Quantum computing - lay of the land, + corporate sponsorship

Much has been written about quantum computers and their prospects for doing remarkable things (see here for one example of a great primer), and Scott Aronson's blog is an incredible resource if you want more technical discussions.   Recent high profile news this week about Microsoft investing heavily in one particular approach to quantum computation has been a good prompt to revisit parts of this subject, both to summarize the science and to think a bit about corporate funding of research.  It's good to see how far things have come since I wrote this almost ten years ago (!!).

Remember, to realize the benefits of general quantum computation, you need (without quibbling over the details) some good-sized set  (say 1000-10000) of quantum degrees of freedom, qubits, that you can initialize, entangle to create superpositions, and manipulate in deliberate ways to perform computational operations.  On the one hand, you need to be able to couple the qubits to the outside world, both to do operations and to read out their state.  On the other hand, you need the qubits to be isolated from the outside world, because when a quantum system becomes entangled with (many) environmental degrees of freedom whose quantum states you aren't tracking, you generally get decoherence - what is known colloquially as the collapse of the wavefunction.  

The rival candidates for general purpose quantum computing platforms make different tradeoffs in terms of robustness of qubit coherence and scalability.  There are error correction schemes, and implementations that combine several "physical" qubits into a single "logical" qubit that is supposed to be harder to screw up.  Trapped ions can have very long coherence times and be manipulated with great precision via optics, but scaling up to hundreds of qubits is very difficult (though see here for a claim of a breakthrough).  Photons can be used for quantum computing, but since they fundamentally don't interact with each other under ordinary conditions, some operations are difficult, and scaling is really hard - to quote from that link, "About 100 billion optical components would be needed to create a practical quantum computer that uses light to process information."   Electrons in semiconductor quantum dots might be more readily scaled, but coherence is fleeting.   Superconducting approaches are the choices of the Yale and UC Santa Barbara groups.

The Microsoft approach, since they started funding quantum computing research, has always been rooted in ideas about topology, perhaps unsurprising since their effort has been led by Michael Freedman.  If you can encode quantum information in something to do with topology, perhaps the qubits can be more robust to decoherence.  One way to get topology in the mix is to work with particular exotic quantum excitations in 2d that are non-Abelian.  That is, if you take two such excitations and move them around each other in real space, the quantum state somehow transforms itself to remember that braiding, including whether you moved particle 2 around particle 1, or vice versa.  Originally Microsoft was very interested in the \(\nu = 5/2\) fractional quantum Hall state as an example of a system supporting this kind of topological braiding.  Now, they've decided to bankroll the groups of Leo Kouwenhoven and Charlie Marcus, who are trying to implement topological quantum computing ideas using superconductor/semiconductor hybrid structures thought to exhibit Majorana fermions.

It's worth noting that Microsoft are not the only people investing serious money in quantum computing.   Google invested enormously in John Martinis' effort.  Intel has put a decent amount of money into a silicon quantum dot effort practically down the hall from Kouwenhoven.  This kind of industrial investment does raise some eyebrows, but as long as it doesn't kill publication or hamstring students and postdocs with weird constraints, it's hard to see big downsides.  (Of course, Uber and Carnegie Mellon are a cautionary example of how this sort of relationship may not work out well for the relevant universities.)

More short items, incl. postdoc opportunities

Some additional brief items:

• Rice's Smalley-Curl Institute has two competitive, endowed postdoctoral opportunities coming up, the J. Evans Attwell Welch Postdoctoral Fellowship, and the Peter M. and Ruth L. Nicholas Postdoctoral Fellowship in Nanotechnology.  The competition is fierce, but they're great awards and come with separate travel funds and research supplies resources.  Applying requires working with a Rice faculty sponsor, and the deadline for applications is June 30, 2017, with a would-be start date around the beginning of September, 2017.
• This may be completely academic, but my colleagues at Rice's Baker Institute, including former NSF director and Presidential science adviser Neal Lane, have prepared a report with recommendations to the next science adviser regarding the Office of Science and Technology Policy and how to integrate science into policy making.  Yeah.  Sigh.
• Check out Funsize Physics!  It's a repository of education and broader outreach products from NSF investigators, started by Shireen Adenwalla and Jocelyn Bosely, related to their NSF MRSEC.
• Anyone have strong opinions about Academic Analytics?  The main questions are whether the quality control on the information is good, and whether the information can actually be useful.

