True Lab Stories: Boris the Magnet Guy

The story about Bloch oscillation gravity measurements reminds me of a True Lab Story about a different sort of sensitive measurement made using cold atoms, made during my grad school days. Sadly, this one wasn’t particularly useful…

The standard technique for accumulating large numbers of cold atoms is a thing called a magneto-optical trap, which uses a combination of laser light and magnetic fields to trap a cloud of atoms and confine them in a small region of space. This works by setting up a magnetic field that goes to zero at a specific point in space, and increases as you move away from that point.

Now, the fields involved are actually quite small, which means that the exact position at which the zero occurs is rather sensitive to stray fields. If you put a big chunk of iron next to your vacuum system, odds are, you’ll shift the position. To account for this, most laser cooling experiments have “trim coils” around the trap chamber: large loops of wire used to generate a magnetic field that exactly cancels the stray field in the vicinity of the trap. Part of the morning routine when I was in grad school was to carefully adjust the current in those coils to account for any shifts in the local field (say, from some dumb-ass grad student leaving a magnetic screwdriver on the table next to the chamber). Once that was done, though, it was good for the rest of the day.

Except when it wasn’t… In my fourth year of grad school, we started having problems with the field. We’d be sitting there working, and all of a sudden, the field at the trap would change, and our lovely cloud of atoms would go drifting off to a new location. We could get it back by changing the trim coils, but a few hours later, it would wander off again.

This drove us nuts for a period of weeks. We bought new, more stable power supplies for the trim coils. We checked the coils for short-circuits. We looked for electrical problems in other coils in the system. We were just about ready to rip out the coils and make new ones, when my supervisor went upstairs and found the answer to the problem on the door of a lab:

Magneto-Raman Spectroscopy

That was the sign on the door of the lab immediately above mine. The guys working right upstairs from us had a six Tesla magnet– that’s 120,000 times the strength of the Earth’s magnetic field, give or take. Even at ten meters distance from our trap, that’s an awfully big field.

In fact, they had had a big magnet up there for years, and we had never noticed. That was because their previous magnet took several hours to turn on and off, so they would turn it on, and leave it on continuously for a couple of weeks. Every now and then, we’d need to make a big adjustment to the trim coils, but we worte that off as just one of those things.

The new problem we had suddenly noticed was because they bought a better magnet, that they could turn on and off in about half an hour. Suddenly, they were turning it on in the morning, and off at night, and our fields were changing all the damn time.

Since we could run just fine with their magnet on, provided the magnet stayed on at a constant level, we worked out a deal where they would call us before turning the magnet on. We’d let them know when we finished a data run, they’d start the magnet up, and I’d take a half-hour break. When I came back, the field would be steady, and I could adjust the trim coils to compensate, and resume taking data, until they shut down, when I would take another half-hour break.

It worked really well, except for one angry Russian post-doc. This guy was perpetually in a bad mood, and resented having to call us, so he usually didn’t make the call until the magnet was already starting up. I’d be sitting in the lab, and the results would start to go screwy, and then the phone would ring. I’d pick it up, and hear “Magnet is on!” in his thick Boris-and-Natasha accent, and then he’d slam the receiver down…

I never did figure out what his real name was, but I always thought of him as “Boris,” because of the accent and the unpleasant demeanor.