The central bank may enact quantitative easing by purchasing a predetermined number of bonds or other assets from financial institutions without reference to the interest rate. The goal of this policy is to increase the money supply rather than to decrease the interest rate, which cannot be decreased further.This is often considered a last resort to stimulate the economy.
Quantitative easing, and monetary policy in general, can only be carried out if the central bank controls the currency used. The central banks of US, for example, cannot unilaterally expand their money supply and thus cannot employ quantitative easing. They must instead rely on the Central Bank of US to set monetary policy.