## Thursday, April 15, 2021

### NSF Workshop on Quantum Engineering Infrastructure

I spent three afternoons this week attending a NSF workshop on Quantum Engineering Infrastructure.  This was based in part on the perceived critical need for shared infrastructure (materials growth, lithographic patterning, deposition, etching, characterization) across large swaths of experimental quantum information sciences, and the fact that the NSF already runs the NNCI, which was the successor of the NNIN.  There will end up being a report generated as a result of the workshop, hopefully steering future efforts.  (I was invited because of this post.)

The workshop was very informative, touching on platforms including superconducting qubits, trapped ions, photonic devices including color centers in diamond/SiC, topological materials, and spin qubits in semiconductors.  Some key themes emerged:

• There are many possible platforms out there for quantum information science, and all of them will require very serious materials development to be ready for prime time.  People forget that our command of silicon comes after thousands of person-years worth of research and process development.  Essentially every platform is in its infancy compared to that.
• There is clearly a tension between the need for exploratory research, trying new processes at the onesy-twosy level, and the requirements for work at larger scale, which needs dedicated process expertise and control at a level not typically possible in a shared university facility.  Everyone also knows that progress is automatically slow if people have to travel off-site to some user facility to do part of their processing.  Some places are well situated - MIT, for example, has an exploratory fab facility here, and a dedicated 200 mm substrate superconducting circuit fab at Lincoln Labs.  Life is extra complicated when running an unusual process in some tool like a PECVD system or an etcher can "season" the gadget, leaving an imprint on subsequent process runs.
• Whoever really figures out how to do wafer-scale heteroepitaxy of single-crystal diamond will either become incredibly rich or will be assassinated by DeBeers.
• Fostering a healthy relationship between industrial materials growers and academic researchers would be very important.  Industrial expertise can be fantastic, but there is not necessarily much economic incentive to work closely with academia compared with large-scale commercial pressures.  There may be a key role for government encouragement or subsidy.
• It's going to be increasingly challenging for new faculty to get started in some research topics at universities - the detailed process knowhow and the need to buildup expertise can be expensive and slow to acquire compared to the timescale of, e.g., promotion to tenure.  An improved network that supports, curates, and communicates process development expertise might be extremely helpful.

## Thursday, April 08, 2021

### "Fireside Chat" about Majoranas

Along with Zeila Zanolli, tomorrow (Friday April 9) I will be serving as a moderator for a "fireside chat" about Majorana fermions being given by Sergey Frolov and Vincent Mourik.   This is being done as a zoom webinar (registration info here), at 11am EDT.   Should be an interesting discussion - about 20 minutes of presentation followed by q & a.

Update:  Here is a youtube link to a version that includes the intro talk piece from the second (April 16) chat, and the Q&A from both the April 9 and April 16 events.  Alas, this edit means that you miss my and Zelia's glittering introduction, but I bet you'll get over it.

## Monday, April 05, 2021

### Place your bets. Muon g-2....

Back in the early 20th century, there was a major advance in physics when people realized that particles like the electron have intrinsic angular momentum, spin, discussed here a bit.  The ratio between the magnetic dipole moment of a particle (think of this like the strength of a little bar magnet directed along the direction of the angular momentum) and the angular momentum is characterized by a dimensionless number, the g-factor.  (Note that for an electron in a solid, the effective g-factor is different, because of the coupling between electron spin and orbital angular momentum, but that's another story.)

For a free electron, the g-factor is a little bit larger than 2, deviating from the nice round number due to contributions of high-order processes.  The idea here is that apparently empty space is not so empty, and there are fluctuating virtual particles of all sorts, the interactions of which with the electron leading to small corrections related to high powers of (m/M), where m is the electron mass and M is the mass of some heavier virtual particle.   The "anomalous" g-factor of the electron has been measured to better than one part in a trillion and is in agreement with theory calculations involving contributions of over 12000 Feynman diagrams, including just corrections due to the Standard Model of particle physics.

A muon is very similar to an electron, but 220 times heavier.  That means that the anomalous g-factor of the muon is a great potential test for new physics, because any contributions from yet-undiscovered particles are larger than the electron case.  Technique-wise, measuring the g-factor for the muon is complicated by the fact that muons aren't stable and each decays into an electron (plus a muon neutrino and an electron antineutrino).  In 2006, a big effort at Brookhaven reported a result (from a data run that ended in 2001) that seems to deviate from Standard Model calculations by around 3 $\sigma$.

The experiment was moved from Brookhaven to Fermilab and reconstituted and improved, and on Wednesday the group will report their latest results from a new, large dataset.  The big question is, will that deviation from Standard Model expectations grow in significance, indicating possible new physics?  Or will the aggregate result be consistent with the Standard Model?   Stay tuned.

UpdateHere is the FNAL page that includes a zoom link to the webinar, which will happen at 10 AM CST on Wednesday, April 7.