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Aside from the clickbait title, can anyone please ELI5, TL;DR this paper? From abstract: Here we show that, while in nature the complex conjugation needed for

Aside from the clickbait title, can anyone please ELI5, TL;DR this paper? From abstract: Here we show that, while in nature the complex conjugation needed for time reversal is exponentially improbable, one can design a quantum algorithm that includes complex conjugation and thus reverses a given quantum state.

krastanov 8y ago
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.
- harias 8y ago
  Hasn't quantum chemistry calculations already been simulated using quantum computers? https://sydney.edu.au/news-opinion/news/2018/07/25/first-ever-quantum-simulation-of-chemical-bonds-with-trapped-ion.html https://sydney.edu.au/news-opinion/news/2018/07/25/first-eve...
  - krastanov 8y ago
    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).
- otoburb 8y ago
  Another physics layperson question here: Can this technique of studying a past state reversal apply to compromising one-way hash functions in some manner?
  - drinane 8y ago
    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.
  - krastanov 8y ago
    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.
- maccio92 8y ago
  This is the best and most understandable explanation I've seen in the whole thread. Thank you!
edmack 8y ago
Here's my attempt at distilling it: "Normally in a quantum computer, if we apply measurement function F to input x so y=F(x), it's impossible to reverse F and turn y into x. However, if we constrain y, we can make a special function G_Y such that x=G_Y(y)" Whilst I don't have time to really dive through all the sections, this seems weirdly click-baity and IBM-promoting for an academic article.

krastanov 8y ago

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.

harias 8y ago
Hasn't quantum chemistry calculations already been simulated using quantum computers? https://sydney.edu.au/news-opinion/news/2018/07/25/first-ever-quantum-simulation-of-chemical-bonds-with-trapped-ion.html https://sydney.edu.au/news-opinion/news/2018/07/25/first-eve...
- krastanov 8y ago
  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).
otoburb 8y ago
Another physics layperson question here: Can this technique of studying a past state reversal apply to compromising one-way hash functions in some manner?
- drinane 8y ago
  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.
- krastanov 8y ago
  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.
maccio92 8y ago
This is the best and most understandable explanation I've seen in the whole thread. Thank you!

edmack 8y ago

Here's my attempt at distilling it: "Normally in a quantum computer, if we apply measurement function F to input x so y=F(x), it's impossible to reverse F and turn y into x. However, if we constrain y, we can make a special function G_Y such that x=G_Y(y)" Whilst I don't have time to really dive through all the sections, this seems weirdly click-baity and IBM-promoting for an academic article.