short items

A handful of brief items:

• A biologist former colleague has some good advice on writing successful NSF proposals that translates well to other disciplines and agencies.
• An astronomy colleague has a nice page on the actual science behind the much-hyped supermoon business.
• Lately I've found myself recalling a book that I read as part of an undergraduate philosophy of science course twenty-five years ago, The Dilemmas of an Upright Man.  It's the story of Max Planck and the compromises and choices he made while trying to preserve German science through two world wars.  As the Nazis rose to power and began their pressuring of government scientific institutions such as the Berlin Academy and the Kaiser Wilhelm Institutes, Planck decided to remain in leadership roles and generally not speak out publicly, in part because he felt like if he abrogated his position there would only be awful people left behind like ardent Nazi Johannes Stark.   These decisions may have preserved German science, but they broke his relationship with Einstein, who never spoke to Planck again from 1937 until Planck's death in 1947.  It's a good book and very much worth reading.

Lenses from metamaterials

As alluded to in my previous posts on metamaterials and metasurfaces, there have been some recently published papers that take these ideas and do impressive things.

• Khorasaninejad et al. have made a metasurface out of a 2d array of very particularly designed TiO₂ posts on a glass substrate.  The posts vary in size and shape, and are carefully positioned and oriented on the substrate so that, for light incident from behind the glass, normal to the glass surface, and centered on the middle of the array, the light is focused to a spot 200 microns above the array surface.  Each little TiO₂ post acts like a sub-wavelength scatterer and imparts a phase on the passing light, so that the whole array together acts like a converging lens.  This is very reminiscent of the phased array I'd mentioned previously.  For a given array, different colors focus to different depths (chromatic aberration).  Impressively, the arrays are designed so that there is no polarization dependence of the focusing properties for a given color.    
• Hu et al. have made a different kind of metasurface, using plasmonically active gold nanoparticles on a glass surface.  The remarkable achievement here is that the authors have used a genetic algorithm to find a pattern of nanoparticle shapes and sizes that somehow, through phased array magic, produces a metasurface that functions as an achromatic lens - different visible colors (red, green, blue) normally incident on the array focus to the same spot, albeit with a short focal length of a few microns. 
•  Finally, in more of a 3d metamaterial approach, Krueger et al. have leveraged their ability to create 3d designer structures of porous silicon.  The porous silicon frameworks have an effective index of refraction at the desired wavelength.  By controllably varying the porosity as a function of distance from the optical axis of the structure, these things can act as lenses.  Moreover, because of designed anisotropy in the framework, they can make different polarizations of incident light experience different effective refractive indices and therefore have different focal lengths.  Fabrication here is supposed to be considerably simpler than the complicated e-beam lithography needed to accomplish the same goal with 2d metasurfaces.

These are just papers published in the last couple of weeks!  Clearly this is a very active field.

What is a metasurface?

As I alluded in my previous post, metamaterials are made out of building blocks, and thanks to the properties of those building blocks and their spatial arrangement, the aggregate system has, on longer distance scales, emergent properties (e.g., optical, thermal, acoustic, elastic) that can be very different from the traits of the individual building blocks.  Classic examples are opal and butterfly wing, both of which are examples of structural coloration.  The building blocks (silica spheres in opal; chitin structures in butterfly wing) have certain optical properties, but by properly shaping and arranging them, the metamaterial comprising them has brilliant iridescent color very different from that of bulk slabs of the underlying material.

Controlling the relative phases between
antennas in an array lets you steer radiation.
By Davidjessop - Own work, CC BY-SA 4.0,
https://commons.wikimedia.org/
w/index.php?curid=48304978 

This works because of wave interference of light.  Light propagates more slowly in a dielectric (\(c/n(\omega)\), where \(n(\omega)\) is the frequency-dependent index of refraction).  Light propagating through some thickness of material will pick up a phase shift relative to light that propagates through empty space.  Moreover, additional phase shifts are picked up at interfaces between dielectrics.  If you can control the relative phases of light rays that arrive at a particular location, then you can set up constructive interference or destructive interference.

This is precisely the same math that gives you diffraction patterns.  You can also do this actively with radio transmitter antennas.  If you set up an antenna array and drive each antenna at the same frequency but with a controlled phase relative to its neighbors, you can tune where the waves constructively or destructively interfere.  This is the principle behind phased arrays.

