Quantum entanglement is essential for quantum repeaters needed for any of the proposals for quantum information processing to have a chance. The goal of this project is to obtain a quantum pancake that is good enough to achieve quantum entanglement. "Good" means one with a large dipole moment and radiatively dominated lifetime and decoherence time. A large dipole moment allows one to optically control the pancake with the vacuum field or an applied optical pulse faster than nonradiative decoherence times. Based on the growth of narrow-linewidth InGaAs/GaAs quantum wells and extraction of the radiative width and dipole moment, this proposal would take the next step: make a quantum pancake by laterally confining a 100- to 200-nm diameter portion of such a quantum well. A good pancake could be used in a small 3D microcavity to achieve quantum entanglement between a transition of the pancake and the optical mode.
The approach will begin with a search for the best portions of a single quantum well with 0.8-micron spatial resolution using conventional optics or 0.25-micron resolution with a solid immersion lens. Various techniques for lateral confinement will be considered, for example, stress from a layer of self-organized dots grown above the quantum well or interdiffusion by local heating using a tightly focused laser beam. Candidate pancakes will be evaluated by a multitude of optical spectroscopic techniques. Differential reflectivity or absorption will determine the integrated absorption and dipole moment. MHz-resolution cw nonlinear spectroscopy and picosecond pump-probe measurements will be used to find out the effect of absorption of a single photon. Finally, streak-camera detection of the forward or backward emission following picosecond resonant excitation will show to what extent the polarization decay is purely radiative.
