When interest rates hit zero, central banks started buying bonds instead

Quantitative easing is a central bank creating new money to buy government bonds directly from the market, pushing down long-term borrowing costs once ordinary interest-rate cuts run out of road. The Bank of Japan launched the first program explicitly under this name in March 2001 to fight deflation. The tool went global after the 2008 financial crisis, when the US Federal Reserve began its own bond-buying program in 2009 to prop up credit markets.

— Wikipedia contributors, Quantitative easing — Bank of Japan program began March 2001