An optical metasurface is an interface that has structures on it that impose particular phase shifts on light that either is transmitted through or reflected off the interface.  Like a metamaterial and for the same wave interference reasons, the optical properties of the interface on distance scales larger than those structures can be very different than those of the materials that constitute the structures.  Bear in mind, the individual structures don't have to be boring - each by itself could have complicated frequency response, like acting as a dielectric or plasmonic resonator.  We now have techniques that allow rich fabrication on surfaces with a variety of materials down to scales much smaller than the wavelength of visible light, and we have tremendous computational techniques that allow us to calculate the expected optical response from such structures.  Put these together, and those capabilities enable some pretty amazing optical tricks.  See here (pdf!) for a good slideshow covering this topic.

What is a metamaterial?

(This is part of a lead-in to a brief discussion I'd like to do of two papers that just came out.) The wikipedia entry for metamaterial is actually rather good, but doesn't really give the "big picture".  As you will hopefully see, that wording is a bit ironic.

"Ordinary" materials are built up out of atoms or molecules.  The electronic, optical, and mechanical properties of a solid or liquid come about from the properties of the individual constituents, and how those constituents are spatially arranged and coupled together into the whole.   On the length scale of the constituents (the size of atoms, say, in a piece of silicon), the local properties like electron density and local electric field vary enormously.  However,  on length scales large compared to the individual constituent atoms or molecules, it makes sense to think of the material as having some spatially-averaged "bulk" properties, like an index of refraction (describing how light propagates through the material), or a magnetic permeability (how the magnetic induction \(\mathbf{B}\) inside a material responds to an externally applied magnetic field \(\mathbf{H}\)), or an elastic modulus (how a material deforms in response to an applied stress).

A "metamaterial" takes this idea a step further.  A metamaterial is build up out of some constituent building blocks such as dielectric spheres or metallic rods.  The properties of an individual building block arise as above from their own constituent atoms, of course.  However, the properties of the metamaterial, on length scales long compared to the size of the building blocks, are emergent from the properties of those building blocks and how the building blocks are then arranged and coupled to each other.   The most common metamaterials are probably dielectric mirrors, which are a subset of photonic band gap systems.  You can take thin layers of nominally transparent dielectrics, stack them up in a periodic way, and end up with a mirror that is incredibly reflective at certain particular wavelengths - an emergent optical property that is not at all obvious at first glance from the properties of the constituent layers.

Depending on what property you're trying to engineer in the final metamaterial, you will need to structure the system on different length scales.  If you want to mess with optical properties, generally the right ballpark distance scale is around a quarter of the wavelength (within the building block constituent) of the light.  For microwaves, this can be the cm range; for visible light, its tens to hundreds of nm.  If you want to make an acoustic metamaterial, you need to make building blocks on a scale comparable to a fraction of the wavelength of the sound you want to manipulate.  Mechanical metamaterials, which have large-scale elastic properties far different than those of their individual building blocks, are trickier, and should be thought about as something more akin to a small truss or origami framework.  These differ from optical and acoustic metamaterials because the latter rely crucially on interference phenomena between waves to build up their optical or acoustic properties, while structural systems rely on local properties (e.g., bending at vertices).

Bottom line:  We now know a lot about how to build up larger structures from smaller building blocks, so that the resulting structures can have very different and interesting properties compared to those of the constituents themselves.

Rice has an opening for Dean of Engineering

Just in case anyone out there is interested, Rice is beginning a search for the next dean of our George R. Brown School of Engineering.

Measuring temperature at the milliKelvin scale

How do we tell the temperature of some piece of material?  I've written about temperature and thermometry a couple of times before (here, here, here).  For ordinary, every-day thermometry, we measure some physical property of a material or system where we have previously mapped out its response as a function of temperature.  For example, near room temperature liquid mercury expands slightly with increasing \(T\).  Confined in a thin glass tube, the length of a mercury column varies approximately linearly with changes in temperature, \(\delta \ell \sim \delta T\).  To do primary thermometry, we don't want to have some empirical calibration - rather, we want to measure some physical property for which we think we have a complete understanding of the underlying physics, so that \(T\) can be inferred directly from the measured quantity and our theoretical expressions, with no adjustable parameters.  This is particularly important at very low temperatures, thousandths of a Kelvin above absolute zero, where the number of things that we can measure is comparatively limited, and tiny flows of power (from our measurements, say) can actually produce large percentage temperature changes.

