That's gotta be a really sharp knife

This is a purely theoretical exercise, from what I could understand:

[They] considered what would happen if a single photon passed through an optical shutter—essentially a very fast mirror that can be switched on and off to block part of a pulse of light. If the shutter was fast enough, it could intercept the photon mid-pulse, snipping off part of this extended wave.

They then did a bunch of calculations to simulate what would happen, but I'm wondering if such a shutter wouldn't have to travel at the speed of light to catch the photon, and if this doesn't make the experiment meaningless...

If you flipped a mirror back and forth fast enough to “split a photon” you’d end up generating a bunch of photons due to the heat energy generated by the system anyway. I’m sure that’s not what they’re talking about but just imagine something oscillating that quickly.

Is energy conserved in this scenario?

Rather than producing a photon on one side and a vacuum on the other, the shutter generates something far more strange and complex: a superposition of states containing infinitely many photons simultaneously.

This happens because, in quantum mechanics, empty space isn't truly empty—in reality, it seethes with fluctuations in the electromagnetic field. By rapidly switching the shutter, the team found that these fluctuations are disturbed—and in doing so, they spontaneously create new photons. Crucially though, if you were to look only at the region immediately either side of where the shutter operated, the state would appear deceptively normal: indistinguishable from a single photon on one side, and a simple vacuum on the other.

"Hey math that represents the universe, what would happen if I did this thing you say is impossible?"

"Well, looking at the math you just broke, which represents the universe, it seems like it would break the universe."

"Wild!"