Tuesday, July 20, 2021

Quantum computing + hype

 Last Friday, Victor Galitski published a thought-provoking editorial on linkedin, entitled "Quantum Computing Hype is Bad for Science".  I encourage people to read it.

As a person who has spent years working in the nano world (including on topics like "molecular electronics"), I'm intimately familiar with the problem of hype.  Not every advance is a "breakthrough" or "revolutionary" or "transformative" or "disruptive", and that is fine - scientists and engineers do themselves a disservice when overpromising or unjustifiably inflating claims of significance.  Incentives often point in an unfortunate direction in the world of glossy scientific publications, and the situation is even murkier when money is involved (whether to some higher order, as in trying to excite funding agencies, or to zeroth order, as in raising money for startup companies).   Nano-related research advances overwhelmingly do not lead toward single-crystal diamond nanofab or nanobots swimming through our capillaries.  Not every genomics advance will lead to a global cure for cancer or Alzheimers.  And not every quantum widget will usher in some quantum information age that will transform the world.  It's not healthy for anyone in the long term for unsupported, inflated claims to be the norm in any of these disciplines.

I am more of an optimist than Galitski.  

I agree that we are a good number of years away from practical general-purpose quantum computers that can handle problems large enough to be really interesting (e.g. breaking 4096-bit RSA encryption).  However, I think there is a ton of fascinating and productive research to be done along the way, including in areas farther removed from quantum computing, like quantum-enhanced sensing.  Major federal investments in the relevant science and engineering research will lead to real benefits in the long run, in terms of whatever technically demanding physics/electronics/optics/materials work force needs we will have.  There is very cool science to be done.  If handled correctly, increased investment will not come at the expense of non-quantum-computing science.  It is also likely not a zero-sum game in terms of human capital - there really might be more people, total, drawn into these fields if prospects for employment look more exciting and broader than they have in the past.  

Where I think Galitski is right on is the concern about what he calls "quantum Ponzi schemes".  Some people poured billions of dollars into anything with the word "blockchain" attached to it, even without knowing what blockchain means, or how it might be implemented by some particular product.  There is a real danger that investors will be unable to tell reality from science fiction and/or outright lying when it comes to quantum technologies.  Good grief, look how much money went into Theranos when lots of knowledgable people knew that single-drop-of-blood assays have all kinds of challenges and that the company's claims seemed unrealistic. 

I also think that it is totally reasonable to be concerned about the sustainability of this - anytime there is super-rapid growth in funding for an area, it's important to think about what comes later.  The space race is a good example.  There were very cool knock-on benefits overall from the post-Sputnik space race, but there was also a decades-long hangover in the actual aerospace industry when the spending fell back to earth.  

Like I said, I'm baseline pretty optimistic about all this, but it's important to listen to cautionary voices - it's the way to stay grounded and think more broadly about context.  


APS Division of Condensed Matter Physics Invited Symposium nominations

Hopefully the 2022 APS March Meeting in Chicago will be something closer to "normal", though (i) with covid variants it's good to be cautious about predictions, and (ii) I wouldn't be surprised if there is some hybrid content.  Anyway, I encourage submissions.  Having been a DCMP member-at-large and seen the process, it's to all of our benefit if there is a large pool of interesting contributions.

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The Division of Condensed Matter Physics (DCMP) program committee requests your proposals for Invited Symposium sessions for the APS March Meeting 2022. DCMP hosts approximately 30 Invited symposia during the week of the March Meeting highlighting cutting-edge research in the broad field of condensed matter physics. These symposia consist of 5 invited talks centered on a research topic proposed by the nominator(s). Please submit only Symposium nominations. DCMP does not select individual speakers for invited talks.

Please use the APS nominations website for submission of your symposium nomination.

Submit your nomination

Nominations should be submitted as early as possible, and no later than August 13. Support your nomination with a justification, a list of five confirmed invited speakers with tentative titles, and a proposed session chair. Thank you for spending the time to help organize a strong DCMP participation at next year’s March Meeting.

Jim Sauls, Secretary/Treasurer for DCMP

Friday, July 16, 2021

Slow blogging + a couple of articles

Sorry - blogging has been slow in recent days because, despite it being summer, it's been a very busy time for various reasons.