This recent paper shows a nice example of applying three different primary thermometry techniques to a single system, a puddle of electrons confined in 2d at a semiconductor interface, at about 6 mK.  This is all the more impressive because of how easy it is to inadvertently heat up electrons in such 2d layers.  All three techniques rely on our understanding of how electrons behave at low temperatures.  According to our theory of electrons in metals (which these 2d electrons are, as far as physicists are concerned), as a function of energy, electrons are spread out in a characteristic way, the Fermi-Dirac distribution.  From the theory side, we know this functional form exactly (figure from that wikipedia link).  At low temperatures, all of the electronic states below a highest-filled-state are full, and all above are empty.  As \(T\) is increased, the electrons smear out into higher energy states, as shown.  The three effects measured in the experiment all depend on \(T\) through this electronic distribution:

Fig. 2 from the paper, showing excellent, consistent agreement

between experiment and theory, showing electron temperatures 

of ~ 6 mK. 

• Current noise in a quantum point contact, the fluctuations in the average current.  For this particular device, where conduction takes place through a small, controllable number of quantum channels, we think we understand the situation completely.  There is a closed-form expression for what the noise should do as a function of average current, with temperature as the only adjustable parameter (once the conduction has been measured).
• "Coulomb blockade" in a quantum dot.  Conduction through a puddle of electrons connected to input and output electrodes by tunneling barriers ("pinched off" versions of the point contacts) shows a very particular form of current-voltage characteristic that is tunable by a nearby gate electrode.   The physics here is that, because of the mutual repulsion of electrons, it takes energy (supplied by either a voltage source or temperature) to get charge to flow through the puddle.  Again, once the conduction has been measured, there is a closed-form expression for what the conductance should do as a function of that gate voltage.
• "Environmental" Coulomb blockade in a quantum dot.  This is like the situation above, but with one of the tunnel barriers replaced by a controlled resistor.  Again, there is an expression for the particular shape of the \(I-V\) curve where the adjustable parameter is \(T\).  

As shown in the figure (Fig. 2 from the paper - open access and also available on the arxiv), the theoretical expressions do a great job of fitting the data, and give very consistent electron temperatures down to 0.006 K.  It's a very impressive piece of work, and I encourage you to read it - look at Fig. 4 if you're interested in how challenging it is to cool electrons in these kinds of devices down to this level.

What do LBL's 1 nm transistors mean?

In the spirit of this post, it seems like it would be a good idea to write something about this paper (accompanying LBL press release), particularly when popular sites are going a bit overboard with their headlines ("The world's smallest transistor is 1nm long, physics be damned").  (I discuss most of the background in my book, if you're interested.)

What is a (field effect) transistor and how does it work?  A transistor is an electronic switch, the essential building block of modern digital electronics.  A field-effect transistor (FET) has three terminals - a "source" (an input), a "drain" (an output) on either side of a semiconductor "channel", and a "gate" (a control knob).  If you think of electrical current like fluid flow, this is like a pipe with an inlet, and outlet, and a valve in the middle, and the gate controls the valve.  In a "depletion mode" FET, the gate electrode repels away charges in the channel to turn off current between the source and drain.  In an "accumulation mode" FET, the gate attracts mobile charges into the channel to turn on current between the source and drain.   Bottom line:  the gate uses the electrostatic interaction with charges to control current in the channel.  There has to be a thin insulating layer between the gate and the channel to keep current from "leaking" from the gate.   People have had to get very clever in their geometric designs to maximize the influence of the gate on the charges in the channel.

What's the big deal about making smaller transistors?  We've gotten where we are by cramming more devices on a chip at an absurdly increasing rate, by making transistors smaller and smaller.  One key length scale is the separation between source and drain electrode.  If that separation is too small, there are at least two issues:  Current can leak from source to drain even when the device is supposed to be off because the charge can tunnel; and because of the way electric fields actually work, it is increasingly difficult to come up with a geometry where the gate electrode can efficiently (that is, with a small swing in voltage, to minimize power) turn the FET off and on.

