A bit hard to wrap my head around it even in its popular simplified form but here it is: a quantum computer could reverse the arrow of time breaking our well established norm of causal asymmetry.
He has analyzed the issue by looking at computational complexity and, critically, the issue of noise. All physical systems are noisy, he argues, and qubits kept in highly sensitive “superpositions” will inevitably be corrupted by any interaction with the outside world. Getting the noise down isn’t just a matter of engineering, he says. Doing so would violate certain fundamental theorems of computation.
Quanta Magazine has a situationer on quantum computing today.