In early July 1943, Bethe invited Rossi to join the Manhattan Project. Within a month, he reported for duty at Los Alamos Laboratory. A few weeks later, Nora and their three-year-old daughter, Florence, joined Rossi in Los Alamos, New Mexico. The laboratory's director, Robert Oppenheimer, asked Rossi to form a group to develop diagnostic instruments needed to create the atomic bomb. He soon realized that there already existed a group with a similar mission headed by the Swiss physicist Hans H. Staub. The two decided to merge their efforts into a single "Detector Group". They were assisted by approximately twenty young researchers, including Matthew Sands an "electronic wizard", who later earned a PhD under Rossi, and David B. Nicodemus, whom Staub brought from Stanford University, who was an expert on particle detectors.