So what is quantum computing?

This is the most dreaded question for anybody involved with this field if posed by a friend or relative without a physics background.  When I am too tired or busy to make an honest effort, I usually answer by quoting Feynman’s quip on quantum mechanics (click here to listen to the man himself – the quote appears about 6:45 min into the lecture):

“A lot of people understand the theory of relativity in some way or other.  (..) On the other hand, I think I can safely say that nobody understands quantum mechanics.”

The crux of the matter is that Quantum Computing derives its power from precisely the same attributes that make this realm of physics so alien to us.

It’s small comfort that greater minds than mine have been mulling this conundrum. For instance, a while back Scott Aaronson described his struggle to write a column for the New York Times that describes Quantum Computing.

Then there is Michael Nielsen’s take on it, a brilliant write-up illustrating why there is really no reason to expect a simple explanation.

But if this hasn’t utterly discouraged you then I have this little treat from D-Wave’s blog. You need to be willing to tolerate a little math, understanding that an expression like this

$ \displaystyle ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ \sum_{i} x_{i} $

means you are summing over a bunch of variables x indexed by i

$ \displaystyle ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ \sum_{i} x_{i} = x_{0}+x_{1}+x_{2}+ … $

Other than that it just requires you to contemplate Schroedinger’s light switches.  Just as his cat can be thought of as dead and alive at the same time, his light switches are in a superposition of On and Off.  Strictly speaking, D-Wave’s description is specific to their particular adiabatic quantum chip design, but nevertheless, if you get your head around this, you will have a pretty good idea why a Quantum Computer’s abilities go beyond the means of a classical Turing machine.