This image shows three 2D coherent spectra of a quantum dot ensemble for different prepulse powers. The lines show the excited state populations as a function of prepulse power. (Photo Credit: Brad Baxley-UM)

ANN ARBOR—U-M Physics Professor Steven Cundiff and research team including U-M Research Fellows Takeshi Suzuki and Rohan Singh found that light can be used to excite matter, which then evolves with its natural frequency much like the sound made by an organ pipe. Deterministic excitation of matter, known as coherent control, has applications ranging from quantum computing and simulation to controlling chemical reactions. These applications will benefit significantly from coherent control of large ensembles of particles/oscillators. However, much like the different sounds made by organ pipes of different lengths, in general, distinct particles have different natural frequencies, which is referred to as the inhomogeneous broadening. Inhomogeneous broadening masks the effect of coherent control on an ensemble. This effect is especially significant in semiconductor quantum dots, which are equivalent to artificial atoms, due to a spread in their sizes introduced during the manufacturing process. Just as for organ pipes, a quantum dot’s natural frequency depends on its size. Studying single dots is one approach to avoid this limitation. In this work, we measure the effect of coherent excitation of ten million quantum dots with ultrafast pulses using pre-pulse two-dimensional coherent spectroscopy.

The concept of coherent control is broadly applicable to a range of material systems, not just the quantum dots used in this work. For example, coherent control has been proposed as a method for driving chemical reactions of molecules and it is at the heart of quantum information science. The methods demonstrated in this work can impact these applications as well, which might entail applying it to molecules in solution or atoms in a vapor.