May I ask why „30 years ago“?

No OP, but another former physicist. There's not much career opportunity in that field, but physicists are eminently hireable in software engineering or investment banking.

Current Physics PhD student here.

Pretty much this is the case. Trying to not get spun out of academia is hard work, due to the limited opportunities. Especially is you're unable to move beyond your own country.

Then the temptation of other jobs that easily pay three, four, fives times the salary creep in.

And FAANG or investment banking pays even higher than then that.

Potentially, there's a lower glass ceiling in the UK

In engineering, probably not so much in banking. Unless you mean class barriers.

I mean SE salaries aren't what you'd get in the US for FAANG.

Banking in the city is probably the higher paid job in the UK

Did my PhD, when I started my studies there were 90 new students each year. Upon my promotion there were 30, and the faculty had merged(!). One of the groups (experimental, not my cup of tea) had been cancelled/stopped. I went into IT. A standard joke I used to say is that everyone who knows something about garden gnomes could go into IT. I knew nothing about garden gnomes (but learned myself C++ to [numerically] solve some nasty path integrals using object orientation) and easily adopted to the IT industry. (Started as a programmer, through DBA work (datawarehouse!), different software/data/business architecture, now a ‘portfolio manager’). I stopped being a physicist in 1997, so perhaps 30 year is overdoing it (26th year now, I guess).

Apparently there can be interference effects between two beams (4th order interference) so maybe such an effect could show up in this case. See Ghosh/Mandel 1987 "Observation of Nonclassical Effects in the Interference of Two Photons"

How does one go about not being a physicist anymore?

Username almost checks out

Yes, my name is another anekdote. (I had a summer school in Krete, Greece; there were people from the Max Planck institute. And yes, I probably made an impression as the organisers used the spelling of my last name to write down the emailadresses of those from the PlanCk institute;-0. But no, no known relation with Dr. Planck, would have been honored (did do my PhD in quantum optics, so in a related field I guess).

Was hoping it was something close to that. DAMN quantum optics. That dissertation must have been a pile of work.