CERN demonstrates coherent control of antiproton spin state -- an antiqubit

BASE, which is a CERN initiative to compare the properties of protons and antiprotons, has announced results wherein an antiproton was kept in a coherent oscillation between spin states for 50 seconds. The linked news release describes this as the first antimatter qubit.

That's quite a mouthful, but as a former physicist myself, I'm a bit in awe!

An antiproton is the antimatter counterpart of a proton, one of the particles found within the nucleus of an atom. CERN's particle accelerators are great at producing antimatter from high-energy collisions, but it's very difficult to store it for any length of time because matter and antimatter famously annihilate on contact. In light of that, keeping an antiproton around for 50 seconds at all is a significant undertaking, which BASE accomplish using so-called Penning traps based on electric and magnetic fields.

The reason why qubits are mentioned is that these building blocks of quantum computers are two-level quantum systems (the two levels representing the 1 and 0 of a traditional computer bit). A spin-½ system like a proton or antiproton has two spin states (up and down), hence the antiproton of this experiment was behaving like a qubit by oscillating coherently between those states. In a hypothetical antimatter quantum computer you could have an array of antiprotons as your qubits, using spin-up and spin-down as 1 and 0.

That said, there's no serious prospect of using an antiproton as a qubit in an actual computer. Rather, there are two other reasons this experiment is exciting: firstly, just in practical terms it's a stunning achievement that demonstrates amazing, world-leading advances in the control of antimatter; and secondly, it has really exciting prospects for research into the Standard Model of particle physics.

As I mentioned at the start, the purpose of BASE is to compare the properties of protons and antiprotons, particularly their magnetic moments. The reason that's important is that the Standard Model, which summarises our current understanding of fundamental particles, says that all the properties of the proton and antiproton have absolutely identical magnitudes, differing only by a minus sign. The property that BASE are interested in comparing is the magnetic moment.

In particular, any difference in magnitude between the two magnetic moments, even if it's tiny, would at least partially invalidate the Standard Model and open the door to all kinds of exciting new physics – it would be a hugely significant, genuinely historic discovery. As stated in the pull-out quote, coherent control of antiproton oscillations will allow measurements of the antiproton magnetic moment with 10 to 100 times more precision, and that could be what's needed to discover a discrepancy that begins a new era of fundamental physics.

I'll be keenly watching to see what else comes out of BASE going forward!

“This represents the first antimatter qubit and opens up the prospect of applying the entire set of coherent spectroscopy methods to single matter and antimatter systems in precision experiments,” explains BASE spokesperson Stefan Ulmer. “Most importantly, it will help BASE to perform antiproton moment measurements in future experiments with 10- to 100-fold improved precision.”

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