What did the LBL team do?  The investigators built a very technically impressive device, using atomically thin MoS₂ as the semiconductor layer, source and drain electrodes separated by only seven nm or so, a ZrO₂ dielectric layer only a couple of nm thick, and using an individual metallic carbon nanotube (about 1 nm in diameter) as the gate electrode.  The resulting device functions quite well as a transistor, which is pretty damn cool, considering the constraints involved.   This fabrication is a tour de force piece of work.

Does this device really defy physics in some way, as implied by the headline on that news article?  No.  That headline alludes to the issue of direct tunneling between source and drain, and a sense that this is expected to be a problem in silicon devices below the 5 nm node (where that number is not the actual physical length of the channel).   This device acts as expected by physics - indeed, the authors simulate the performance and the results agree very nicely with experiment.

If you read the actual LBL press release, you'll see that the authors are very careful to point out that this is a proof-of-concept device.  It is exceedingly unlikely (in my opinion, completely not going to happen) that we will have chips with billions of MoS₂ transistors with nanotube gates - the Si industry is incredibly conservative about adopting new materials.  If I had to bet, I'd say it's going to be Si and Si/Ge all the way down.   (You will very likely need to go away from Si if you want to see this kind of performance at such length scales, though.)   Still, this work does show that with proper fabrication and electrostatic design, you can make some really tiny transistors that work very well!

This year's Nobel in physics - Thouless, Kosterlitz, Haldane

Update:  well, I was completely wrong!  Topology ruled the day.  I will write more later about this, but Congratulations to Thouless, Kosterlitz, and Haldane!

Real life is making this week very busy, so it will be hard for me to write much in a timely way about this, and the brief popular version by the Nobel Foundation is pretty good if you're looking for an accessible intro to the work that led to this.  Their more technical background document (clearly written in LaTeX) is also nice if you want greater mathematical sophistication.

Here is the super short version.  Thouless, Kosterlitz, and Haldane had major roles to play in showing the importance of topology in understanding some key model problems in condensed matter physics.

Kosterlitz and Thouless (and independently Berezinskii) were looking at the problem of phase transitions in two dimensions of a certain type.  As an example, imagine a huge 2d array of compass needles, each free to rotate in the plane, but interacting with their neighbors, so that neighbors tend to want to point the same direction.  In the low temperature limit, the whole array will be ordered (pointing all the same way).  In the very high temperature limit, when thermal energy is big compared to the interaction between needles, the whole array will be disordered, with needles at any moment randomly oriented.  The question is, as temperature is increased, how does the system get from ordered to disordered?  Is it just a gradual thing, or does it happen suddenly in a particular way?  It turns out that the right way to think about this problem is in terms of vorticity, a concept that comes up in fluid mechanics as well (see this wiki page with mesmerizing animations).  It's energetically expensive to flip individual needles - better to rotate needles gradually relative to their neighbors.  The symmetry of the system says that you can't spontaneously create a pattern to the needles that has some net swirliness ("winding number", if you like).  However, it's relatively energetically cheap to create pairs of vortices with opposite handedness (vortex/antivortex pairs).  Kosterlitz, Thouless, and Berezinskii showed that these V/AV pairs "unbind" collectively at some finite temperature in a characteristic way, with testable consequences.  This leads to a particular kind of phase transition in a bunch of different 2d systems that, deep down, are mathematically similar.  2d xy magnetism and superconductivity in 2d are examples.  This generality is very cool - the microscopic details of the systems may be different, but the underlying math is the same, and leads to testable quantitative predictions.

Thouless also realized that topological ideas are critically important in 2d electronic systems in large magnetic fields, and this work led to understanding of the quantum Hall effect.  Here is a nice Physics Today article on this topic.   (Added bonus:  Thouless also did groundbreaking work in the theory of localization, what happens to electrons in disordered systems and how it depends on the disorder and the temperature.)

Haldane, another brilliant person who is still very active, made a big impact on the topology front studying another "model" system, so-called spin chains - 1d arrangements of quantum mechanical spins that interact with each other.  This isn't just a toy model - there are real materials with magnetic properties that are well described by spin chain models.  Again, the questions were, can we understand the lowest energy states of such a system, and how those ordered states go away as temperature is increased.  He found that it really mattered in a very fundamental way whether the spins were integer or half-integer, and that the end points of the chains reveal important topological information about the system.  Haldane has long contributed important insights in quantum Hall physics as well, and in all kinds of weird states of matter that result in systems where topology is critically important.  (Another added bonus:  Haldane also did very impactful work on the Kondo problem, how a single local spin interacts with conduction electrons.)

