Quantum computers are real and are starting to be used for some interesting applications. As well as many applications in finance, organic chemistry and complex dynamical systems there is an ugly elephant in the room. That elephant is Shor's algorithm.
Given a sufficiently powerful quantum computer, Shor's algorithm can factorise numbers in polynomial time. I have implemented it on a quantum simulator and it has been used on real quantum computers. When quantum computers are powerful enough nearly all the encryption techniques that we currently rely on will be useless.
The time is still far off when RSA will be useless but I will share some compelling reasons why we need to be taking action right now to avoid potential catastrophe in the not too distant future.