The quantum switch only works if each operation is used…
0 reactions
The quantum switch only works if each operation is used exactly once — I got this wrong at firs
Say you have two machines that each change something, and the order you
use them in matters. Normally you'd just try it both ways — but that means
running the experiment twice.
The quantum switch does it in one run, by not fixing the order at all. The
catch is that you only get that advantage if each machine is used exactly
ONCE in total. Draw it as a standard Mach-Zehnder — A then B on one arm,
B then A on the other — and you've quietly built two copies of each. Now
you could have tested it in a fixed order anyway, and the whole point is
gone. Took me embarrassingly long to spot.
The second thing that caught me: both branches have to come out in the
same final state. If the two orders left the target visibly different, the
target itself would record which order happened, coherence dies, and
there's nothing left to interfere. With X and Z you get ZX|H> = -|V> and
XZ|H> = +|V> — same state, opposite sign. Unmeasurable on its own, which
is exactly why it survives.
I make science documentaries and just finished one on this. Link below if
it's useful, but the above is the part I actually wanted to share.
https://youtu.be/wVvOj-5JJGI
#science
Say you have two machines that each change something, and the order you
use them in matters. Normally you'd just try it both ways — but that means
running the experiment twice.
The quantum switch does it in one run, by not fixing the order at all. The
catch is that you only get that advantage if each machine is used exactly
ONCE in total. Draw it as a standard Mach-Zehnder — A then B on one arm,
B then A on the other — and you've quietly built two copies of each. Now
you could have tested it in a fixed order anyway, and the whole point is
gone. Took me embarrassingly long to spot.
The second thing that caught me: both branches have to come out in the
same final state. If the two orders left the target visibly different, the
target itself would record which order happened, coherence dies, and
there's nothing left to interfere. With X and Z you get ZX|H> = -|V> and
XZ|H> = +|V> — same state, opposite sign. Unmeasurable on its own, which
is exactly why it survives.
I make science documentaries and just finished one on this. Link below if
it's useful, but the above is the part I actually wanted to share.
https://youtu.be/wVvOj-5JJGI
#science
| 1,000 mlx | |
| 2 | |
| 0 |