Quantitative Easing (QE) by the Federal Reserve has a goal of increasing liquidity and the circulation of money and reducing interest rates. Negative real interest rates are used by Central Banks with the goal to stimulate economy by pressuring people to start a business so they can earn a yield. The problem is that marketing or finding customers is the hardest part of running a business. An investment advisor would not recommend buying a business simply because financing was attractive; instead they factor is demand for goods and services.
In many other countries loans are granted to businesses that are not creditworthy because it is a matter of government policy. These banks are called “policy” banks instead of real banks because they don’t operate with a constraint to loan only to credit worthy businesses. In Japan during their great real estate bubble of the 1980’s the banks would persuade businesses to borrow more than they needed with a promise to buy office buildings from them at predetermined appreciated prices. This is an example of negative interest rates creating an expansion of loans and the money supply. But that behavior is not allowed in the U.S. banking system.
Thus offering loans at negative real rates will only have a minimal impact on the economy in terms of creating more growth.
It may be true that when yields are too low that investors will invest in junk bonds which are issued by lending company that seeks to find extra borrowers by lending too much to high risk borrowers. But those businesses would be a non-bank shadow bank type of lender. Their financing is more constrained and unreliable, so if they get into trouble their access to funds gets shut of quickly. These lenders are only interested in lending at high rates. So the impact of negative real rates makes them more willing to lend yet they only want to lend at high rates. This negates much of the benefit of artificially low rates.
Very low interest rates could induce more investor to become landlords. This would result in a surplus of rental properties, thus cutting the market value of rents, since there is a finite supply of tenants. As rents were cut then the owners would find their leveraged investments didn’t produce the projected profit and then real estate prices would go down.