Earlier today I posted a chart of the flattening US yield curve. I thought for reference it might be interesting to look at Japan's yield curve over the last few decades. The curve hasn't ever quite inverted, but has gotten to a 0.5% spread on a few occasions.
It's important to note that it's not clear what the maturity of the short term bond is from the data source that I pulled this from, so this isn't necessarily an apples to apples comparison to the US chart. It looks to me like the short term chart may be more equivalent to a fed funds rate than a 2 yr bond. It's an interesting chart in its own right and is posted below.
Japan has been in ZIRP for over a decade. There remains no empirical evidence that ZIRP is effective in stimulating economic growth.