A technique to generate single photons with tailored properties from laser-quality solid-state devices could bring us a step closer to a quantum “Internet.” Single photons will form an integral part of distributed quantum networks, or a quantum Internet, as flying units of quantum information called qubits. The qubits can carry information quickly and reliably across long distances and can take part in quantum logic operations, provided that all photons in the network are identical. The quality of photons generated from solid-state qubits, including quantum dots, however, can be low because of decoherence mechanisms within the materials. With each emitted photon being distinct from the others, developing a quantum photonic network has proved difficult. But researchers from the Cavendish Laboratory at Cambridge University have developed a solution to the problem: a technique that generates consistent single photons. As their photon source, the investigators developed a semiconductor Schottky diode device containing individually addressable quantum dots. The quantum dots’ transitions were used to generate single photons via resonance fluorescence — a technique demonstrated previously by the same team. An artist’s impression of distributed qubits (the bright spots) linked to each other via photons (the light beams). The colors of the beams signify that the optical frequency of the photons in each link can be tailored to the needs of the network, according to researchers at Cambridge University. Courtesy of Mete Atature/University of Cambridge. Under weak excitation, also known as the Heitler regime, the main contribution to photon generation is through elastic scattering. This form of operation avoided photon decoherence and enabled the scientists to quantify how similar these photons are to lasers in terms of coherence and waveform — and it turned out they were identical. "Our research has added the concepts of coherent photon shaping and generation to the toolbox of solid-state quantum photonics," said Dr. Mete Atature from the department of physics. "We are now achieving a high rate of single photons which are identical in quality to lasers with the further advantage of coherently programmable waveform — a significant paradigm shift to the conventional single-photon generation via spontaneous decay." Protocols for quantum computing and communication relying on the photon- generation scheme have been proposed, and plans to extend the method to other single photon sources such as single molecules, color centers in diamond and nanowires are possible, the researchers said. "We are at the dawn of quantum-enabled technologies, and quantum computing is one of many thrilling possibilities," Atature said. "Our results in particular suggest that multiple distant qubits in a distributed quantum network can share a highly coherent and programmable photonic interconnect that is liberated from the detrimental properties of the chips. Consequently, the ability to generate quantum entanglement and perform quantum teleportation between distant quantum-dot spin qubits with very high fidelity is now only a matter of time." For more information, visit: www.cam.ac.uk