A device that is able to generate single photons on demand in groups of several dozen or more could help scientists overcome one of the fundamental obstacles facing the construction of quantum computers. The heart of the system to generate groups of photons is a glass cell filled with hot gas vapor. Illuminating the cell with a laser results in the emission of photons with a wavelength in the infrared spectrum range. Courtesy of UW Physics/Mateusz Mazelanik. Physicists from the Faculty of Physics at the University of Warsaw (UW) have invented holographic atomic memory. Wojciech Wasilewski, physicist and professor at UW Physics, said their device enables the production of a precisely controlled group of photons, as opposed to just a single photon. "Compared to existing solutions and ideas, our device is much more efficient and allows for integration on a larger scale,” said Wasilewski. “In the functional sense, one can even think of it as a first equivalent of a small 'integrated circuit' operating on single photons." Single photons can be successfully used in quantum communication protocols that guarantee full confidentiality. However, to be able to perform complex quantum computations, entire groups of photons are needed. The simplest method of generating groups of photons is to use a sufficiently large number of sources. The devices in widespread use today utilize the phenomenon of spontaneous parametric down-conversion (SPDC). Under certain conditions, a photon generated by a laser can split into two new ones, each with half the amount of energy, and with all other properties linked by the principles of conserving energy and momentum. When information is recorded on one of the photons from the pair we also find out about the existence and properties of the other photon, which remains undisturbed by observation and therefore perfectly suitable for quantum operations. The problem with this is that every SPDC source generates single photons rather slowly and quite randomly. As a result, for a simultaneous emission from even as few as 10 sources, it could take several years. In 2013, a team of physicists from the Universities of Oxford and London proposed a much more efficient protocol for generating groups of photons. The idea was to place a quantum memory at each source, which would be capable of storing emitted photons. The photons stored in the memories could be released at the same moment. This shortened emission time from years down to microseconds. Wojciech Wasilewski (left) and Michal Dabrowski from the Faculty of Physics at the University of Warsaw demonstrate the single-photon generator based on holographic quantum memory. Here, the gas-filled glass cell is located inside the magnetic shield used to eliminate external disturbances. Courtesy of UW Physics/Mateusz Mazelanik. The source from the University of Warsaw physicists represents the first implementation of this concept, and one that's more integrated. The photons are created immediately within the quantum memory as a result of the laser pulse, which lasts only microseconds. External sources of single photons are no longer needed at all, and the necessary number of quantum memories has dwindled to just one. "Our entire experimental setup takes up about two square meters of our optical table surface. But the most important events take place in the memory itself, in a glass cylinder measuring approximately 10 cm in length and with a diameter of 2.5 cm,” said Michal Dabrowski, a Ph.D. student at UW Physics. The new memory, which was built with the support of PRELUDIUM and SONATA grants from Poland's National Science Centre and the resources of the PhoQuS@UW project, is a spatially multimode memory where individual photons can be placed, stored, processed and read in different areas inside the cylinder, acting as separate memory drawers. The write operation, performed with a laser beam, works by preserving a certain spatial model, a hologram, in the form of atomic excitations. Illuminating the system with the laser allows the physicists to reconstruct the hologram and read the memory's content. In the conducted experiments, the new source generated a group of 60 photons. Calculations show that in realistic conditions, the use of higher-power lasers would help to increase this number even up to several thousand. Theoretically, the new quantum memory will be able to perform several hundred operations on a single photon, which is sufficient for quantum communication and information processing. The research has been published in the journal Physical Review Letters (doi.org/10.1103/PhysRevLett.118.063603).