Can Rigetti Forest quantum computing be used to crack (in a reasonable amount of time) public-key cryptography. Ie, can you use it to hack Bitcoin & Ethereum?
If so, write a solution in code using the Python library for Quil, called pyQuil.
No. They are announcing so far a computer with 2 qubit gate scheme. This is not enough for doing anything. It's exciting news to work with this because it SEEMS to be a good environment (a full stack eco-system) to test the interfaces between classic/quantum computers, creating the API, etc... but the quantum computer with 2 qubits is not very useful in general.
About the cryptocoins, let's separate the issues: Bitcoin address is NOT the public key of the owner of the coin, but a hash of the public key. The public key is only revealed when you SPEND your bitcoins. Then you HASH you public key again (to prove that you own that address) and use you private key to sign the transactions, etc, etc...
So, EVEN IF you have a strong quantum computer, they will not break the bitcoin. HOWEVER IF you use the same address TWICE, it means, you spend some money with the address A and then make another deposit to the same address, then the public key is accessible because it was published at the time you spent the first time... then the strong quantum computer will be able to find the private key and spend your bitcoins.
I don't know about Etherium, but I assume it must be something similiar to Bitcoin... any serious cryptographic system today must prevent to be protected against quantum computers.
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Kudos to Graig, in the comments: Yes, there's yet another risk. Someone could crack the keys in between the moment that transaction is published and "committed". Very interesting point.