By harnessing aspects of quantum mechanics, communication and information processing could be radically transformed. Promising forms of quantum information technology include optical quantum cryptographic systems and computing using photons for quantum logic operations. As with current information processing systems, some form of memory will be required. Quantum repeaters, which are required for long distance quantum key distribution, require quantum optical memory as do deterministic logic gates for optical quantum computing. Here, we present results from a coherent optical memory based on warm rubidium vapour and show 87% efficient recall of light pulses, the highest efficiency measured to date for any coherent optical memory suitable for quantum information applications. We also show storage and recall of up to 20 pulses from our system. These results show that simple warm atomic vapour systems have clear potential as a platform for quantum memory.

Photons with a twisted phase front carry a quantized amount of orbital angular momentum (OAM) and have become important in various fields of optics, such as quantum and classical information science or optical tweezers. Because no upper limit on the OAM content per photon is known, they are also interesting systems to experimentally challenge quantum mechanical prediction for high quantum numbers. Here, we take advantage of a recently developed technique to imprint unprecedented high values of OAM, namely spiral phase mirrors, to generate photons with more than 10,000 quanta of OAM. Moreover, we demonstrate quantum entanglement between these large OAM quanta of one photon and the polarization of its partner photon. To our knowledge, this corresponds to entanglement with the largest quantum number that has been demonstrated in an experiment. The results may also open novel ways to couple single photons to massive objects, enhance angular resolution, and highlight OAM as a promising way to increase the information capacity of a single photon.quantum entanglement | orbital angular momentum | photonic spatial modes | quantum foundations P hotonic systems are an excellent platform to test the foundations of quantum physics (1). Photonic technologies used to generate, manipulate, and measure one of its key features, quantum entanglement, have matured to an unprecedented level. In various experiments the limit of extending quantum mechanical predictions to the macroscopic regimes have been investigated by increasing the distance between the entangled photons (2, 3), the numbers of involved photonic systems (4), or the dimensionality of the entanglement (5). In another approach, the property of orbital angular momentum (OAM), which is related to the helical phase structure of the photons (6) in the paraxial regime and can be used to rotate particles around the optical axis (7), has been explored in quantum entanglement experiments (8). Interestingly, according to quantum theory this angular momentum can, in principle, be arbitrarily large even for entangled quantum states of single photons. If large enough, this angular momentum can conceivably be transferred to macroscopic particles, which could open novel ways of investigating light-matter interactions. Additionally, large OAM quanta of photonic quantum states are an interesting property to investigate the fundamental question regarding the existence of a quantum-classical transition. It is still believed by many people and often cited in textbooks in connection to Bohr's correspondence principle (see e.g., ref. 9) that the quantum-classical transition has to occur when the quantum number of the investigated state becomes very large. However, in the opinion of the authors and many others, such a simple relation is not correct (e.g., see also ref. 10). Therefore, generating quantum states with large quantum numbers (e.g., high OAM values) are important experimental tests to challenge and clarify these fundamental questions. A recent experiment toward these directions d...

Color centers in solid state crystals have become a frequently used system for single photon generation, advancing the development of integrated photonic devices for quantum optics and quantum communication applications. In particular, defects hosted by two-dimensional (2D) hexagonal boron nitride (hBN) are a promising candidate for next-generation single photon sources, due to its chemical and thermal robustness and high brightness at room temperature. The 2D crystal lattice of hBN allows for a high extraction efficiency and easy integration into photonic circuits. Here we develop plasma etching techniques with subsequent high temperature annealing to reliably create defects. We show how different fabrication parameters influence the defect formation probability and the emitter brightness. A full optical characterization reveals 1 arXiv:1711.10246v2 [quant-ph] 5 Mar 2018 the higher quality of the created quantum emitters, represented by a narrow spectrum, short excited state lifetime and high single photon purity. We also investigated the photostability on short and very long timescales. We utilize a wet chemically-assisted transfer process to reliably transfer the single photon sources onto arbitrary substrates, demonstrating the feasibility for the integration into scalable photonic quantum information processing networks.