Hacker News Reader

Comment by currenciessfe | Hacker News Reader

Something related.

In classical computers you can do:

  X = 4
  X = 5

In quantum computers this sequence of operations is impossible, because the program needs to respect unitarity and need to be able to be undone/run backwards.

But you can't run this program backwards because the second operation (=5) completely erases the information from the first step (=4)

View on HN

Classically, isn't the reverse program just (?):

X = 5

X = 4

I feel like 2 things are being mixed. In QM/QC, the operators must be unitary, which roughly means they "rotate" states and their inverse "rotation" exists, so (in principle) the reverse operation can be performed; so (in principle), we can construct the reverse operations, execute them, and kinda reverse the time evolution. But it doesn't mean that the quantum state (at time t) of the system encodes the time history (t'<t) of the state.

> But it doesn't mean that the quantum state (at time t) of the system encodes the time history (t'<t) of the state.

it does if you take an everettian perspective on QM, the universal wavefunction at time T can always be unwound to time T'.

I should have been more specific, the 4 and 5 values are dynamic runtime values, not constants known ahead of time.

Reply on news.ycombinator.com