Not all existing crypto would be broken. Symmetric encryption algorithms like AES will be fine. Asymmetric encryption (public-key) algorithms like RSA/DSA will

Not all existing crypto would be broken. Symmetric encryption algorithms like AES will be fine. Asymmetric encryption (public-key) algorithms like RSA/DSA will be in trouble due to Shor's algorithm, but other public-key systems will be fine. Hash functions like SHA-2 and SHA-3 will be mostly fine, because Grover's algorithm is at best sub-exponential, not quadratic. Also, quantum cryptography mostly provides superior key establishment/management capabilities, not superior confidentiality guarantees. It's a misnomer to state that we can achieve "stronger encryption" by "leveraging entanglement states"; most of quantum computing is useful for cryptanalytic attacks right now, not cryptographic construction.