If you walked into my atomic physics lab at the University of California Berkeley two weeks ago, you would have found me in a pensive pose. Left hand nestled inside my right elbow, right hand clasping my chin, with an intensely furrowed brow, I was staring off into space while leaning up against the counter. I spend a lot of time like this these days. I spend a lot of time in the dark. I mean that proverbially, but the lab I was standing in was also rather dimly lit to avoid disturbing my light-sensitive sample. I was standing in the shadow of my experiment, an SUV-sized mess of cables and lasers and mysterious boxes. There was a problem somewhere in there. The machine wasn’t working as it should — something in that morass of cables and sensors was different than it was the day before. And I had to find it.
I’m an Atomic Physics PhD student. That essentially means I orchestrate an army of lasers and electrical circuits to freeze the motion of atoms and put their quantum properties to work. If my research were easy, I’d already have used them to try to uncover an elusive substance no physicist has yet observed directly: dark matter.
Now, I would love to tell you that the reason I so often search for answers by staring off into space with a furrowed brow is because I’ve stumbled on some intriguing dark matter data, the kind that brings me closer to deep insights into what dark matter is and how it behaves. Alas, I can’t claim such a lucky position (yet!).
The machine I’m building is called an atom interferometer. It sits on a large metal table, pock-marked with screw-holes that I use to fix components to the table. There are lasers at the corners of the table, mirrors, lenses, and a jungle of optical fibers that guide the cherry red laser to where I need it to go. Vibrating crystals and homebuilt circuits fed by beefy power supplies make sure the lasers operate at exactly the right wavelength, and a whopping steel vacuum chamber shaped like a starfish houses my delicate sample. When this is choreographed perfectly, the machine turns atoms inside the vacuum chamber into a sensor — one of the most precise sensors in the world.
The problem is that it is rarely choreographed perfectly. That’s the real reason I was gazing off into space on that day two weeks ago, chin in hand. The success I’d had the day before had given way to chaos — my interferometer had stopped working — and I had to figure out why. In all honesty, that’s what a typical day in my life as an atomic physicist looks like, clawing to stay one step ahead of every next malfunctioning component.
For the prior few days, everything had seemed to be going smoothly. My work aims to sense the effects of dark matter using atoms. (I say “aims” because I haven’t actually done it yet — it’s really hard.) Atoms at room temperature move faster than the speed of sound, so I need to slow them down (cool their temperature) so they’ll sit still long enough for me to perform a measurement. That’s the function of most of my lasers. As the lasers pelt the atoms, they slow them down to nearly absolute zero temperature (the absence of all heat), trapping them in something called a Magneto-Optical Trap (MOT). The cloud of atoms within the MOT, when happy, looks like a bright red orb levitating at the center of the chamber. A month ago, I decided to replace one of my older lasers, which had been on its last legs for more than a year. I soldered its connections carefully to prevent static discharges from destroying the laser, installed it into its box, drilled a hole in the side of the box to let the beam out because it was now pointing in a new direction, and began aligning it to slow and trap my atoms. A brighter shining orb means there are more atoms inside, and within a week with the new laser, I was trapping the brightest red orb I had ever seen in my life — twice as many atoms as my experiment had ever held at once! I started tuning up the next stages of the experiment over the following days, working to re-assemble the full sensor that I’d had before replacing the laser.
Then I rolled into lab two weeks ago, optimistic about a few ideas to improve my signal. I glanced expectantly into the steel vacuum chamber to greet that beautifully bright orb I’d already grown accustomed to. But it wasn’t there. It was gone. In its place floated a pitifully dim red puff. I checked all the usual finicky components, but nothing looked out of the ordinary. The lasers were powerful enough. The magnetic field, by all metrics, switched to the right strength at the right time. For two weeks I checked and rechecked all my assumptions. Finally, today, my colleague found an unnecessary, nefarious electrical connection. Once the connection was broken, we recovered that cathartically brilliant red cloud of atoms in a matter of hours. I will sleep better tonight than I have in days.
That’s the way of experimental physics: to inch closer and closer to a functioning specialized machine that can test a hypothesis. Often I take one step forward and two steps back. However, it’s the awesome days when I take a huge leap forward that keep me going. Uncovering the mysteries of dark matter, however, requires about a thousand steps forward. So I’ve dropped in for the long haul. Despite the inevitable feeling of being held back at every turn, I have to remain grateful for what I learn along the way.