Given how important topological ideas are to physics these days, it is not surprising that these three have been recognized.   In a sense, this work is a big part of the foundation on which the topological insulators and other such systems are built.


Original post:  The announcement this morning of the Nobel in Medicine took me by surprise - I guess I'd assumed the announcements were next week.  I don't have much to say this year; like many people in my field I assume that the prize will go to the LIGO gravitational wave discovery, most likely to Rainer Weiss, Kip Thorne, and Ronald Drever (though Drever is reportedly gravely ill).    I guess we'll find out tomorrow morning!

Mapping current at the nanoscale - part 1 - scanning gates

Inspired by a metaphor made by our colloquium speaker, Prof. Silke Paschen, this past week, I'd like to try to explain to a general audience a couple of ways that people have developed for mapping out the flow of charge in materials on small scales.

Eric Heller's art piece "Dendrite", based
on visualization of branching current flow.

Often we are interested in understanding how charge flows through some material or device.  The simplest picture taught in courses is an analogy with water flowing through a pipe.  The idea is that there is some input for current, some output for current, and that in the material or device, you can think of charge moving like a fluid flowing uniformly along.  Of course, you could imagine a more complicated situation - perhaps the material or device doesn't have uniform properties; in the analogy, maybe there are obstacles that block or redirect the fluid flow.  Prof. Eric Heller of Harvard is someone who has thought hard about this situation, and how to visualize it.  (He's also a talented artist, and the image at right is an example of artwork based on exactly this issue - how the flow of electrons in a solid can branch and split because of disorder in the material.)

There's a different analogy that might be more useful in thinking about how people actually map out the flow of current in real systems, though.  Suppose you wanted to map out the roads in a city.  These days, one option would be to track all GPS devices (especially mobile phones) moving faster than, say, a few km/h.  If you did that you would pretty quickly resolve a decent map of the streets of a city, and you'd find where the traffic is flowing in high volume and at what speed.  Unfortunately, with electronic materials and devices, we generally don't have the option of tracking each individual mobile electron.  

Some condensed matter experimentalists (like Bob Westervelt, for example) have developed a strategy, however.  Here's the traffic analogy: You would set up traffic cameras to monitor the flow of cars into and out of the city.  Then you would set up road construction barrels (lanes blocked off, road closures) in known locations in the city, and see how that affected the traffic flow in and out of town.  By systematically recording the in/out traffic flow as a function of where you put in road closures, you could develop a rough map of the important routes.  If you temporarily close a road that hardly carries any cars, there won't be any effect on the net traffice, but if you close a major highway, you'd see a big effect.  

The experimental technique is called scanning gate microscopy.  Rather than setting up traffic cones, the experimentalists take a nanoscale-sharp conductive tip and scan it across the sample in question, mapping the sample's end-to-end conduction as a function of where the tip is and what it's doing.  One approach is to set the tip at a negative potential relative to the sample, which would tend to repel nearby electrons just from the usual like-charges-repel Coulomb interaction.  If there is no current flowing near the tip, this doesn't do much of anything.  If the tip is right on top of a major current path, though, this can strongly affect the end-to-end conduction.   It's a neat idea, and it can produce some impressive and informative images.  I'll write further about another technique for current mapping soon.

Nanovation podcast

 Michael Filler is a chemical engineering professor at Georgia Tech, developing new and interesting nanomaterials.  He is also the host of the outstanding Nanovation podcast, a very fun and informative approach to public outreach and science communication - much more interesting than blogging :-) .  I was fortunate enough to be a guest on his podcast a couple of weeks ago - here is the link.  It was really enjoyable, and I hope you have a chance to listen, if not to that one, then to some of the other discussions.

Deborah Jin - gone way too soon.

As was pointed out by a commenter on my previous post, and mentioned here by ZapperZ, atomic physicist Deborah Jin passed away last week from cancer at 47.   I don't think I ever met Prof. Jin (though she graduated from my alma mater when I was a freshman) face to face, and I'm not by any means an expert in her subdiscipline, but I will do my best to give an overview of some of her scientific legacy.  There is a sad shortage of atomic physics blogs....  I'm sure I'm missing things - please fill in additional information in the comments if you like.

