Advice for struggling PhD student
I have found this graphic really helpful for staying motivated in the PhD and reminding me that many, many people go through this. Sounds like you have hit the trough :)
The most concrete advice I've gotten is that the PhD is an endurance sport rather than a sprint. So if you want to finish it sounds like you might need to just persist and stick it out. But I know how you're feeling and it is awful.
Concretely, I can see a few things that might help:

Ask your advisors to help you work out a timeline for graduating in 6 years (or whatever number of years -- just make it fixed). That way you will have clear milestones and deadlines to work towards. It can change later, but write something down now.

Try to take concrete action towards developing a proposal. I find it strange that they don't let you propose by the end of the fourth year; we actually can't advance past it without proposing. It does take some time to pick a topic and build related experience, but in theory, a proposal is what you will do. Is there something intermediate -- like a 1 page outline of the proposal, or a list of three proposal pitches, or a plan for preliminary experiments -- that you can work with them on? (the equipment you're building ostensibly has a purpose). Particularly now, that you can't fully run your experiments, it seems like a good time to take stock and figure out a direction of what will happen, so that you can run with it as soon as things normalize.

Why can't you publish your review paper? Try to understand if this is just a recommendation or a firm no. Is there something you can improve? Is there something you can salvage from it (e.g. an intro chapter of the thesis?) If you're not working on anything else, you might as well be trying to get this out. It's possible that they think it's not good enough for a top publication, but it might help motivate you to have one publication through the pipeline, even if not the most competitive.

Try to find a support community. I have found that meeting other students through summer schools or conferences has helped me a lot. You might be able to undertake some related collaborations to help motivate you, and as a bonus publish that way. For experimental work, maybe you could try to find a lab that you could visit or intern with (CERN? DOE? I am not a physicist) -- joining existing experiments might enable you to publish without setting up from scratch.

Take some time off. If you are burnt out and experiments aren't running, maybe just block off a week or two for complete rest and no physics.


It sounds like your advisor might be thinking longer-term than you are -- for him the priority is probably setting up the lab, rather than graduating you. Hopefully, once the equipment is set up, you'll be first in the queue (and get credited on the publications of the other younger students), but I can see that it would be hard to guarantee this in advance and you risk being taken advantage of. For now, I'd try to be clear that you're having a hard time and need some sort of framework or plan for moving forward. Hopefully that will take the focus back to what you want -- an end goal.
I haven't commented on your outside offer because I think that's really a personal choice. But it sounds like you want to take it because you are having trouble with the PhD, so I tried to think of ideas to solve that. The fact that you're asking this question suggests that you're not quite ready to give up on PhD...?