Why do you need a quantum computer to attack a bitcoin - I don't understand at all. Even the old asymmetric cryptography on elliptical curves, with a 4-fold increase in the length of the key - will remain a dream to crack the known algorithms on quantum computers.
Because the power of a QC scales exponentially due to superposition and entanglement. Superposition meaning that a qubit can be - to simplify somewhat - both 0 and 1 at the same time. Entanglement meaning that multiple qubits can be combined into a single state. So the number of classical outcomes that can be assessed scales 2^n. The nature of QCs means that they are strong on integer factorisation and the discrete logarithm problem (both normal and ECC). Shor's algorithm can dismantle
current asymmetric cryptography.
QC scaling as 2
n is a common misconception. As n grows, the system scales worse and worse. At certain point, for n<50, the noise dominates, no signal is left. For example, last year Google claimed "quantum supremacy". It was supremacy in generating noise.