Bright quantum light source by spontaneous four-wave mixing in a hollow-core fiber

• Applicant: Professor Dr. Thomas Halfmann
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410249930

Optical implementations of quantum technologies necessarily require single photons (or pairs of entangled photons) as carriers to encode, store, or transfer information. In particular quantum communication relies on photonic technologies. As major advantages, photons travel with the speed of light and low loss over large distances, are not affected by electric or magnetic perturbations from the environment, have only negligible interaction with each other, and can be manipulated by linear optics. Moreover, optical quantum communication networks can use the already available and very powerful photonic infrastructure for classical data transfer on the global scale, i.e., optical fiber networks, extending it by quantum repeaters (e.g., based on quantum memories for light). Thus, the availability of bright, deterministic or at least heralded, and potentially integrable sources for single photons is a crucial prerequisite for quantum communication technologies. During the runtime of the previous project we performed experimental studies on single-photon sources, implemented in dense ensembles of cold atoms. As a particular highlight, we demonstrated spontaneous four-wave mixing (SFWM) in cold atoms loaded in a hollow-core fiber. With this approach we achieved a world record generated spectral brightness, exceeding the previous record by an order of magnitude, at 100-fold reduced pump power and 10-fold lower bandwidth. The latter is already compatible with atomic quantum memories, driven by broader-bandwidth protocols. The aim of this renewal proposal is to improve our unique SFWM photon-pair source towards larger duty cycle, larger brightness, and longer coherence time (thus, smaller spectral bandwidth). We will implement technical improvements such as an optimized loading procedure from the magneto-optical trap into the hollow-core fiber, additional cooling of atoms in the fiber, further reduction of reabsorption losses by optimized conditions of electromagnetically-induced transparency (EIT) during SFWM, and reduction or even laser-controlled compensation of perturbing dynamic Stark shifts. We will also apply a novel conceptual approach, i.e., super-fluorescence in the SFWM source to further boost the brightness. Finally, we will study the potential of SFWM to generate multi-photon states (correlated photon pairs). As a strategic long-term goal (beyond the proposal runtime) we intend to apply the single-photon source after frequency conversion with a quantum memory in a rare-earth ion doped solid. These combine large storage efficiency, long storage time, integrability and scalability. Our team performed several world-record experiments on such memories. Beyond this specific purpose, the bright SFWM source is expected to find applications also in in other fields of quantum technologies.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Thorsten Peters