What positive effects can Quantum Computers have on the Blockchain technology and hence Bitcoin Quantum Computers
We always fear what we do not know. This statement has been proven right throughout the history of mankind and has resulted in significant evolution, even though there was always speculation of singularity (existential crisis).
Not hopping the &quot;Quantum Computers will destroy Bitcoin&quot; wagon; I'd like to explore the positive outcomes or the use cases that Quantum computers could be used for, in blockchain technology.
Edit 1:
Just to clarify, this is not a duplicate question as it discovers only positive impacts that quantum computing can bring to the technology.
A couple very hypothetical examples:


Quantum channels: These channels allow wiretap resistant transmission of data. Bitcoin users can share secret information such as keys over quantum channels in such a way that any wiretapping would be detectable.
Database queries: Quantum computers can search through a database in time O(N^(1/3)). This could potentially speed up lookups in a blockchain. Say I want to transactions that correspond to my public key. For one billion transactions this can be done in only a thousand steps (ignoring gigantic constants probably). 
Quantum simulation could perhaps lead to better hardware so that's always nice.


However, the biggest real impact that quantum computers have right now on cryptography and thus cryptocurrencies is something different: They encourage cryptographers to explore new and interesting schemes and cryptographic assumptions. The study of these new assumptions and schemes could lead to more useful cryptographic tools which we couldn't build from other assumptions. The prime example for this is fully homomorphic encryption which can be built from ideal lattices. These lattices were first used in cryptography to build quantum resistant cryptography.