Consider the vacuum (I)

While I’m sure there will be a lot of chatter around here in the next few weeks about the vacuum (or, God help me, vacua), I feel like I should lay the groundwork by talking about laboratory vacuum. I know I’m here to talk about cold atoms and the hot stuff going in in experimental physics right now, but I’ve spent a lot of time in the last couple days dealing with vacuum, and I want to tell you about it.

If you want to do most sorts of atomic physics experiment, it’s essential that you isolate what you’re interested in studying (atoms) from stuff that will either knock those atoms away, or obscure the physics you’re trying to observe. If, for example, you’re studying cold atoms, you’ve got to suspend the atoms somehow far away from any heat source, which is everything in the laboratory, including this annoying stuff that permeates the place. I suppose you need it to breathe and whatnot, but omnipresent air is really more trouble than it’s worth.

So how serious do we need to be about this? Turns out, very. If you’ve rigged up a chamber with some sort of magnetic or electromagnetic trap to suspend atoms in the middle of it, any stray molecule that hits those trapped atoms will take out at least one as it smashes through. Those stray molecules are in thermal equilibrium with the walls of the chamber, which are at room temperature, and thus moving quite fast. If you start with a sample of trapped atoms (thousands or millions or billions), and you look at how many of them are left as a function of time, you see the number decay exponentially with some time constant T. A sample plot of this here, or the more illustrative log plot here.

While even the Mona Lisa is falling apart, decay is still bad, and in any cold atom experiment we want the characteristic lifetime T due to residual gases in the chamber to be as large as possible.

The kicker is that T, which in those graphs is forty seconds, is inversely proportional to the pressure, and a forty-second lifetime ends up requiring in the neighborhood of 1e-11 torr. For some perspective, atmospheric pressure is 760 torr, orbital space is around 1e-9 torr, and interstellar space (the real stuff) is down around 1e-16 torr, where you end up with hydrogen atoms per cubic meter. So I like to think of the vacuum that I use everyday as being somewhere between Mars and the Oort cloud.

Now, you may think that forty seconds is unnecessary, and in some ways it is; but long lifetimes like that were necessary in the early days of BEC, because it took many tens of seconds to evaporate down to phase transition temperature (and still does, in any lab like mine that uses classic magnetic traps, with large coils and tens of kilowatts of dissipation.) But even the latest and greatest techniques for making BEC are still on the second-ish timescale, so vacuum requirements aren’t ever going to go away, especially if you want to do things with your cold atoms.

So how do we get there, and what are the cool widgets that we need to do it? Vacuum is such a large part of any practicing experimentalist’s life that I know I’m going to ramble, so to save your eyes from completely glazing over, I think I’ll save it for a part II.

And if you’re anywhere near DC today and reading this, go outside! It’s an unreasonably beautiful day. I’ll be in the lab, pretending that I don’t know that.