Here are a couple of articles that I've come across that seem interesting.  On the news/popular writing front:

On the science front, there have been several cool things that I haven't had time to look at in depth.  A couple of quantum info papers:

  • In this Nature paper, the google quantum AI team have used their 53 qubit chip to do proof-of-concept demonstrations of two different quantum error correction approaches.  Perhaps someone more knowledgable that me can chime in below in the comments about how the ratio of physical qubits to logical qubits depends on the fidelity and other properties of the physical qubits.  Basically, I'm wondering if, e.g., ion trap-based schemes would be able to make even better advantage out of the 1D error correction approach here.
  • Meanwhile, in China a large group has demonstrated a 66 qubit system similar in design to the google/Martinis approach.  

Tuesday, July 06, 2021

Infrastructure and competitiveness

With the recent passage in the US Senate of an authorization that would potentially boost certain scientific investments by the US, and the House of Representatives version passing its versions for NSF and DOE, talk of "competitiveness" is in the air.  It took a while, but it seems to have dawned on parts of the US Congress that it would be broadly smart for the country to invest more in science and engineering research and education.  (Note that authorizations are not appropriations - declaring that they want to increase investment doesn't actually commit Congress to actually spending the money that way.  A former representative from my area routinely voted for authorizations to double the NSF budget, and then did not support the appropriations, so that he could claim to be both pro-science and anti-spending.) 

Looking through my old posts on related topics, I came across this one from 2014, about investment in shared research equipment at universities and DOE labs.  Since then, the NSF's former National Nanotechnology Infrastructure Network has been replaced by the National Nanotechnology Coordinated Infrastructure organization, but the overall federal support for this fantastic resource has actually gone down in real dollars, since its annual budget is unchanged since then at $16M/yr.  As I wrote back in 2014, in an era when one high end transmission electron microscope can cost $8M or more, that seems like underinvestment if the goal is to maximize innovation by making top-flight shared research instruments available to the broadest cross-section of universities and businesses.   

I reiterate my suggestion:  Companies (google? Intel? Microsoft? SpaceX? Tesla? 3M? Dupont? IBM?) and wealthy individuals who truly want to have a more competitive science and engineering workforce and innovation base should consider establishing an endowed entity to support research equipment and staffing at universities.   A comparatively modest investment ($300M) could support more than the entire NNCI every year, in perpetuity.  


Sunday, June 27, 2021

Quantum coherence and classical yet quantum materials

Because I haven't seen this explicitly discussed anywhere, I think it's worth pointing out that everyday materials around us demonstrate some features of coherence and decoherence in quantum mechanics.

Quantum mechanics allows superposition states to exist - an electron can be in a state with a well-defined momentum, but that is a superposition of all possible position states along some wavefront.   As I mentioned here, empirically a strong measurement means coupling the system being measured to some large number of degrees of freedom, such that we don't keep track of the detailed evolution of quantum entanglement.  In my example, that electron hits a CCD detector and interacts locally with the silicon atoms in one particular pixel, depositing its charge and energy there and maybe creating additional excitations.  That "collapses" the state of the electron into a definite position.  This kind of measurement is a two-way street - a quantum system leaves its imprint on the state of the measuring apparatus, and the measurement changes the quantum system's state.

One fascinating aspect of the emergence of materials properties is that we can have systems that act both very classically (as I'll explain in a minute) and also very quantum mechanically at the same time, for different aspects of the material.  

If I have a piece of aluminum sitting in front of me (like the case of my laptop) that hunk of metal does not show up in a superposition of positions or orientations.  It surely seems to have a definite position and orientation, and if I looked closely at a given moment I would find the aluminum atoms arranged in crystal lattices, with clear atomic positions.  Somehow, the interactions of the aluminum with the broader environment have washed out the quantumness of the atomic positions.  (Volumes have been written about interpretations of quantum mechanics and "the measurement problem", as I touched on here.  In the many-worlds view, we live in a particular branch of reality, while there are other branches that correspond to other possible positions and orientations of the aluminum piece, one for each possible outcome of a positional or orientational measurement.  I'm not going to touch on the metaphysics behind how to think about this here, except to say that somehow the position of the aluminum empirically acts classically.)

