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Comment by krastanov | Hacker News Reader

A physics simulation usually simulates the future, but there is nothing stopping you from running the code "backwards" in order to study the past state of the system. If we want to sound fancy we call this "reversing time".

In the case of a quantum computer simulating something (we can not really do this yet, but we are hard at work building the hardware) that type of reversal might require us to calculate complex conjugates of quantum states. For various reasons this is nontrivial and this paper describes ways to do that.

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Hasn't quantum chemistry calculations already been simulated using quantum computers?

https://sydney.edu.au/news-opinion/news/2018/07/25/first-eve...

A bunch of non-scalable examples do exist, and they are very exciting, but we have nothing that can yet reliably simulate molecules of practical interest (we have only "toy" examples that are small enough to be already solvable with a classical computer).

This is the best and most understandable explanation I've seen in the whole thread. Thank you!

Another physics layperson question here: Can this technique of studying a past state reversal apply to compromising one-way hash functions in some manner?

No! Contrary to what the sibling comment says, there are plenty of one-way hash functions that are resistant to quantum computers. Two comments though:

1. This particular work is explicitly relying on the fact that many quantum operations (including the type of time evolution they are considering) are a one-to-one map (not the one-to-many "irreversible" hash functions). Hence this work is explicitly not applicable.

2. Quantum computers do break some form of public key encryption, but this is a solved problem, as there are many other public key encryption protocols that are can not be broken by a quantum computer. Quantum computers do provide modest speedup in all types of brute force searches, like breaking symmetric encryption, but this is trivial to defend against by using a slightly bigger encryption key.

Yes that is one of the reasons for big gubment interest in quantum machines, but allegedly those algorithms require control of millions of qubits and right now the highest qubit machine publicly disclosed is 50 I think.

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