Spatially localized optical excitations in a doped solid using robust broadband composite pulses

We experimentally demonstrate spatial confinement of optically driven atomic excitation by broadband composite pulses in a rare-earth ion-doped <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msup> <a:mtext>Pr</a:mtext> <a:mrow> <a:mn>3</a:mn> <a:mo>+</a:mo> </a:mrow> </a:msup> <a:mtext>:</a:mtext> <a:msub> <a:mtext>Y</a:mtext> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mtext>SiO</a:mtext> <a:mn>5</a:mn> </a:msub> </a:mrow> </a:math> crystal. The experimental data confirm that such sequences enable spatially localized excitation well below the diameter of the applied laser beam, with a substantial improvement compared to previous developments and applications of other classes of composite pulses. In particular, we derived and applied broadband composite sequences, which also work efficiently in inhomogeneously broadened media, while also maintaining robustness with regard to fluctuations of the driving laser intensity. We perform extensive measurements to assess and compare the performance of several classes of broadband composite sequences. The findings are confirmed by numerical simulations, which agree very well with the experimental data. As the concepts are also applicable to spatially confined excitations below the diffraction limit of the laser beam, the results serve as an important step towards high-resolution imaging, preparation and addressing in quantum technologies and classical applied optics.