Search for CPT violation with protons/antiprotons using quantum logic

[Twigg]

TL;DR Summary

Found a cool proposal on the arxiv from the BASE collaboration. Put a proton in a Penning trap with a beryllium ion. Let the proton spin precess for a while. SWAP the proton spin state with the beryllium spin state a la quantum logic techniques. Readout the beryllium state. Get a g-2 measurement. Repeat the process for an antiproton. Look for CPT violation.

Just wanted to share a cool proposal paper from the BASE collaboration. I found this article dense and the theoretical aspects are way above my pay grade, so please chime in if you think I get anything wrong. For the record, I have no ties to this group and hadn't heard of them before this paper.

Why:
Perform g-2 measurements for protons and anti-protons sourced from CERN. Any difference in the g-2 measurements (protons vs anti-protons) would be a deviation from the Standard Model.

What:
Load a proton (or anti-proton) in a Penning trap. Cool it down with a combination of resistive and sympathetic cooling with a co-trapped beryllium ion (which can be laser cooled). Use quantum logic techniques (3 SWAP gates, each with a very different implementation) to exchange the spin state of the proton (antiproton) and the beryllium ion. Read out the spin state of the beryllium with good ol' laser spectroscopy. A previous paper from this group shows numerical simulations for the state exchange with 1% error rate (it will surely be higher in a real implementation).

How:
Check out figure 2 in the paper. The top panel of the figure is a cartoon of the trap layout. There are 4 stages, starting from the right and propagating to the left. I'm not sure where cooling takes place on this figure. The first stage is for precession, and the last 3 are for quantum logic and readout.

When:
I imagine they'll be ready to start taking data in something like 4-8 years. This is one heck of a build they're talking about. There are a lot of processes that need to be characterized. One of the oft-overlooked difficulties of multi-trap systems is their sheer mechanical complexity. I count 28 precision machined electrodes in the cartoon, plus who knows how many fasteners, electrical contacts, etc. That's hundreds of failure points. And remember that fixing anyone of these means breaking vacuum and having to do a vacuum bake (at least a week of just slow-cooking the chamber). This doesn't include magnetic shielding, or any cryogenic shields for blackbody effects (not clear to me if that will be necessary or not).

Overall, I find this a really exciting proposal, and these folks have a lot of work to do (which is normal, in precision measurement). Part of what I like is that it showcases a lot of advanced techniques developed over the last two decades. This is definitely a complicated measurement.

 

[Twigg]

Figured I'd add a comment on a technical aspect that might not be clear: Why do they use 3 SWAP gates if they're just exchanging the spin state of the proton (or antiproton) onto the beryllium ion? It's because you can't get the ion and the proton close enough for their spins to interact directly due to Coulomb repulsion. Spins interact via dipole fields, so they go as $\frac{1}{r^3}$, whereas the coulomb potential goes as $\frac{1}{r}$ (much stronger at long range). Even if you could get them close enough, it'd be a highly uncontrolled process and you'd be extremely like to see your ions and/or protons skedaddle out of the trap. So instead, they SWAP the proton's spin state with it's motional state, then SWAP the proton's motional state with the beryllium's motional state via coulomb interaction, then SWAP the beryllium's motional state with its spin state with a sideband-resolved laser pulse. It's just a detour.

Sorry, I'm totally nerding out over this.