Similar questions have been asked before:
However, none of these answers give me a clear indication as to why Grover's algorithm can't be utilized for bitcoin mining. If the current difficulty of bitcoin mining is about 15,466,098,935,554 (approximately 2^44), which means that it would take about 2^44*2^32=2^76 SHA256 hashes before a valid block header hash is found.
However, by implementing Grover's algorithm, it would take only sqrt(2^76)=2^38 hashes to discover a valid block header hash. A 38-qubit quantum computer should be sufficient in this case. Quantum computers of this size have already been built (the largest quantum computer is 72 qubits, to my knowledge).
Therefore, why isn't it feasible to use a quantum computer to mine bitcoin by implementing Grover's algorithm? Did I get something wrong? I'm not too familiar with encryption and quantum algorithms, so please correct me if I missed something......