In August 2019 I set up a short box spread on the EOE index (AEX). I used options with 200 and 800 strike prices with strike date in December 2023. Setting up the box yielded EUR 61230,- per box and in 2023 the box will settle at EUR -60000,-. This yield was possible due to the negative interest rate so this was an interesting way to borrow money with negative interest. This way I built a virtual deposito with positive yield for my cash.
This box is intrinsically risk free up to a certain level of market stress. When Covid hit the markets in March 2020 the volatility rised skyhigh resulting in crazy spreads which made trading impossible and which caused my broker to change their risk management. This resulted in devaluation of my stock portfolio which I use for coverage. I admit that the way this broker calculated risk was slightly irrational but it's their call. I covered the margin calls by depositing extra cash into my account.
Another risk I encountered is that interest rates may change. In 2020 my broker announced that they would apply negative interest rates for cash positions larger than EUR 100000,-. This may consume the yield which would tempt you to invest cash which may not always be the best choice.
There are upsides too: Today, (Feb 2022) the interest rate (EUR) is rising (NLGGB10J.BONDS) which results in an increasing result for this box as it now yields less than EUR 60000,-. As it looks now this box will develop succesfully, I expect to liquidate this box before strike date. A truly free lunch.
I don't recommend borrowing money to invest it.
In today's market I would not set up the virtual deposito I mentioned above, it would not work.
At this moment (Feb 8 2022) the short box spread AEX 200/800 DEC 2023 yields EUR 59944,-, with settlement in 2023 at EUR 60000,- this would result in a loan at 0,09% interest. This is nowhere near the negative % I got in 2019. However, with investing in a CD at 1,7% you may think that gives a nice leverage.
By investing the yield in a CD you won't have the broker's cash interest risk. But as long as a broker decides about risk management, stock portfolio valuation and margin rates I would not consider this a safe loan for long term investments. There is a serious risk that should (can?) be covered. Interesting stuff.
Again, I don't recommend borrowing money to invest it, actually, I don't recommend investing at all ;-)