After we've concluded that 64-bit is an insecurity for classical computers, I think many would like to know, how secure in a perspective view, is 64 quantum bits security.
As we know, the Grover's algorithm halves the security strength offered by symmetric key primitives - a 128-bit-key block cipher would effectively require only $2^{64}$ trial decryption to recover key from a pair of plaintext and ciphertext on a quantum computer.
But how to put that into perspective? On classical computers, we have massively parallel processing units and hi-speed spinning fans providing air conditioning, we can have dedicatedly fabricated ASICs for extra speed up.
Q1) What equivalent can we find on a quantum computer?
We've always heard such obstacles as "quantum decoherence", and difficulty with entangling large number of particles, and some even question the possibility of even medium scale quantum computers. So
Q2) Is quantum computer really that impossible a dark art?
When NIST started their post-quantum cryptography standardization, they at first proposed having 128-bit classical security equivalent to 64 quantum bits security, they've later changed to measuring circuit depth.
Q3) What good translation between quantum security and classical security do we currently know.
Update
Per @fgrieu, 64 quantum bits security should be defined as: require equivalent to $2^{64}$ "black-box" evaluation.
