Israel's development of cutting-edge technologies in software, communications and the life sciences have evoked comparisons with Silicon Valley. Israel is first in the world in expenditure on research and development as a percentage of GDP. It is ranked 13rd in the Global Innovation Index in 2020, down from 10th in 2019 and 5th in the 2019 Bloomberg Innovation Index. Israel has 140 scientists, technicians, and engineers per 10,000 employees, the highest number in the world, for comparison the U.S has 85 per 100,000. Israel has produced six Nobel Prize-winning scientists since 2004 and has been frequently ranked as one of the countries with the highest ratios of scientific papers per capita in the world. Israel has led the world in stem-cell research papers per capita since 2000. Israeli universities are ranked among the top 50 world universities in computer science (Technion and Tel Aviv University), mathematics (Hebrew University of Jerusalem) and chemistry (Weizmann Institute of Science).