Energy storage in the internal states of molecules - old and new

A science story first, then a US research ecosystem story later.

When we think about using molecules to store energy, it's usually in the context of food or fuel, so that chemical reactions take place - bonds are broken and remade, and in an exothermic reaction, the products end up with more kinetic energy (center of mass motion, molecular vibrations and rotations) than the initial reactants.  However, there are other ways that molecules can store energy.  I read about a cool example of this last week, but first I want to give tell you an old and very quantum mechanical story that I learned about in grad school when I did very low temperature physics.

Diatomic hydrogen, H2, is the simplest molecule there is, just two electrons and two protons.  Roughly speaking, the \(1s\) orbitals of the H atoms hybridize to form \(\sigma\) bonding and \(\sigma*\) antibonding molecular orbitals.  The lowest electronic state is the two electrons in a spin singlet, \((1/\sqrt{2})(|\uparrow \downarrow\rangle - |\downarrow \uparrow\rangle)\) in the \(\sigma\) molecular orbital.  Remember, the electrons are fermions, so the electronic wavefunction has to be antisymmetric (pick up a minus sign) under exchange of the electrons. The spin singlet is antisymmetric under exchange, the \(\sigma\) orbital is spatially symmetric under exchange, so the full electronic wavefunction (product of the spin and spatial components) is appropriately antisymmetric.  

That's not all there is to it, though, as explained thoroughly here.  The protons (while being made up of quarks and gluons, etc.) are (composite) fermions, so we have to think about the quantum wavefunction that describes them, too.  There are two possibilities.  In the "para" configuration, the proton spins are in a singlet (antisymmetric), meaning that the spatial wavefunction of the protons must be symmetric under exchange.  The spatial state of the bound protons can have some orbital angular momentum \(\mathbf{L}\), and the simplest, lowest energy situation is with quantum numbers \(\ell =0\) and therefore \(m_{\ell} = 0\).  In contrast, in the "ortho" configuration, the proton spins form a triplet state (symmetric under exchange), meaning that the spatial wavefunction must be antisymmetric, \(\ell = 1\).  Approximating the H2 molecule as a rigid barbell-like rotor with some moment of inertia \(I\), then ortho molecule has a rotational energy \(\hbar^2/2I\) larger than the para case.  That works out to about 15 meV of energy per molecule.  So, para-hydrogen is the true ground state.  It turns out that the ortho/para spin isomer energy difference makes liquefying hydrogen a challenge, since the latent heat of vaporization for H2 is only 9.4 meV.  That is, every time an ortho-hydrogen molecule converts to para-hydrogen through some collisional process, it releases enough energy to kick a hydrogen molecule out of the liquid.  I learned about this in my thesis work playing around at ~ 1 mK temperatures - any H2 adsorbed or otherwise stuck in the apparatus could result in detectable long-term heating effect as it slowly converted from ortho to para.  Bottom line:  Energy can be stored in the internal states of molecules.

From Fig. 1 of this paper.

This seems very esoteric, but the idea of storing energy in some internal state of a molecule for later release shows up elsewhere.  Last week, in this article in Science, for example, the authors report a molecule inspired by aspects of DNA that can be put via UV exposure into a distorted form ("Dewar isomer") where it is metastable at room temperature (half-life of 481 days).  It can be induced to pop back into the undistorted isomer by heat, acid exposure, or via a catalyst, and when it does, each molecule releases the stored energy (2.36 eV per molecule!) into vibrations and rotations that heat its surroundings.  The stored energy density in this stuff is about 4% of the releasable energy density of gasoline, which is not too shabby.  The authors propose a system where exposure to sunlight can store energy in the molecules, and this can later be released on demand via catalyst.  They demonstrated that the heat release from enough dissolved molecules can readily boil water.  Very neat stuff.

News items and essays to read

First, some inside-baseball US funding discussion.  Apologies to my international readers, who likely don't care much about this except in the abstract.

• Breaking news:  According to journalist Dan Garisto, as of April 25, 2026, the president has fired the entire National Science Board.  The NSB helps oversee the National Science Foundation.   From the outside, it had sure looked to me like the NSB had tried verrrrrrry hard to stay on the administration's good side.  That was fraying recently, as this February article in Science included comments implying that not everyone was thrilled with the executive branch strongly shifting NSF priorities.  It sure looks like their reward for not speaking out strongly about the importance of continued support for research was apparently to be terminated with prejudice.  If any readers have first-hand knowledge of what happened, please post in the comments.
• I had already been planning to point out this article in Nature, which says that NSF funding may finally be about to flow, after a long, murky back-and-forth between the agency, Congress, and OMB.  It's worth noting that Congress had stated in their guidance that no directorate within NSF should be cut by more than 5%, while OMB has mandated (apparently) different spending levels which, among other things, would cut Mathematics and Physical Sciences by 15% and Engineering by 18%.  It sure doesn't look obvious to me, with everything else going on right now, that Congress is willing to truly push back on this.  Surrendering microscopic spending authority to OMB seems like a complete abdication of congressional authority, but what do I know.

One bit of science:

• I participated in a virtual workshop this week, Targeted Questions: Strange Metals.  I have written about strange metals before (see here and here).  The workshop was streamed, including the discussion sessions, and here is the youtube link if you are interested.

Some essays this week:

• There was a report by a Yale University committee about the erosion of trust in higher education in the US earlier this month.  It certainly spurred a lot of conversation, and it raises many important issues.  That said, it does seem to downplay the fact that there has been a decades-long campaign by some with the goal of eroding trust in higher education, as pointed out in this essay.
• A friend pointed me to this essay by Santiago Schnell about higher education in the era of AI - I found it very thoughtful, even though I don't agree with every aspect.
• Speaking of AI, this essay ("The future of everything is lies, I guess: Bullshit about bullshit machines") by Kyle Kingsbury was provocative and worth reading, again even if I don't agree with every aspect.
• Speaking of thoughtful commentary, here is an essay by a billionaire that is worth reading.

Floating magnets to sense magnetic fields

We've all seen a traditional compass.  A ferromagnetic, magnetized needle is mounted on a rotating bearing (or floated on the surface of a liquid) so that it can rotate in the \(x-y\) plane.  If there is an in-plane magnetic field \(\mathbf{B}\), the needle will rotate to align with that component of the field.  (It stops in the aligned state because of friction; otherwise it would "librate", oscillating back and forth about the field direction.)  In first-year undergrad physics, we learn a simple model of why this happens.  The magnetized needle can be modeled as a magnetic dipole \(\mathbf{m}\).  We learn that a magnetic dipole in a uniform magnetic field generates a torque \(\boldsymbol{\tau} = \mathbf{m}\times \mathbf{B}\).  If both \(\mathbf{m}\) and \(\mathbf{B}\) are in the \(x-y\) plane, any torque must be directed along \(z\), and the torque goes to zero when \(\mathbf{m} || \mathbf{B}\).  The simplest result of \(\boldsymbol{\tau} || z\) is an angular acceleration that would cause an otherwise at-rest compass needle to rotate in the plane counterclockwise about the \(z\) axis. 

