Sunday, May 09, 2021

Catching up

As may be obvious from my pace of posting, the last couple of weeks have been very busy and intense for multiple reasons.  I hope that once the academic year really ends I can get back into more of a routine.

Two notable stories this week:

  • Two papers were published back-to-back in Science (here and here, with commentary here) that demonstrate (a) that comparatively macroscopic mechanical oscillators - drumheads - can be operated as true quantum objects (cooled down to the point where the thermal energy scale \(k_{\mathrm{B}}T\) is small compared to the quantum energy level spacing \(\hbar \omega\) (this has been done before); and that these resonators can be quantum mechanically entangled, so that the two have to be treated as a single quantum system when understanding measurements performed on each individually.   This can be used, in the case of the second paper, to allow clever measurement schemes that shift measurement back-action (see here for a nice tutorial) away from a target system, enabling precision measurements of the target better than standard quantum limits.  
  • IBM has demonstrated 300 mm wafer fabrication of integrated circuits with features and techniques for the upcoming "2 nm node".  As I've mentioned before, we have fully transitioned to the point where labeling new semiconductor manufacturing targets with a length scale is basically a marketing ploy - the transistors on this wafer do not have 2 nm channel lengths, and the wiring does not have 2 nm lines and spaces.  However, this is a very impressive technical demonstration of wafer-scale success in a number of new approaches, including triple-stacked nanosheet gate-all-around transistors.

Monday, April 26, 2021

Brief items

 As we careen toward the end of the spring semester, here are a few interesting links for perusal:

  • My colleagues at the Rice Center for Quantum Materials are running a mini-workshop this week about topology and correlations in condensed matter.
  • More broadly, there is a new site for all things quantum at Rice.  More news in the coming weeks....
  • Speaking of quantum, I thought that this paper was pretty impressive as a technical achievement.  The authors are able to cool a mechanical resonator (a suspended aluminum drumhead, essentially) down to 500 microKelvin (!), so cold that \(k_{\mathrm{B}}T\) is smaller than the harmonic oscillator energy levels - down to the quantum ground state for its center of mass motion.  As someone who built a nuclear demagnetization stage as part of my PhD, I have to respect achieving that temperature for a sample in vacuum.  Likewise, as someone who studied tunneling two-level systems in solids, it's impressive to see the logarithmic temperature dependence of sound speed in the aluminum extend smoothly down to below 1 mK.  
  • On a more general thermodynamic topic, this paper really surprised me. It's a review article about the existence of a dynamical crossover (the "Frenkel line") that exists above the critical temperature and pressure for a number of fluids - basically a separation into different regimes of response (not true phases per se).  Embarrassingly, I'd never heard of this, and I need to find the time to read up on it.
  • I'm late to the party on this, as it got quite a bit of press, but this paper is really interesting - special engineered light modes that are designed to propagate without distortion (though with attenuation) through scattering media.  There are many potential applications, such as medical imaging (with light or with ultrasound).
  • Anyone want a dinner plate-sized chip with 2.4 trillion transistors

Saturday, April 24, 2021

Lecturer position, Rice Physics & Astronomy

The Department of Physics and Astronomy at Rice University invites applications from recent Ph.D. graduates for a lecturer position in physics and astronomy, commencing July/August 2021.  Familiarity with and/or interest in physics education research, undergraduate teaching at the introductory level, pedagogy, and curricular issues is preferred. This is a non-tenure-track position for a two-year term with the possibility of reappointment for additional three-year terms.  This is a full-time, 9-month academic calendar position.  There would also be opportunities to develop innovative teaching methods and pursue independent research or collaborations with existing research programs (see web page ).  Evaluation of applications will begin May 15 and continue until the position is filled. Applications for this position must be submitted electronically at  Applicants should submit (1) a curriculum vitae, (2) a statement of teaching interests, (3) a statement on diversity and outreach, (4) a list of publications, and (5) the names, affiliations, and email addresses of three professional references.  Applicants must be eligible to work in the U.S. Rice University is committed to a culturally diverse intellectual community. In this spirit, we particularly welcome applications from all genders and members of historically underrepresented groups who exemplify diverse cultural experiences and who are especially qualified to mentor and advise all members of our diverse student population.

Rice University is an Equal Opportunity Employer with a commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We encourage applicants from diverse backgrounds to apply.

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.

Tuesday, March 30, 2021

Amazingly good harmonic oscillators

One way that we judge the "quality" of a harmonic oscillator by how long it takes to ring down.  A truly perfect, lossless harmonic oscillator would ring forever, so that's the limiting ideal.  If you ding a tuning fork, it will oscillate about 1000 times before its energy falls by a factor of around \(\exp(-2\pi) \approx 1/535\).  That means that its quality factor, \(Q\), is about 1000.  (An ideal, lossless harmonic oscillator would have \(Q = \infty\).   In contrast, if you ding the side of a coffee mug, the sound dies out almost immediately - it doesn't seem bell-like at all, because it has a much lower \(Q\), something like 10-50.  The quality is limited by damping, and in a mechanical system this is the lossy frictional process that, in the simplest treatment, acts on the moving parts of the oscillator with a force proportional to the speed of the motion.  That damping can be from air resistance, or in the case of the coffee mug example, it's dominated by "internal friction".

So, how good of a mechanical oscillator can we make?  This paper on the arxiv last night shows a truly remarkable (to me, anyway) example, where \(Q \sim 10^{8}\) in vacuum.  The oscillators in question are nanofabricated (drumhead-like) membranes of silicon nitride, with resonant frequencies of about 300 kHz.  To put this in perspective, if a typical 1 kHz tuning fork had the same product of \(Q\) and frequency, it would take \(3 \times 10^{10}\) seconds, or 950 years, for its energy content to ring down by that 1/535 factor.  The product of \(Q\) and frequency is so high, it should be possible to do quantum mechanics experiments with these resonators at room temperature.  
A relevant ad from a favorite book.

That's impressive, but it's even more so if you know a bit about internal friction in most solids, especially amorphous ones like silicon nitride.  If you made a similar design out of ordinary silicon dioxide glass, it would have a \(Q\) at room temperature of maybe 1000.  About 15 years ago, it was discovered that there is something special about silicon nitride, so that when it is stretched into a state of high tensile stress, its internal friction falls dramatically.  This actually shows a failure of the widely used tunneling two-level system model for glasses.  The investigators in the present work have taken this to a new extreme, and it could really pave the way for some very exciting work in mechanical devices operating in the quantum regime.  

update:  In resonators made from silicon nitride beams with specially engineered clamping geometries, you can do even better.  How about the equivalent of a guitar string that takes 30000 years to ring down?  “Listen to that sustain!