The advent of optical trapping and laser cooling (relevant Nobel here) transformed atomic physics from what had been a comparatively sleepy specialty, concerned with measuring details of optical transitions and precision spectroscopy (useful for atomic clocks), into a hive of activity, looking at the onset of new states of matter that happen when gases become sufficiently cold and dense that their quantum statistics start to be important.  In a classical noninteracting gas, there are few limits on the constituent molecules - as long as they don't actually try to be in the same place at the same time (think of this as the billiard ball restriction), the molecules can take on whatever spatial locations and momenta that they can reach.  However, if a gas is very cold (low average kinetic energy per molecule) and dense, the quantum properties of the constituents matter - for historical reasons this is called the onset of "degeneracy".  If the constituents are fermions, then the Pauli principle, the same physics that keeps all 79 electrons in an atom of gold from hanging out in the 1s orbital, keeps the constituents apart, and keeps them from all falling into the lowest available energy state.   In contrast, if the constituents are bosons, then a macroscopic fraction of the constituents can fall into the lowest energy state, a process called Bose-Einstein condensation (relevant Nobel here); the condensed state is a single quantum state with a large occupation, and therefore can show exotic properties.

Prof. Jin's group did landmark work with these systems.  She and her student Brian DeMarco showed that you could actually reach the degenerate limit in a trapped atomic Fermi gas.  A major challenge in this field is trying to avoid 3-body and other collisions that can create states of the atoms that are no longer trapped by the lasers and magnetic fields used to do the confinement, and yet still create systems that are (in their quantum way) dense.  Prof. Jin's group showed that you could actually finesse this issue and pair up fermionic atoms to create trapped, ultracold diatomic molecules.  Moreover, you could then create a Bose-Einstein condensate of molecules (since a pair of fermions can be considered as a composite boson).  In superconductors, we're used to the idea that electrons can form Cooper pairs, which act as composite bosons and form a coherent quantum system, the superconducting state.  However, in superconductors, the Cooper pairs are "large" - the average real-space separation between the electrons that constitute a pair is big compared to the typical separation between particles.  Prof. Jin's work showed that in atomic gases you could span between the limits (BEC of tightly bound molecules on the one hand, vs. condensed state of loosely paired fermions on the other).  More recently, her group had been doing cool work looking at systems good for testing models of magnetism and other more complicated condensed matter phenoma, by using dipolar molecules, and examining very strongly interacting fermions.   Basically, Prof. Jin was an impressively creative, technically skilled, extremely productive physicist, and by all accounts a generous person who was great at mentoring students and postdocs.   She has left a remarkable scientific legacy for someone whose professional career was tragically cut short, and she will be missed.

Alan Alda Center for Communicating Science, posting

Tomorrow I'll be a participant in an all-day workshop that Rice's Center for Teaching Excellence will be hosting with representatives from the Alan Alda Center for Communicating Science - the folks responsible for the Flame Challenge, a contest about trying to explain a science topic to an 11-year-old.  I'll write a follow-up post sometime soon about what this was like.

I'm in the midst of some major writing commitments right now, so posting frequency may slow for a bit.  I am trying to plan out how to write some accessible content about some recent exciting work in a few different material systems. 

 

Professional service

An underappreciated part of a scientific career is "professional service" - reviewing papers and grant proposals, filling roles in professional societies, organizing workshops/conferences/summer schools - basically carrying your fair share of the load, so that the whole scientific enterprise actually functions.  Some people take on service roles primarily because they want to learn better how the system works; others do so out of altruism, realizing that it's only fair, for example, to perform reviews of papers and grants at roughly the rate you submit them; still others take on responsibility because they either think they know best how to run/fix things, or because they don't like the alternatives.   Often it's a combination of all of these.

More and more journals proliferate; numbers of grant applications climb even as (in the US anyway) support remains flat or declining; and conference attendance continues to grow (the APS March Meeting is now twice as large as in my last year of grad school).  This means that professional demands are on the rise.  At the same time, it is difficult to track and quantify (except by self-reporting) these activities, and reward structures give only indirect incentive (e.g., reviewing grants gives you a sense of what makes a better proposal) to good citizenship.  So, when you're muttering under your breath about referee number 3 or about how the sessions are organized nonoptimally at your favorite conference (as we all do from time to time), remember that at least the people in question are trying to contribute, rather than sitting on the sidelines.

