So this means once quantum computers become cheap enough to buy. Private keys will be useless that is unless Bitcoin can move with this new technology.

"Post-quantum cryptography refers to cryptographic algorithms (usually public-key algorithms) that are thought to be secure against an attack by a quantum computer. As of 2017, this is not true for the most popular public-key algorithms, which can be efficiently broken by a sufficiently large hypothetical quantum computer."

I think that once quantum computers become viable, one of the first uses will be quantum encryption. It won't be the case that hackers will immediately be able to compromise everything - encryption will have developed either in tandem or ahead as a preventative measure.

Incidentally, quantum encryption itself is by nature absolutely unhackable - regardless of any future developments (unless we uncover new laws of physics). This is because of quantum entanglement, Einstein's famous 'spooky action at a distance'. This is well into the realms of things that our mere human brains just aren't built to comprehend. We can know the maths, but not really understand its full implication.

Yeah this is a very interesting subject indeed. Years ago people were talking about quad core processors and gigabyte hard drives then terabyte hardrives and at the time I thought it would never happen while I was still alive. It happened a lot faster than i could have ever imagined and I do think that we will see quatum computers in most people houses within 10-20 years  I also think that the software will have to change and so will encryption like you say. Lets hope Bitcoin grasps this technology becuase if they dont Bitcoin will be a thing of the past .

As far as I know, quantum computers will not be available to humanity in the next five years. I believe that 5 years is enough to prepare for this.

Lets hope so lol. I think it will be a lot longer tbh because of the cost

Yep as systems move forward so does most software and as long as Bitcoin are on top of it it should be fine ..I hope

PS:   I like this part though " but a new physics that could crack entangled encryption would likely confer untold power and turn us into a post-scarcity star-trek society in any case "  

It is impossible to have such a computer in a single instance. Any powerful computer is always a lot of the same powerful computers. Against any poison there is always an antidote. I believe that nothing can stop bitcoin. But for that bitcoin needs to be able to respond quickly to external threats. Now this is not happening.

A True quantum computer will break any modern encryption even these so-called "quantum resistant" blah blah blah... Ill easily explain it in simple terms
How?

Example:
Lets say for the poops and laughs your string is (that is only letters and only capital letters):
ABCDEFGHIJKLMNOPQRSTUVWXYZ (broken down into binary code):
01000001 01000010 01000011 01000100 01000101 01000110 01000111 01001000 01001001 01001010 01001011 01001100 01001101 01001110 01001111 01010000 01010001 01010010 01010011 01010100 01010101 01010110 01010111 01011000 01011001 01011010

A typical modern computer trying to brute force its way in would have to crack 26 characters exponetially 26 times (26^26) or 6.156 Decillion (A.k.A  6,156,119,580,207,157,310,796,674,288,400,203,776 times) and that is just with the 26 uppercase letters!  This is because current computer bits are either 1's or 0's and it can only test one pathway at a time which could potentially take (26^26) tries to solve the password.


Seems impossible but not with quantum computers

Since at a quantum level the bits exist as both a 1 and a 0 the encryption basically grabs its ankles and takes it.  The Quantum attempt looks at the encryption then makes the attempt and succeeds immediately. 

This is where new cryptography needs to occur and when complete it will be truly quantum proof.  You need a crypto-algo that changes when observed, accessed, or altered.  Some call this Quantum Cryptography but an algo could be written right now that would be quantum proof.  The problem is even Watson on a dedicated pipe to the internet would have trouble handling it.  Think of 10000tx per second.  Each one changing the blockchain... (I can smell the fried circuits now... ahhhhh)

So ya this is very layman explanation and I could get more detailed but it should sum up.

Ok so as somebody said above this will affect a lot of software including the blockchain for cryptos  ? so what if they do not use blockchains like IOTA is doing. Im thinking that blockchains in the future will be useless and if that is so Bitcoin will be left behind unless they get rid of the blockchain 

Yes and no, Quantum Cryptography blockchain will make it unhackable, but by the time QC gets to the point of being everywhere to run a decentralized block-chain the first QC's will have annihilated the primitive cryptography and drove their values to nothingness.  You can't institute a QC based solution without have QC's everywhere.  If you ran a primary blockchain on using entanglement on a 0 and 1's architecture it wouldn't understand what was going on.  SO the modified softfork would only run on a few sparse QC computers in the beginning until the QC's are everywhere.

I understand what you are saying but there are layers upon layers to this.

And IOTA is quantum resistant in such that the Merkle signature scheme is a one time code that only gets one attempt at a crack one-way.  But that doesn't mean that a Quantum computer can't solve the code.  Think of a water resistant watch vs a water proof watch... same concept here.  The QC can and would eventually crack this and it wouldn't take years months or days either.  The ability for a QC to be able to access all the possible entry points of the IOTA chain and try multi variant codes all at once on every POE will allow the code to be cracked in minutes. 


I agree with Styca to a point, the only truly quantum proof POE is an entangled POE.  And this only happens for a decentralized blockchain when all POE's can run quantum entanglement.  It can't be backwards compatible at all or you have the weak POE.