelectronvolts, from atoms that have ...