First off, just to reassure everybody, I have no motive other than pure curiosity for asking this question.

I don't want my bank account hacked any more than you want the same done to yours.

My question, if it makes sense to those in the quantum computing area, is how many qubits would it take for a quantum computer to crack, if it's possible in the first place, an RSA 2048 or 4096 bit encryption code, in a reasonable time, say under a year or two?

In other words, around how long before I need to really worry about this as a real prospect?

Google's Quantum Computer

  • $\begingroup$ There is an entire stackexchange site devoted to IT security and questions like these would probably get more expert attention there than here. For example, this answer probably goes a long way... $\endgroup$ – Floris Jun 11 '15 at 0:53

This paper talks about factoring with 1.5n qubits, so you'd need 3072 qubits to factor a 2048 bit semiprime. It says you might be able to do it faster. I'm willing to bet you can't do it with less than n qubits since you need to store the number. You might be able to do it with one or two fewer qubits, since the first and last bits will always be one, but that's it.

There's already research into post-quantum cryptography, so it shouldn't be too big a deal if quantum computers become feasible.

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  • $\begingroup$ Are those logical qubits or physical ones though? (On mobile; can't read paper. ) A real QC will need redundancy for error correction. $\endgroup$ – zeldredge Jun 11 '15 at 3:42
  • $\begingroup$ Logical qubits. The number of physical qubits will be way higher, but that crucially depends on the coherence length of the qubits and the error correction being used, so I guess it can hardly be answered. $\endgroup$ – Martin Jun 11 '15 at 16:42

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