Left: A compass needle out of alignment with an in-plane 
magnetic field produces a torque along \(z\), and angularly
accelerates to reorient toward the field . Right: Under the 
right circumstances, the spin angular momentum of the 
needle could also precess around the field.

So, how sensitive of a magnetic field detector could you make using this basic approach?  This week I came upon this paper from a few years ago, which looks at this problem from the theoretical modeling side, and then this paper from last year that does the experiment.  The magnet in question is a little (21 \(\mu\)m diameter) sphere of Nd2Fe14B, a rare-earth magnet.  The authors put that inside a lead chamber with a rounded bottom, and they cool the lead down to 4.2 K, well below its superconducting transition temperature.  As a result, the sphere is magnetically levitated inside thanks to the Meissner effect, with its magnetization lying in the \(x-y\) plane.  There is some residual magnetic flux trapped in the setup that does lead to a preferred field direction.  The authors can use cleverly wound pickup coils inside the chamber to detect the orientation of the sphere, as well as apply AC magnetic fields.  The authors are primarily concerned in thinking about energy resolution of detection, because they are thinking about detecting unusual particles (e.g. dark matter, axions), but they point out that it should be possible to achieve tens of atto-Tesla per Hz\(^{1/2}\) field sensitivity per unit bandwidth - pretty wild.

But wait, there's more!  The magnetic moment of the magnetized needle originates from the spins of electrons in there.  This is gyromagnetism, so \(\mathbf{m} \propto \mathbf{S}\), the total spin angular momentum of the electrons in the magnet.  This means that in the presence of \(\boldsymbol{\tau} || z\), if mechanically possible the needle could start swinging up out of the \(x-y\) plane to project a component of \(\mathbf{S}\) along \(z\).  This is gyroscopic precession.  For macroscopic magnets, it's hard to be in the regime where this is the dominant effect, because that would require the precessional angular momentum to be small compared to \(\mathbf{S}\), and that's tough to achieve.  Maxwell (!) tried to do it in 1861 (!!), with no success.

In a very recent paper, this precessional response was finally observed, again in Nd2Fe14B microspheres.  (For a uniformly magnetized sphere of radius \(R\), the moment of inertia \(I \propto R^{5}\), and \(|\mathbf{S}| \propto R^{3}\), so it's easier to get into the precessional regime with smaller \(R\).)  This precession approach is a pathway to even higher sensitivity measurements of magnetic fields.

I think this is very cool, and it is a strong reminder that spin angular momentum is just as real as any "mechanical rotation of solids" angular momentum.  

Disorder and illumination

No, this is not another grim post about the chaotic US research funding environment.  Instead I wanted to write a bit about a good example of empiricism in experimental condensed matter physics, the use of illumination to (somewhat but not entirely mysteriously) improve electronic transport in 2D electronic systems.  

This story goes back decades, and it's all about the role of "disorder" and its effects on electronic conduction.  It's been appreciated since the 1930s that, at low temperatures so that lattice vibrations are frozen out, conduction in ordinary crystalline metals and semiconductors is limited by the charge carriers (let's work with electrons rather than holes to make discussion simpler) scattering from disorder, deviations from an infinite periodic crystal lattice.  Grain boundaries, vacancies, impurities - these all can scatter electrons that would otherwise propagate ballistically through the material, and this is often modeled as a "disorder potential", a spatially varying potential energy \(V(\mathbf{r})\).  If you want the best transport properties (longest elastic mean free path), you want \(V(\mathbf{r})\) to be small in magnitude and as smooth as possible.  This is even more important if you want to study some delicate many-body state that is expected to arise at very low temperatures - you need the disorder potential to be small compared to the energy scale of that state to avoid messing it up.

In semiconductors, where the carrier density is low and screening is therefore not as good, charged defects are particularly effective at scattering.  In modulation doping, the dopants that are the source of the charge carriers in some nearby semiconductor 2D interface or quantum well are spatially distant from where the current is going to be flowing, to minimize the scattering from those ionized donors.  

For decades it has been known that, to get the best transport properties in GaAs-based (and other) semiconductor structures, it can be good to illuminate the devices at cryogenic temperatures with a red LED.  See, for example, this paper trying to explore the upper limits of charge mobility in GaAs 2D electron gas (2DEG), where the authors say "For measurement, our samples are loaded into a 3He cryostat, where a red light-emitting diode (LED) is used to illuminate the samples for 5 min at \(T \sim \) 10 K. Following illumination, we wait for 30 min at \(T  \sim \) 10 K after the LED has been turned off before resuming the cool down to base temperature."   The qualitative explanation for this is that the photons provide enough energy to excite charge carriers, and those mobile carriers can occupy trap states, rearrange themselves, and generally set up a better screened disorder potential.  In GaAs 2DEG, the result is higher mobilities (as inferred from conductivity + Hall effect) and much cleaner fractional quantum Hall effect data, showing that the post-illumination disorder is now sufficiently weak that more delicate states can form - see Fig. 1 of this paper (arXiv version) for the before/after.  As far as I know, there is not a deep, rigorous theory of how this works, but it is known empirically.  

Fig. 2 from this paper, showing electronic magnetotransport

before/after illumination by a UV LED at low temperatures.

This preprint on the arXiv this week shows that a similar improvement in transport properties can be found in structures where graphene is encapsulated by hexagonal boron nitride (hBN).  Sandwiching graphene and other 2D materials in hBN has been known since 2010 as a way of drastically improving the charge disorder situation compared to just putting 2D materials on top of SiO2.  (That paper has 8800+ citations on google scholar btw.)  Now, it is shown that if you shine 5 eV photons (deep UV = 248 nm wavelength) on such a sandwiched structure, the already-good charge environment can become even better.  Even though that energy scale is below the band gap of hBN, the light is able to kick enough charges around to smooth out some residual disorder.  Very cool.  

FY27 Presidential budget request

To the surprise of no one at all, the 2027 presidential budget request is extremely bad for science.  Remember, this is largely a political document, and Congress does not have to follow this.  In the past year, Congress largely ignored the recommendations and appropriated a much flatter budget (though agency priorities are still set by the PBR for executive agencies).  This new request shows that Vought et al. still would prefer to kill much public funding for science.

