afaik, without a need for error correction a quantum computer with 256 bits could break an old 256 bit RSA key. RSA keys are made by taking 2 (x-1 bit) primes and multiplying them together. It is relatively simple algorithms to factor numbers that size on both classsical and quantum computers, However, the larger the number/bits, the more billions of billions of years it takes a classical computer to factor it. The limit for a quantum computer is how many "practical qubits" it has. OP's article did not answer this, and so far no quantum computer has been able to solve factoring a number any faster than your phone can in under a half second.