An interesting question posted on the platform Quora.

I made the analysis and post on the platform itself but will quote it here in full detail as it concerns the basic understanding of how blockchain works.

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How can Blockchain be safe?

In order to understand this point we need to look at what blocks and block-chain is.
The following image contains the information needed to understand what block and block chain is.

This is as straight-forward as it gets.

Now, how does this relate to 51%-attacks?

Fairly simple. As indicated in step 3 and 4, the network does the validation of the transaction. A transaction is validated if AT LEAST 51% of the network approve of the transaction.

The longer and more adaptation a coin has, the more nodes (users) make part of the network and the more secure it becomes. Hence why many talk about the power of “decentralization”.

What about Quantum-computing?

Quantum-computing has a lot of calculating power.
It has the ability to calculate fast.

When it concerns Crypto-mining, Quantum-computers will hold the advantage.
Their raw calculating-power trumps the power of many mining pools and they might start getting all the rewards that, at the moment, are fairly distributed.

To the security of the blockchain they would not make a dent.
As you can see from the image, it makes part of a distributed network.
It is but one node and still has to account for the approval of the rest of the network.
I.e. it cannot overrule other nodes/users even if they have a potato-computer.

HOWEVER!

Quantum-computers, at this point, may pose a danger to many alt-coins.
Due to their raw power they can easily take-over the mining part and amass all coins to themselves.

When it concerns Ethereum, Bitcoin, Litecoin (or any of the bigger coins with decent market-capitalization), chances are slim with each passing day.

This has to do the established mining pools and amount of interlocked computers.
Some mining-pools have already the combined raw power of a quantum-computer.
So if, for example, a quantum computer would enter the fray, it wouldn’t even make a dent in the functioning of a coin like Bitcoin.