• NSF: request to cut from $8.8B (FY26 enacted) to $4B, a 56% cut that would eviscerate the agency.
• DOE: request to cut $1.1B from $8.4B (FY26 enacted) Office of Science budget.
• NASA: request to cut $5.6B from $24.4B (FY26 enacted), including $3.7B from science programs and $1.1B from the ISS.
• Commerce: This one shocks me. Request to cut $993M from NIST's $1.184B (FY26 enacted) budget. That would be an 84% cut (!!), seemingly destroying NIST. This needs to get headlines.  Either the people making this recommendation have no idea what NIST does (seems plausible), or someone has a personal grudge against the standard kilogram. Update:  Dan Garisto on bluesky points out that the enormous cut topline number is not consistent with the budget appendix, which implies a much smaller cut.  Unclear what the answer is here - it'd be quite a goof to have a topline number that far off.
• NIH: Proposed $5.5B cut from $47.2B (FY26 enacted).
• DOD: It's very hard to tell, especially since they're proposing hundreds of billions of dollars in additional spending including for missile defense. The proposed DOD increases vastly outweigh the cuts described above. 

These cuts are proposed despite constant fretting that China is surpassing the US scientifically. This past year it took aggressive lobbying to ensure that Congress pushed back against these kinds of cuts. For those who favor continued public investment in science and engineering research in the US, the task of arguing against these kinds of cuts begins again now.  As I've written before, this is a marathon not a sprint, and this will be an annual exercise under this administration.

Brief science items and news

As I distract myself from work-related writing that I really need to do, here are a few science-related links:

• This article in ars technica is about this paper, in which a quantum interference experiment is performed that, in the most straightforward interpretation, involves superpositions of states where "event A preceded event B" and "event B preceded event A".  This is in the same mind-zapping vein as quantum eraser experiments.  I haven't read this in detail, but my typical takeaway from these things is two-fold:  (1) the math of the usual quantum mechanics formalism is in excellent agreement with experiment every time it's been checked, and (2) nature doesn't care about our hang-ups about interpretation, especially when phrasing questions about quantum in terms of, "what classical past history should have happened here, when we weren't actually making observations?"
• This paper in Advanced Materials is a very nice overview about application-relevant magnetic materials (e.g., rare-earth magnets like Nd2Fe14B) and where we might want to look to find new ones.  
• This youtube video is a great discussion and experimental proof of the Feynman inverse sprinkler problem.  The short version:  suppose an ordinary water sprinkler rotates clockwise when it's spewing water out of its arms, the momentum flux of the water generating a torque.  If instead you immersed the sprinkler in water and had suction bring water in through the arms, which way would it spin, if it spins at all?  I don't want to spoil it for you.  Watch the video - it's very nicely done.
• I'm not sure the link will work for everyone, but there is an article in The Atlantic called "The Shocking Speed of China's Scientific Rise".  For anyone paying the slightest bit of attention for the last decade, this must be some new definition of "shocking" with which I was previously unfamiliar.  
• Info is beginning to trickle out about the likely presidential budget request for FY27, and guess what:  some of the same folks who freak out that China might surpass the US as a science power are again going to pitch big budget cuts (article is about NIH, but it's hard to imagine other agencies won't be similarly viewed).  Congress largely pushed back against the PBR this past year, but this is an annual exercise in the US - it never stops.

AI and the practice of theoretical physics

Matthew Schwartz of Harvard has made a big recent splash, between his public Aspen talk "10000 Einsteins" a year ago about the role of AI and the future of physics, and his talk last week at the APS Global Physics summit on the same topic, and now with this essay, "Vibe Physics:  The AI Grad Student", on the website of Anthropic (producers of the AI tool Claude).

The essay talks about how Prof. Schwartz used Claude to write this paper, and he states that the AI tool functions roughly like a 2nd year grad student (one who also doesn't get tired or complain, but does need close checking and supervision).  The claim is that with this approach to doing calculations and writing papers, he was able to come out with a piece of work that would've taken literally ten times longer if done by working with a human student.  Note that he's not exactly unbiased, and he concludes his essay (on anthropic's site) saying you should spend the $20/month Claude subscription fee and it will change your life.  

There is no doubt that AI tools can speed up certain kinds of work, and there is a every hope that applying this in science will lead to increased pace of progress.  That said, right now these tools are (unsurprisingly) best at working in areas that are well-known and explored - one of my colleagues has tried applying these to really underexplored higher dimensional problems, and they're much less effective there.  The essay's claim that "LLMs are profoundly creative" is provocative.   There is also no discussion here about the cost of these tools, in financial, energy, and environmental terms.  

Still, Schwartz raises many questions about the future of the field and graduate education in general.   (His paragraph about how human beings will still be needed in science for getting experimental data, at least for a while, is really something.)  University research is not just about answering scholarly questions; it's about educating people.  Maybe some faculty will revel in writing papers without that kind of interaction, but somehow I don't think we're quite at the stage yet where we don't need to worry anymore about training experts in technical fields.  I do agree that it's good advice for everyone to pay close attention to where these capabilities are going.  We certainly live in interesting times.

APS March Meeting 2026, Day 4 and wrap-up

Since I headed home early this afternoon, I was only able to go to a couple of talks this morning.  Here are those highlights, and a couple of general observations about the meeting.

• Piers Coleman gave a very interesting talk that put me onto an experimental puzzle I'm sorry to say I had not seen previously.  Some context:  It is now well-established that one can do spin-polarized scanning tunneling microscopy, which (given certain constraints) can image magnetic contrast in conductors down to the atomic scale.  The mechanism is basically the same as tunneling magnetoresistance:  there is a difference in the density of states for spin-up and spin-down electrons, and so a spin-polarized (magnetic) tip results in a tunneling current into/out of a magnetic sample that depends on the local magnetization.  That is, the sign of the current doesn't affect the sign of the magnetic contrast.  I had missed this 2022 Science paper, where instead of a magnetic tip, the investigators used a tip made from a nanowire of SmB6.  That peculiar material is widely (though not universally) viewed as a topological Kondo insulator that can host special surface states in which the spin direction is locked to the current direction.  With that tip, they see magnetic contrast (!) that flips sign with the sign of the current (!!), which is at least hand-wavingly what you'd expect if the direction of the tunneling electron's spin is tied to the current direction.  A more recent paper does something similar with a (BiBr)4 tip (another topologically nontrivial material).  In the talk and related paper, the argument is made that something special happens to the surface states (the effect in SmB6 turns on below about 10 K) and that this tied to the condensed matter analog of axion physics.
• On a completely different note, I saw a talk by John Davis about a new, clever kind of continuously running refrigerator that has a base temperature of around 500 mK and uses only a couple of gas liters of 3He.  One can pump on liquid 3He and get down to about 270 mK in one-shot mode, or about 450 mK if recondensing the 3He gas with a heat exchanger to get continuous operation, but 3He is very expensive.  The new design works with a mixture that's mostly 4He.  After condensing, pumping on this can cool it sufficiently that the 3He phase separates and rises to the top of the liquid, and then the 3He can be preferentially pumped (and recirculated back in).  Very cute.
• Tangentially, one nice feature of conferences is that you can stumble upon facts you didn't know.  For example, during that talk, Prof. Davis mentioned, off-handedly, that in 2D turbulence as studied in things like helium films, you can end up with long-time persistent vortices, and that this is similar to how cyclonic storms persist for centuries on Jupiter.
• Regarding the meeting in general, the APS is aware that there were some AV issues, including some of the rooms having 50" monitors rather than projectors.  This was a surprise to the organizers.  I'm still not sure how much I like the merger of the March and April meetings into one super-meeting.  On the plus side, there are opportunities for cross-over events (e.g., the Kavli symposium, which I didn't see this time), and there are some financial benefits to the society via economies of scale.  Still, 14,000 attendees makes things unwieldy for sure.
• I don't understand some of the choices re the meeting website and the meeting app.  For example, people can upload their slides and make them available.  However, on the meeting website, even when you're logged in, there's apparently no way to get to them.  You can only find the files using the APS meetings app, and even then it's not trivial.  