What about the electrons in the piece of crystalline aluminum?  Well, we've learned about band structure.  The allowed quantum states of electrons in a periodic potential consists of bands of states.  Each of these states has an associated crystal momentum \(\hbar \mathbf{k}\), and there is some relationship between energy and crystal momentum, \(E(\mathbf{k})\).  There are values of energy between the bands that do not correspond to any allowed electronic quantum states in that periodic lattice.  In aluminum, the electronic states are filled up to states in the middle of a band.  (One can be more rigorous that this, but it's beside the point I'm trying to make.)  Interestingly, the electrons in those filled states energetically far away from the highest occupied states are coherent - they are wavelike and extended, and indeed the Bloch waves themselves are a direct consequence of quantum interference throughout the periodic lattice.  Why haven't these electrons somehow decohered into some classical situation?   If you imagine some dynamic interaction that would "measure" the location, say, of one of those electrons, you have to consider some final state in which the electron would end up.  Because all of the states at nearby energies are already occupied, and the electrons obey the Pauli Principle, there is no low-energy (on the scale of, say, the thermal energy available, \(k_{\mathrm{B}}T\)) path to decoherence.  You'd need much larger energy/higher momentum/shorter wavelength processes to reach those electrons and scatter them to empty final states (as in ARPES).

By that argument, though, the electrons that are energetically close to the Fermi level in metals should be vulnerable to decoherence - they have energetically nearby states into which they can be scattered, and a variety of comparatively low energy scattering processes (electron-electron scattering, electron-phonon scattering).   Is  that true?  Yes.  This is exactly why you can't see quantum interference effects in electrical conduction in metals at room temperature, but at low temperatures you can see interference effects like universal conductance fluctuations and understand the effects of decoherence on those effects quantitatively.

I find it remarkable that a piece of aluminum can show both the emergence of classical physics (the piece of aluminum is not spatially delocalized) while having quantum coherent degrees of within.  Understanding how to engineer robust quantum coherent systems despite the tendency toward environmental decoherence is key to future quantum information science and technology.

Wednesday, June 16, 2021

Nanoscale Views on the Scientific Sense podcast

I recently had the opportunity to be interviewed for the Scientific Sense podcast, available on a variety of platforms.  It was a fun discussion, and it's now available here (youtube link) or here (spotify link).  

Tuesday, June 15, 2021

Brief items

 

Some news items:

  • Big news yesterday was the announcement at Condensed Matter Theory Center conference (I'll put up the link to the talk when it arrives on the CMTC youtube channel) by Andrea Young that ABC-stacked trilayer graphene superconducts at particular carrier densities and vertically directed electric field levels.  There are actually two superconducting states, with quite different in-plane critical fields (suggesting different pairing states).  Note that there is no twisting or moirĂ© superlattice here, which suggests that superconductivity in stacked graphene may be more generic than has been thought.  Here is a relevant article in Quanta magazine.
  • Here is a talk by Padmanabhan Balaram, about greed in the academic publishing industry.  Even open-access journals apparently have profit margins of 30-40% (!!).  Think about that when publishers claim that production costs and their amazing editorial experience really justify that authors pay $5K per open-access publication.  (Note to self:  get around to putting manuscripts up on the arxiv....)  The talk is also an indictment of fixation on publication metrics.
  • On a lighter note, my very talented classmate, Yale chem professor Patrick Holland with a song about Reviewer 3.  It's more mellow than another famous response to Reviewer 3.
  • I was going to write a blog post about the physics motivating the use of sticky substances on baseballs, only to discover that someone already wrote that piece.  The time is ripe for someone to try to go to the other extreme:  Some kind of miracle superomniphobic coating on the ball so that the no-slip condition for air at the surface is violated, and every pitch then travels more like a knuckleball.



Friday, June 11, 2021

The power of computational materials theory

With the growth of computational capabilities and the ability to handle large data volumes, it looks like we are entering a new era for the global understanding of material properties.  

As an example, let me highlight this paper, with the modest title, "All Topological Bands of All Stoichiometric Materials".  (Note that this is related to the efforts reported here two years ago.) These authors oversee the Topological Materials Database, and they have ground through the entire Inorganic Crystal Structure Database using electronic structure methods (density functional theory (see here, here, here) with VASP both with and without spin-orbit coupling) and an automated approach to checking for topologically nontrivial electronic bands.  This allows the authors to look at essentially all of the inorganic crystals that have reliable structural information and make a pass at characterizing whether there are topologically interesting features in their band structure.  The surprising conclusion is that almost 88% of all of these materials have at least one topologically nontrivial band somewhere (though it may be buried energetically far away from the electronic levels that affect charge transport, for example).  Considering that people didn't necessarily appreciate that there was such a thing as topological insulators until relatively recently, that's really interesting.  

This broad computational approach has also been applied by some of the same authors to look for materials with flat bands - these are systems where the electronic energy depends only very weakly on (crystal) momentum, so that interaction effects can be large compared to the kinetic energy.

The ability to do large-scale surveys of predicted material properties is an exciting development!