If one looks at the 10 and 30 year US government bond yields going back to the beginning of 2008, so just before the "great recession" started, from the chart below you can see that 10 year bonds were at 4% and 30 year bonds were at 5%. What is astonishing about this chart is the big fall in US long bond yields that occurred in the late summer of 2011, and you might immediately assume that this marked the beginning of one of the "QE" programs of quantitative easing mounted by the Fed. But you'd be entirely wrong here - it was essentially the beginning of "operation twist", where the Fed committed to buying more longer term Treasuries, or moving further down the maturity spectrum by buying more longer term bonds and selling shorter term ones.
Let's have a look at real bond yields for the same maturity bonds (courtesy of the US Treasury's bond pages here). What's clear is that the "operation twist" announcement sent US 10 year real bond yields into negative territory for all of 2012 and the first half of 2013. It's also noticeable that US real long bond yields are now not negative ( - but of course short term bond rates are). But it is also clear that it would be pretty exceptional circumstances that would send the US 10 year real bond yield into negative territory. In other words, to quote the pop band Yazz - "the only way is up"!