For those at the meeting the rest of today and Friday, if there are big stories that I missed because of my travel, please feel free to discuss in the comments.

APS March Meeting 2026, Day 3

It was another eclectic day at the APS Global Physics Summit.  Here is a selection of highlights based on my stochastic sampling of talks.

• I've written before about CISS (the chirality-induced spin selection effect).  Joe Subotnik gave a neat invited talk related to this, based on something I'd never really considered.  In physics we learn about the Born-Oppenheimer approximation, which basically says that electrons are fast and nuclei are slow, so we can often solve electronic problems without worrying about nuclear motion.  In practice, as usually done, B-O theory does not strictly conserve momentum or angular momentum, so it cannot explain something like the Einstein-de Haas effect, where flipping electronic spins eventually results in actual mechanical rotation of a solid.  Similarly, ordinary Marcus theory of electron transfer doesn't worry about angular momentum conservation.   The talk focused on a recent approach (and here) that looks carefully at wavefunctions, involves the equivalent of Berry phase and quantum geometry and recaptures the key physics, and this may explain CISS.
• Javad Shabani presented his group's recent work on growing epitaxial layers of germanium substitutionally doped with gallium, at carrier densities around \(5 \times 10^{21}\) carriers per cc, basically around 1 Ga atom in each 8-atom unit cell.   This hole-dopes the material enormously.  The resulting films superconduct with a \(T_{c}\sim\) 3 K and good critical fields, and look very nice structurally.  This is potentially a route toward creating arrays of millions of epitaxially nice Josephson junctions.
• I attended the AI Town Hall, which featured Hal Finkel from DOE talking about the Genesis Mission; Rachel Burley, chief publication officer of the APS, speaking about the challenges that AI presents to all facets of journals and scientific publishing; and Sarah Demers, chair of the physics department at Yale and chair of the APS's Panel on Public Affairs, discussing the community's effort to formulate an enduring position on physics and AI in this rapidly changing landscape.  
• In the last session of the day, I attended the DCMP prize session, and it was very interesting to hear from this year's Buckley Prize winners about their journeys and what they've been doing lately.  

In addition to a lot of fun conversations, I popped in and out of a few other talks, and apologies for not covering everything.

APS March Meeting 2026, Day 2

Today was again a bit random, as I had talks for both one of my students and me, and meetings with folks.  Some highlights:

• Edoardo Baldini gave a very nice talk about exotic phases and collective excitations in van der Waals magnets.  This included using second harmonic generation microscopy and polarimetry to look at the evolution of magnetic phases in NiPS3 as a function of thickness, ending up at the monolayer which acts like a 2D XY magnet.  In the paper, they see clear evidence of a BKT transition, plus a second lower temperature ordering of some kind.
• After some AV issues (seem like quite a few of those this year), Barry Zink gave an interesting presentation about using Cr as a spin detector in spin Seebeck measurements on YIG, and looking at how the antiferromagnetism of the Cr affects the measurement (see here).  In new results, they have been adding in an intervening layer of antiferromagnetic IrMn, looking at how magnons in the IrMn affect the results.
• This was followed by a talk by Romain LeBrun all about spintronics in the GHz and THz, using hematite (Fe2O3) as an example antiferromagnetic insulator.  They see some interesting nonlinear dynamics and rectification in antiferromagnetic resonance in Fe2O3.  In NiO, they use optical excitation to drive coherent phonons, exciting a spin current, which then leads to a THz pulse when the spin current hits an inverse spin Hall detector (Pt or W).  Similar experiments in BiFeO3 show that THz generation in that multiferroic system can arise just from oscillating the ferroelectric polarization.
• Andrew Dane from IBM gave a great presentation to a more-than-packed room about their recent studies of two-level systems in qubits.  From waaaaay in the back, I learned about their use of nearby suspended electrodes to apply electric fields to try to shift the energies of some of the TLS (the ones with electric dipole moments and at the surface of the devices).  TLS drastically suppress the coherence of superconducting qubits, and understanding their origins and ways to work around them (to characterize fab processes, for example) is very important.  As I said in that post linked above, once again we see that TLS are everywhere, and they are evil.  I need to think about whether there's anything I could contribute on this.  The real highlight of the talk was the use of "percussive maintenance" (banging the side of the cryostat) to alter the not-field-tunable TLS distribution via some unknown mechanism.
• Bonking the experiment was taken to a new level in this talk about mechanoluminescence, which involved shooting the sample with an airsoft pellet gun under controlled conditions.

There were other talks as well - some fun stuff.  I also want to give a shoutout to the free-to-play vintage arcade games in the exhibit hall.  Galaga and the stand-up vector graphics Star Wars game were great consumers of my time and my quarters back in the day.

APS March Meeting 2026, Day 1

I hit a pretty random assortment of talks on my first day at the APS Global Physics Summit, after catching a very early flight to get to Denver.  Here are a few highlights:

• My colleague Hanyu Zhu gave a nice talk about the coupling between chiral phonons (vibrational excitations of atomic motion that carry net orbital angular momentum) and their coupling to electronic spins.  For example, chiral ionic motion can effectively generate enormous local magnetic fields (see here).
• I went to part of a session about magnons (quantized spin waves) and their connection to quantum information.  There was a theory talk by Silvia Viola Kusminskiy about cavity manipulation of magnons, and there was an experimental talk by Mathias Weiler about using surface acoustic waves plus magnetoelastic coupling to set up all sorts of interesting nonreciprocal magnetoacoustic devices. 
• My former postdoc Longji Cui gave a talk about molecular phononics - measuring thermal transport (by phonons) down to the single molecule level.  For a nice review of the overall topic, see here.  He then discussed extending this to measurements of polymers.
• There was a session about strange metals and the cuprates, which included a talk by Dragana Popovic about how there is evidence for persistent vortex-liquid-like phase fluctuations in these materials even into the normal state.  This was followed by Nigel Hussey showing systematic studies of the magnetoresistance in both electron-doped and hole-doped cuprates.  The upshot is that in the electron-doped materials, there is a clear anisotropic inelastic scattering rate (from spin fluctuations) that scales with the superconducting transition, implying that spin fluctuations are the "glue".  In contrast, the hole-doped system has different systematics, implying that perhaps the strange metal fraction of material is what leads to superconductivity.
• For maybe the second time in my long attendance at the meeting, I attended the APS prize session, where they present the certificates associated with the various honors.  It was very nice.

