Always fun waiting for Scott Aaronson to explain why the latest QC paper is bullshit. Until then can any Quantum Information experts here provide some background? I read the paper but don’t trust my intuition with these kind of things.
If there’s anything interesting here, I suppose it’s just that “scientists use a quantum computer to reverse time” is one of the purest examples I’ve ever seen of a scientific claim that basically amounts to a mind-virus or meme optimized for sharing on social media—discarding all nontrivial “science payload” as irrelevant to its propagation.
Essentially it seems like they are exploring ways in which for certain computations a solution can be found before the computation has actually finished, approaching something like O(1) performance.
>The exemplary outcome probabilities 𝑃𝑖𝑗=|⟨𝑏𝑖𝑏𝑗|𝜓̃ 0⟩|2 and 𝑃𝑖𝑗𝑘=|⟨𝑏𝑖𝑏𝑗𝑏𝑘|𝜓̃ 0⟩|2, 𝑖,𝑗,𝑘=0,1 obtained in a real experiment for the 2- and 3-qubit models are shown on the Fig. 3E. One can see that the probability for observing the correct final state |0…0⟩ is less than 100% and for 2- and 3-qubit experiment are given by 85.3±0.4% and 49.1±0.6% correspondingly.
Does this mean that the 3 qubit model is predicted to only get the correct result for less than half the times it is asked?
Note that "time reversal" is a common accurate technical term. The problem is that you get one or two clases in an advanced QM class explaining what it mean and what it doesn't mean. Using it in a title in a non technical publication is misleading (or clickbaity).
It's easy to explain what they did with an unrelated experiment:
Let's imagine that you make an experiment where you have a few ball laying on a billiard table. You hit one, it colide with the other and you wait tome time "t". Let's assume that the walls are magic and the ball bounce perfectly without loosing energy.
Now with some very accurate device you measure the position, speed and spin off all the balls, and then with some combination of magic and very precise actuators you create an scenario where all the ball have that positions, with the speed in the exact oposite direction, and spinning with the oposite spin.
If you wait again a time "t" you will see that everything goes in the other direction and the ball return to the initial position, like a movie shown backwards. So it's like "reversing the time", but you don't reverse the time in the "clock in the wall", you only make the system apparently move backwards.
With a quantum system with particles or pseudoparticles it is easier to have perfect collisions and perfect walls that don't loose energy, but you can't measure the position and speed at the same time, so you can't recreate the scene like in a classical system. Also, the balls may be entangled and have other nasty quantum stuff.
The trick they did is to use a quantum system to simulate a quantum system. The operation to reverse the direction is related to complex conjugation. IIRC they simulate the real and imaginary part separately to make it posible to reverse the imaginary part and simulate the complex conjugation.
The good part of the complex conjugation is that it work without measuring the position and speed at the same time that is impossible in a quantum system.
So they star an experiment, wait "t" time, take the complex conjugate, wait "t" time and (most of the time) they return to the inicial configuration.
The second part where things move backwards is call the "time reversal". They are not making the wall clock go backward, only the part of the system move backward.
Lmao. Your reply should be the top comment on a lot of articles that get filtered before they make it to the front page. Apparently, some that do make it, too.