Conference for Undergraduate Women in Physics!

Over January 13-15, 2017, Rice is going to be hosting one of the American Physical Society's Conferences for Undergraduate Women in Physics.  Registration is now open - please click on the link in the previous sentence, and you will be taken to the meeting website.  This is one of about 10 regional CUWiP meetings, and our region encompasses Texas, Mississippi, Alabama, Florida, Arkansas, and Louisiana.  Many thanks to my faculty colleagues Prof. Marj Corcoran and Prof. Pat Reiff for leading the way on this, and to our staff administrator and our excellent SPAS undergraduates for their efforts.

Gulf Coast Undergraduate Research Symposium!

Rice University's schools of Natural Sciences and Engineering want to make sure that when talented science and engineering undergraduates in the US are deciding where to apply for graduate school, we are on their radar, so to speak.  To that end, we are hosting our second annual Gulf Coast Undergraduate Research Symposium.  To quote the webpage,

The Gulf Coast Undergraduate Research Symposium (GCURS) is a forum for undergraduate researchers to present original research discoveries.... GCURS fosters intercollegiate interactions among students and faculty who share a passion for undergraduate research. We expect several hundred speakers from about half of the states. The event also offers a friendly and supportive environment to students who would be giving their first formal research presentation, and faculty will provide written constructive feedback.

The registration deadline is Sept. 29.  Breakfast, lunch, and dinner will be provided on Saturday, and travel expenses for students (hotel, mileage, airfare if preapproved) will be covered by Rice's Office of Graduate and Postdoctoral Studies.  Please pass this along - it's a fun time.  If you want more details and contact information either for our department's role or the meeting as a whole, please let me know.

Amazon book categories are a joke

A brief non-physics post.  Others have pointed this out, but Amazon's categorizations for books are broken in such a way that they almost have to be designed to encourage scamming.  As an example, my book is, at this instant (and that's also worth noting - these things seem to fluctuate nearly minute-to-minute), the number 30 best seller in "Books > Science & Math > Physics > Solid State Physics".  That's sounds cool, but it's completely meaningless, since if you click on that category you find that it contains such solid state physics classics as "Ugly's Electrical References, 2014 ed.", "Barron's 500 Flash Cards of American Sign Language", "The Industrial Design Reader", and "Electrical Motor Controls for Integrated Systems", along with real solid state books like Kittel, Simon, and Ashcroft & Mermin.  Not quite as badly, the Nanostructures category is filled "Strength of Materials" texts and books about mechanical structures.  Weird, and completely fixable if Amazon actually cared, which they seem not to.

Proxima Centauri's planet and the hazards of cool animations

It was officially announced today that Proxima Centauri has a potentially earthlike planet.  That's great, especially for fans of science fiction.  Here is a relevant video by Nature:

Did you spot the mistake?  The scientists discovered the planet by seeing the wobble in the star's motion (measured by painstaking spectroscopy of the starlight, and using the Doppler shift of the spectrum to "see" the tiny motion of the star).  The animation tries to show this at 0:55-1:12.  The wobble is because the star and planet actually orbit around a common center of mass located on the line between them.  Instead, the video seems to show the center of mass of the star+planet tracing out a circle around empty space.  Whoops.   Someone should've caught that.  Still an impressive result.

Update:  The makers of the video have updated with a link to a more accurate animation of the Doppler approach:  https://youtu.be/B-oZYm3L1JE.

Statistical and Thermal Physics

Eight years ago I taught Rice's undergraduate Statistical and Thermal Physics course, and now after teaching the honors intro physics class for a while, I'm returning to it.   I posted about the course here, and I still feel the same - the subject matter is intellectually very deep, and it's the third example in the undergraduate curriculum (after electricity&magnetism and quantum mechanics) where students really need to pick up a different way of thinking about the world, a formalism that can seem far removed from their daily experience.

One aspect of the course, the classical thermodynamic potentials and how one goes back and forth between them, nearly always comes across as obscure and quasi-magical the first (or second) time students are exposed to it.  Since the last time I taught the course, a nice expository article about why the math works has appeared in the American Journal of Physics (arxiv version).  

Any readers have insights/suggestions on other nice, recent pedagogical resources for statistical and thermal physics?