Now I just need to get some sleep and figure out what to see tomorrow....

Some science leading into the APS Global Physics Summit

Next week is the annual APS conference that was once the March Meeting and is now the combined March/April "Global Physics Summit".  As I've done annually, I will try to give some impressions of interesting talks that I see, hopefully at an understandable level.  This year I'm only there from Monday through late Thursday morning, so I may miss exciting things - hopefully people will still discuss such things here as has happened in past years.

A few science tidbits in the meantime:

• People sometimes time arXiv submissions to coincide with the APS meeting, and sometimes it's just coincidence.  Two preprints (here and here) popped up very recently, both experiments on interferometry and braiding of anyons in bilayer graphene.  There are many subtleties in such experiments.  The colorized electron microscope images of the devices show how sophisticated fabrication has become in these systems, where very small amounts of disorder can disrupt the fragile many-body quantum states of interest.
• On a much more classical physics note, this preprint uses some sophisticated multiscale modeling to address the question, why is ice so slippery?  A super-thin layer of water on the surface of the ice under sliding conditions is crucial, and the roles of frictional heating and heat transfer have been tricky to quantify.
• Meanwhile, across town from me at the University of Houston, Paul Chu and company have published this paper in PNAS, where they have demonstrated ambient pressure superconductivity in a mercury-based cuprate at 151 K, breaking the old ambient pressure record by 18 K (!).  The trick here has been pressure annealing.  Many superconductors, particularly the cuprates, tend to have higher transition temperatures at elevated pressures.  One idea is that pressure distortion of certain bond angles favors superconductivity in this system, and Chu et al. have been exploring the idea of cycling pressure and temperature to "lock in" the altered crystal structure.
• Moving away from condensed matter and turning to science used in the aid of history:  When Vesuvius erupted in 79 CE, the pyroclastic flow swept through Herculaneum and a nearby Roman villa, housing a library of more than 1800 now-carbonized scrolls.  Using 3D x-ray tomography, it is hoped that these scrolls may actually be read without trying to physically unroll them, prompting the Vesuvius Prize.  This effort, involving x-ray imaging and AI methods, seems to be bearing fruit.  There may be many more scrolls still buried as well.  It would be amazing if great lost works of ancient Greek and Roman literature could be recovered.
• Tangentially related to science, the arXiv is looking for a CEO - here is the position description.  It's hard to overstate the impact of the arXiv and its relations in terms of open science, and in the chaotic world of scientific publishing, it's more important than ever.
• If you need evidence of how screwed up scientific publishing is, apparently Springer-Nature has been surveying people to see how willing they would be to pay an up-front fee (e.g. $299) just for the privilege of submitting an article.  

  

RIP Tony Leggett

It's been an extremely busy time, and there are all kinds of distressing events afoot.  Talking about new science results or the funding situation can seem self-indulgent when there are ongoing global events of huge impact.  That said, it's important not to lose sight of the humanity in the global physics community.  This past weekend, Tony Leggett passed away (wikipedia page here).   

(image from UIUC)

Prof. Leggett was a soft-spoken, kind person who was also a brilliant theoretical physicist.  I was fortunate enough to first meet him back when I was a graduate student working in Doug Osheroff's lab.  Doug had discovered (along with his thesis advisors Bob Richardson and Dave Lee) the superfluid phases of the rare isotope of helium, 3He in 1972.  

[Science digression:  3He atoms are fermions - if you add up the spin angular momentum from the two protons, the neutron, and the two electrons, you end up with a net spin of 1/2.  To condense into a superfluid state, by analogy with electrons in superconductors, the 3He atoms need to pair up, and it's the pairs that condense into the superfluid.  This pairing ends up being quite complicated; the pair of 3He atoms end up having \(\ell = 1\) orbital angular momentum, and this implies that the nuclear spins of the 3He atoms in the pair have to form a triplet.  Prof. Leggett figured out a ton of the insights on this topic - see here for an early paper on this, and here for a definitive review c. 1975.]

Prof. Leggett made many contributions beyond 3He.  For example, he and others studied the problem of a tunneling particle coupled to some dissipative environment (like phonons, say), and similarly of a two-state quantum system coupled to a "bath", as in this paper with several thousand citations.  These both had close connections to the "measurement" problem in quantum mechanics - how in detail do you go from a highly quantum system (e.g., a particle tunneling out of a bound state, or a particle coherently oscillating back and forth) and end up with more classical-looking outcomes due to coupling to "baths" with large numbers of degrees of freedom?  He was interested in these kinds of foundational quantum issues all the way along (see this 1980 paper) and was still writing about them within the last couple of years.  Prof. Leggett also wrote important tutorial reviews of superfluidity and of Bose-Einstein condensation in ultracold gases.  When I got to meet him on a trip through Stanford, I was introduced to the ideas that he and Clare Yu developed about tunneling two-level systems in solids - looking at the big question of why the properties of TLS in disordered solids are so universal even though the materials can be very different at the microscopic level.  He was a great scientist while also being a kind person.

AI/ML, multiscale modeling, and emergence

I've been attending a lot of talks lately about AI/machine learning and multiscale modeling for materials design and control.  This is a vast, rapidly evolving research area, so here is a little background and a few disorganized thoughts.  

For a recent review article about AI and materials discovery, see here.  There is a ton of work being done pursuing the grand goal of inverse design - name some desired properties, and have AI/ML formulate a material that fits those requirements and is actually synthesizable.  Major companies with publicly known efforts include Google Deepmind and GNoME,  Microsoft, Meta working on catalysts, Toyota Research Institute, IBM, and I'm certain that I'm missing major players.  There are also a slew of startup companies on this topic (e.g. Periodic).  

In addition to materials design and discovery, there is enormous effort being put into using AI/ML to bridge across length and timescales.  Quantum chemistry methods can look at microscopic physics and chemistry, for example, but extending this to macroscopic system sizes with realistic disorder is often computationally intractable.  There are approaches like time-dependent DFT and DMFT to try to capture dynamics, but following dynamics even as long as picoseconds is hard.  Using microscopic methods and ML to try to compute and then parametrize force fields between atoms (for example), one can look at larger systems and longer timescales using molecular dynamics for atomic motions.  However, getting from there to, e.g., the Navier-Stokes equations or understanding phase boundaries, is very difficult.  (At the same time, there are approaches that use AI/ML to learn about the solutions of partial differential equations, so that one can, for example, compute good fluid flows quickly without actually having to solve the N-S equations - see here.) 

