Actually no. Long answer: Yes, the outgoing transaction from a ledger to another one is done using crypto that can be broken by quantum computers. But a ledger

Actually no. Long answer: Yes, the outgoing transaction from a ledger to another one is done using crypto that can be broken by quantum computers. But a ledger that has never been used to send money is only visible by it's hash value. So it's security is based on sha256 which is quantum secure (There is Grover's algorithm, but it should be OK for a while). BC would just need to change the signing algorithm. So unless quantum computers pop up just over night everything should be fine. There are many threats to Bitcoin, IMO QC is one of the smallest.