Citadel Securities

Do not confuse it with the hedge fund. Citadel Securities is a separate company that shares a founder and little else: it manages no client money, takes no outside capital, and exists to make markets with its own balance sheet across equities, options, Treasuries, FX and ETFs. Ken Griffin launched it in 2002, and Peng Zhao has run it as CEO since 2017.

It behaves more like an engineering shop than a trading desk. The firm publishes openly on C++ for production systems, staffs hundreds of PhDs across fields from geophysics to bioinformatics, and helped found MEMX, an all-electronic stock exchange built to pressure the NYSE and Nasdaq on cost. In 2022 it moved its headquarters from Chicago to Miami.