We want to keep coarse-graining (looking at larger scales), while maintaining the microscopic physics constraints so that the results are accurate.  There seems to be a lot of hope that either by design or by the action of the AI/ML tools themselves we can come up with descriptors that are good at capturing the essential physics as we move to larger and larger scales.  To use a fluids example, somehow we are hoping that these tools will naturally capture that at scales much larger than one water molecule, it makes sense to track density, temperature, velocity fields, surface tension, liquid-vapor interfaces, etc.  

From the always fun xkcd

One rough description of emergence is the idea that at larger scales and numbers of constituents, new properties appear for the collective system that are extremely difficult to predict from the microscopic rules governing the constituents.  For example, starting from the Schroedinger equation and basic quantum mechanics, it's very hard to determine that snowflakes tend to have 6-fold symmetry and ice will float in water, even though the latter are of course consequences of the former.  A nice article about emergence in physics is here.  

It feels to me like in some AI/ML endeavors, we are hoping that these tools will figure out how emergence works better than humans have been able to do.  This is certainly a worthy challenge, and it may well succeed in a lot of systems, but then we may have the added meta-challenge of trying to understand how our tools did that.  Physics-informed and structured ML will hopefully take us well beyond the situation in the xkcd comic shown here.  

Updates: The US government and STEM research

Now that we're 6 weeks into the new year, I think it's worth it to do an incomplete roundup of where we are on US federal support of STEM research.  Feel free to skip this post if you don't want to read about this.  

• Appropriators in Congress largely went against the FY26 presidential budget request, and various spending bills by and large slightly-less-than level-funded most US science agencies. A physics-oriented take is here. The devil is in the details.  The AAAS federal R&D dashboard lets you explore this at a finer level.  Nature has an interactive widget that visualizes what has been cut and what remains.
• Bear in mind, that was just year 1 of the present administration.  All of the effort, all of the work pushing back against proffered absolutely draconian, agency-destroying cuts?  That likely will have to be done again this year.  And in subsequent years, if the administration still invests effort in pushing enormously slashed budgets in their budget requests.
• There is an issue of Science with the whole news section about how the past year has changed the science funding and pipeline in the US.
• In NSF news, the rate of awards remains very low, though there is almost certainly a major delay because of the lateness of the budget, coping with reduced staffing levels, and restructuring now that Divisions no longer exist.  How greater emphasis on specific strategic priorities (beyond what is in the program calls) will affect operations remains unclear, at least to me.
• Also, some NSF graduate research fellowship applications, especially in the life sciences, seem to be getting kicked back without review - see here (sorry about the paywall).  This seems to be a broad research area issue, despite no information to applicants about this (that lack of information flow is perhaps unsurprising).  
• I'm not well-immersed in the world of NIH and the FDA, but I know things are bad.  Fifteen out of 27 of the NIH institutes have vacant or acting director positions.  The FDA declined to even take the application for Moderna's mRNA flu vaccine, a move not popular even with the Wall Street Journal.  Moderna has also decided to shelve promising vaccines for a number of diseases because they no longer think the US will be a market for them, and it practically seems like someone wants to bring back polio.  (Note:   I will not have the comments become a back-and-forth about vaccines.)
• The back and forth about indirect cost rates continues, along with the relevant court cases.  The recent appropriations have language to prevent sudden changes in rates.  The FAIR model is not yet passed.
• Concerns still loom about impoundment.
• There has been an exodus of technically trained PhDs from government service.
• I could go on.  I know I've left out critical areas, and I haven't talked about DOE or NASA or DOD or EPA or NOAA explicitly.  

Honest people can have discussions about the right balance of federal vs state vs industrial vs philanthropic support for research.  There are no easy answers in the present time.  For those who think that robust public investment in science and engineering research is critical to societal good, economic competitiveness, and security, we need to keep pushing and not let fatigue or fatalism win the day.  

  

Data centers in space make no sense to me

There seems to be a huge push lately in the tech world for the idea of placing data centers in space.  This is not just coming from Musk via the merging of SpaceX and XAi.  Google has some effort along these lines.  NVIDIA is thinking about it. TED talks are being given by startup people in San Francisco on this topic, so you know we've reached some well-defined hype level.    Somehow the idea has enough traction that even the PRC is leaning in this direction.  The arguments seem to be that (1) there is abundant solar power in space; (2) environmental impact on the earth will be less, with no competition for local electricity, water, real estate; (3) space is "cold", so cooling these things should be do-able; (4) it's cool and sounds very sci-fi/high frontier.  

At present (or near-future) levels of technology, as far as I can tell this idea makes no sense.  I will talk about physics reasons here, though there are also pragmatic economic reasons why this seems crazy.  I've written before that I think some of the AI/data center evangelists are falling victim to magical thinking, because they come from the software world and don't in their heart of hearts appreciate that there are actual hardware constraints on things like chip manufacturing and energy production.  

Others have written about this - see here for example.  The biggest physics challenges with this idea (beyond lofting millions of kg of cargo into orbit):

• While the cosmic microwave background is cold, cooling things in space is difficult, because vacuum is an excellent thermal insulator.  On the ground, you can use conduction and convection to get rid of waste heat.  In space, your only option (beyond throwing mass overboard, which is not readily replenishible) is radiative cooling.  The key physics here is the Stefan-Boltzmann law, which is a triumph of statistical physics (and one of my favorite derivations to discuss in class - you combine the Planck result for the energy density of a "gas" of photons in thermal equilibrium at some temperature \(T\) with a basic kinetic theory of gases result for the flux of particles out of a small hole).  It tells you that the best you can ever do is for an ideal black body, the total power radiated away is proportional to the area of the radiator and \(T^{4}\), with fundamental constants making up the proportionality constant with zero adjustable parameters.  

A liquid droplet radiator, from this excellent site

Remember, data centers right now consume enormous amounts of power (and cooling water).  While you can use heat pumps to try to get the radiators up to well above the operating temperatures of the electronics, that increases mass and waste power, and realistically there is an upper limit on the radiator temperature below 1000 K.  An ideal black body radiator at 1000 K puts out about 57 kW per square meter, and you probably need to get rid of tens of megawatts, necessitating hundreds to thousands of square meters of radiator area.  There are clever ideas on how to try to do this.  For example, in the liquid droplet radiator, you could spray a bunch of hot droplets out into space, capitalizing on their large specific surface area.  Of course, you'd need to recapture the cooled droplets, and the hot liquid needs to have sufficiently low vapor pressure that you don't lose a lot of material.  Still, as far as I am aware, to date no one has actually deployed a large-scale (ten kW let alone MW level) droplet radiator in space.  

