> But in case of QC, one is powerful enough to compute everything and we might not even need two, except for spare. Say a 1000 qubits QC exist, Work that it can do is the sum of all computering power in human history. You cannot even verify if the result is right or not because simply you don't have any classical alternative for that workload.
A 1000-qubit QC can't break a RSA-2048 key, let alone a lot of other interesting tasks. Quantum computers aren't magical things that provide exponential speedups on absolutely everything; they can only provide exponential speedups on some algorithms, and those algorithms generally require linear numbers of qubits to the problem size, so 1000 qubits is greatly limiting to problem size.
Neither any classical computer can. We don't even have enough harddrive to store all quantum information in a 100 qubit QC, let alone 1000qubit. QC is limited to solve a subset of problems do not automatically equals to QC is useless comparing to classical ones. Also not able to invalidate the powerfulness of QC beyond 100 qubits.
A 1000-qubit QC can't break a RSA-2048 key, let alone a lot of other interesting tasks. Quantum computers aren't magical things that provide exponential speedups on absolutely everything; they can only provide exponential speedups on some algorithms, and those algorithms generally require linear numbers of qubits to the problem size, so 1000 qubits is greatly limiting to problem size.