• High end computational hardware is vulnerable to radiation damage.  There are no rad-hard GPUs.  Low earth orbit is a pretty serious radiation environment, with some flux of high energy cosmic rays quite a bit higher than on the ground.  While there are tests going on, and astronauts are going to bring smartphones on the next Artemis mission, it's rough.  Putting many thousands to millions of GPUs and huge quantities of memory in a harsh environment where they cannot be readily accessed or serviced seems unwise.  (There are also serious questions of vulnerability to attack.  Setting off a small nuclear warhead in LEO injects energetic electrons into the lower radiation belts and would be a huge mess.)

I think we will be faaaaaaar better off in the long run if we take a fraction of the money that people want to invest in space-based data centers, and instead plow those resources into developing energy-efficient computing.  Musk has popularized the engineering sentiment "The best part is no part".  The best way to solve the problem of supplying and radiating away many GW of power for data centers is to make data centers that don't consume many GW of power.  

What is the Aharonov-Bohm effect?

After seeing this latest extremely good video from Veritasium, and looking back through my posts, I realized that while I've referenced it indirectly, I've never explicitly talked about the Aharonov-Bohm effect.  The video is excellent, and that wikipedia page is pretty good, but maybe some people will find another angle on this to be helpful.  

Still from this video.

The ultrabrief version:  The quantum interference of charged particles like electrons can be controllably altered by tuning a magnetic field in a region that the particles never pass through.  This is weird and spooky because it's an entirely quantum mechanical effect - classical physics, where motion is governed by local forces, says that zero field = unaffected trajectories.  

In quantum mechanics, we describe the spatial distribution of particles like electrons with a wavefunction, a complex-valued quantity that one can write as an amplitude and a phase \(\varphi\), where both depend on position \(\mathbf{r}\).  The phase is important because waves can interfere.  Crudely speaking, when the crests of one wave (say \(\varphi = 0\)) line up with the troughs of another wave (\(\varphi = \pi\)) at some location, the waves interfere destructively, so the total wave at that location is zero if the amplitudes of each contribution are identical.   As quantum particles propagate through space, their phase "winds" with distance \(\mathbf{r}\) like \(\mathbf{k}\cdot \mathbf{r}\), where \(\hbar \mathbf{k} = \mathbf{p}\) is the momentum.  Higher momentum = faster winding of phase = shorter wavelength.  This propagation, phase winding, and interference is the physics behind the famous two-slit experiment.  (In his great little popular book - read it if you haven't yet - Feynman described phase as a clockface attached to each particle.)  One important note:  The actual phase itself is arbitrary; it's phase differences that matter in interference experiments.  If you added an arbitrary amount \(\varphi_{0}\) to every phase, no physically measurable observables would change. 

Things get trickier if the particles that move around are charged.  It was realized 150+ years ago that formal conservation of momentum gets tricky if we consider electric and magnetic fields.  The canonical momentum that shows up in the Lagrange and Hamilton equations is \(\mathbf{p}_{c} = \mathbf{p}_{kin} + q \mathbf{A}\), where \(\mathbf{p}_{kin}\) is the kinetic momentum (the part that actually has to do with the classical velocity and which shows up in the kinetic energy), \(q\) is the charge of the particle, and \(\mathbf{A}(\mathbf{r}\)\) is the vector potential.  

Background digression: The vector potential is very often a slippery concept for students.  We get used to the idea of a scalar potential \(\phi(\mathbf{r})\), such that the electrostatic potential energy is \(q\phi\) and the electric field is given by \(\mathbf{E} = -\nabla \phi\) if there are no magnetic fields.  Adding an arbitrary uniform offset to the scalar potential, \(\phi \rightarrow \phi + \phi_{0}\), doesn't change the electric field (and therefore forces on charged particles), because the zero that we define for energy is arbitrary (general relativity aside).  For the vector potential, \(\mathbf{B} = \nabla \times \mathbf{A}\).   This means we can add an arbitrary gradient of a scalar function to the vector potential, \(\mathbf{A} \rightarrow \mathbf{A}+ \nabla f(\mathbf{r})\), and the magnetic field won't change.  Maxwell's equations mean that \(\mathbf{E} = -\nabla \phi - \partial \mathbf{A}/\partial t\).  "Gauge freedom" means that there is more than one way to choose internally consistent definitions of \(\phi\) and \(\mathbf{A}\).

TL/DR main points: (1)  The vector potential can be nonzero in places where \(\mathbf{B}\) (and hence the classical Lorentz force) is zero.  (2) Because the canonical momentum becomes the operator \(-i \hbar \nabla\) in quantum mechanics and the kinetic momentum is what shows up in the kinetic energy, charged propagating particles pick up an extra phase winding given by \(\delta \varphi = (q/\hbar)\int \mathbf{A}\cdot d\mathbf{r}\) along a path.  

This is the source of the creepiness of the Aharonov-Bohm effect.  Think of two paths (see still taken from the Veritasium video), and threading magnetic flux just through the little region using a solenoid will tune the intensity detected on the screen on the far right.  That field region can be made arbitrarily small and positioned anywhere inside the diamond formed by the paths, and the effect still works.  Something not mentioned in the video:  The shifting of the interference pattern is periodic in the flux through the solenoid, with a period of \(h/e\), where \(h\) is Planck's constant and \(e\) is the electronic charge.  

Why should you care about this?

• As the video discusses, the A-B effect shows that the potentials are physically important quantities that affect motion, at least as much as the corresponding fields, and there are quantum consequences to this that are just absent in the classical world.
• The A-B effect (though not with the super skinny field confinement) has been seen experimentally in many mesoscopic physics experiments (e.g., here, or here) and can be used as a means of quantifying coherence at these scales (e.g., here and here).
• When dealing with emergent quasiparticles that might have unusual fractional charges (\(e^*\)), then A-B interferometers can have flux periodicities that are given by \(h/e^*\). (This can be subtle and tricky.)
• Interferometry to detect potential-based phase shifts is well established.  Here's the paper mentioned in the video about a gravitational analog of the A-B effect.  (Quibblers can argue that there is no field-free region in this case, so it's not strictly speaking the A-B analog.)

Basically, the A-B effect has gone from an initially quite controversial prediction to an established piece of physics that can be used as a tool.  If you want to learn Aharonov's take on all this, please read this interesting oral history.   

Update: The always informative Steve Simon has pointed out to me a history of this that I had not known, that this effect had already been discovered a decade earlier by Ehrenberg and Siday.  Please see this arXiv paper about this.  Here is Ehrenberg and Siday's paper.  Aharonov and Bohm were unaware of it and arrived at their conclusions independently.  One lesson to take away:  Picking a revealing article title can really help your impact.

What is superconductivity?

A friend pointed out that, while I've written many posts that have to do with superconductivity, I've never really done a concept post about it.  Here's a try, as I attempt to distract myself from so many things happening these days.

The superconducting state is a truly remarkable phase of matter that is hosted in many metals (though ironically not readily in the pure elements (Au, Ag, Cu) that are the best ordinary conductors of electricity - see here for some references).  First, some definitional/phenomenological points:

• The superconducting state is a distinct thermodynamic phase.  In the language of phase transitions developed by Ginzburg and Landau back in the 1950s, the superconducting state has an order parameter that is nonzero, compared to the non-superconducting metal state.   When you cool down a metal and it becomes a superconductor, this really is analogous (in some ways) to when you cool down liquid water and it becomes ice, or (a better comparison) when you cool down very hot solid iron and it becomes a magnet below 770 °C.
• In the superconducting state, at DC, current can flow with zero electrical resistance.  Experimentally, this can be checked by setting up a superconducting current loop and monitoring the current via the magnetic field it produces.  If you find that the current will decay over somewhere between \(10^5\) and \(\infty\) years, that's pretty convincing that the resistance is darn close to zero. 
• This is not just "perfect" conduction.  If you placed a conductor in a magnetic field, turned on perfect conduction, and then tried to change the magnetic field, currents would develop currents that would preserve the amount of magnetic flux through the perfect conductor.  In contrast, a key signature of superconductivity is the Meissner-Oschenfeld Effect:  if superconductivity is turned on in the presence of a (sufficiently small) magnetic field, currents will develop spontaneously at the surface of the material to exclude all magnetic flux from the bulk of the superconductor.  (That is, the magnetic field from the currents will be oppositely directed to the external field and of just the right size and distribution to give \(\mathbf{B}=0\) in the bulk of the material.)  Observation of the bulk Meissner effect is among the strongest evidence for true superconductivity, much more robust than a measurement that seems to indicate zero voltage drop.  Indeed, as a friend of mine pointed out to me, a one-phrase description of a superconductor is "a perfect diamagnet".  
• There are two main types of superconductors, uncreatively termed "Type I" and "Type II".  In Type I superconductors, an external \(\mathbf{H} = \mathbf{B}/\mu_{0}\) fails to penetrate the bulk of the material until it reaches a critical field \(H_{c}\), at which point the superconducting state is suppressed completely.  In a Type II superconductor, above some lower critical field \(H_{c,1}\) magnetic flux begins to penetrate the material in the form of vortices, each of which has a non-superconducting ("normal") core.  Above an upper critical field \(H_{c,2}\), superconductivity is suppressed. 
• Interestingly, a lot of this can be "explained" by the London Equations, which were introduced in the 1930s despite a complete lack of a viable microscopic theory of superconductivity.
• Magnetic flux through a conventional superconducting ring (or through a vortex core) is quantized precisely in units of \(h/2e\), where \(h\) is Planck's constant and \(e\) is the electronic charge.  
• (It's worth noting that in magnetic fields and with AC currents, there are still electrical losses in superconductors, due in part to the motion of vortices.)

Physically, what is the superconducting state?  Why does it happen and why does it have the weird properties described above as well as others?  There are literally entire textbooks and semester-long courses on this, so what follows is very brief and non-authoritative.  

• In an ordinary metal at low temperatures, neglecting e-e interactions and other complications, the electrons fill up states (because of the Pauli Principle) starting from the lowest energy up to some highest value, the Fermi energy.  (See here for some mention of this.)   Empty electronic states are available at essentially no energy cost - exciting electrons from filled states to empty states are "gapless".  
• Electrical conduction takes place through the flow of these electronic quasiparticles.   (For more technical readers:  We can think of these quasiparticles like little wavepackets, and as each one propagates around the wavepacket accumulates a certain amount of phase.  The phases of different quasiparticles are arbitrary, but the change in the phase going around some trajectory is well defined.)
• In a superconductor, there is some effective attractive interaction between electrons that we have thus far neglected.  In conventional superconductors, this involves lattice vibrations (as in this wikipedia description), though other attractive interactions are possible.  At sufficiently low temperatures, the ordinary metal state is unstable, and the system will spontaneously form pairs of electrons (or holes).  Those pairs then condense into a single coherent state described by an amplitude \(|\Psi|\) and a phase, \(\phi\), shared by all the pairs.  The conventional theory of this was formulated by Bardeen, Cooper, and Schrieffer in 1957.  A couple of nice lecture note presentations of this are here (courtesy Yuval Oreg) and here (courtesy Dan Arovas), if you want the technical details.  This leads to an energy gap that characterizes how much it costs to create individual quasiparticles.  Conduction in a superconductor takes place through the flow of pairs.  (A clue to this is the appearance of the \(2e\) in the flux quantization.)
• This taking on of a global phase for the pairs of electrons is a spontaneous breaking of gauge symmetry - this is discussed pedagogically for physics students here.  Understanding this led to figuring out the Anderson-Higgs mechanism, btw. 
• The result is a state with a kind of rigidity; precisely how this leads to the phenomenology of superconductivity is not immediately obvious, to me anyway.  If someone has a link to a great description of this, please put it in the comments.  (Interestingly google gemini is not too bad at discussing this.)
• The existence of this global phase is hugely important, because it's the basis for the Josephson effect(s), which in turn has led to the basis of exquisite magnetic field sensing, all the superconducting approaches to quantum information, and the definition of the volt, etc.
• The paired charge carriers are described by a pairing symmetry of their wave functions in real space.  In conventional BCS superconductors, each pair has no orbital angular momentum ("\(s\)-wave"), and the spins are in a singlet state.  In other superconductors, pairs can have \(l = 1\) orbital angular momentum ("\(p\)-wave", with spins in the triplet configuration), \(l = 2\) orbital angular momentum ("\(d\)-wave", with spins in a singlet again), etc.  The pairing state determines whether the energy gap is directionally uniform (\(s\)-wave) or whether there are directions ("nodes") along which the gap goes to zero.  

I have necessarily left out a ton here.  Superconductivity continues to be both technologically critical and scientifically fascinating.  One major challenge in understanding the microscopic mechanisms behind particular superconductors is that the superconducting state itself is in a sense generic - many of its properties (like phase rigidity) are emergent regardless of the underlying microscopic picture, which is amazing.

One other point, added after initial posting. In quantum computing approaches, a major challenge is how to build robust effective ("logical") qubits from individual physical qubits that are not perfect (meaning that they suffer from environmental decoherence among other issues).  The phase coherence of electronic quasiparticles in ordinary metals is generally quite fragile; inelastic interactions with each other, with phonons, with impurity spins, etc. can all lead to decoherence.  However, starting from those ingredients, superconductivity shows that it is possible to construct, spontaneously, a collective state with very long-lived coherence.  I'm certain I'm not the first to wonder about whether there are lessons to be drawn here in terms of the feasibility of and approaches to quantum error correction.