Monday, March 13, 2017

APS March Meeting 2017 Day 1

Some talks I saw today at the APS March Meeting in New Orleans:

John Martinis spoke about "quantum supremacy".  Quantum supremacy means achieving performance truly superior to classical situation - in Martinis' usage, the idea is to look at cross-correlations between different qubits, and compare with expectations for fully entangled/coherent systems, to assess how well you are able to set, entangle, and preserve the coherence of your quantum bits.

An optical analog:  Coherent light (laser pointer) incident on frosted glass results in a diffuse spot that is, when examined in detail, an incredibly complicated speckle pattern.  The statistics of that speckled light (correlations over different spatial regions) are very different than if you just had a defocused spot.  In his system, he is taking nine (superconducting, tunable transmon) qubits, where they can control both the coupling between neighboring bits and the energy of each bit.  They set the system in an initial state (injecting a known number of microwave photons into particular qubits); set the energies in a known but randomly selected way, turn on and off the neighbor couplings (25 ns timescale) for some number of cycles, and then look where the microwave photons end up, and take the statistics.  They find that they get good agreement with an error rate of 0.3%/qubit/cycle.  That's enough that they could conceivably do something useful.

As a demo, they use their qubits to model the Hofstadter butterfly problem - finding the energy levels of a 2d electronic system (on a hexagonal lattice, which maps to a 1d problem that they can implement w. their array of nine qubits).  They can get a nice agreement between theory and experiment.  Very impressive.  He  concluded w/ a warning not to believe all hype from qc investigators, including himself.  In general, the approach is basically brute force up to ~ 45 qubits or more (couple of hundred), to think about optimal control and feedback schemes before worrying about truly huge scaling.  The only downside to the talk was that it was in a room that was far too small for the audience.

Alex MacLeod gave a nice talk about using scanning near-field optical microscopy to study the metal-insulator transition in V2O3, as in this paper.   By performing cryogenic near-field scanning optical microscopy in ultrahigh vacuum (!), they measured scattered light from nanoscale scanning tip, giving local dielectric information (hence distinction between metal and insulator surroundings) with an effective spatial resolution that is basically the radius of curvature of the tip.   There is pattern formation at the metal-insulator transition because the two phases have different crystal structures (metal = corundum; insulator = monoclinic), and therefore the transition is a problem of constrained free energy minimization.  This generically leads to pattern formation in the mixed-phase regime.  They see a clear percolation transition in optical measurements, coinciding w/ long distance transport measurements - they really are seeing metallic domains.  Strangely, they find a temperature offset betw/ the structural transition (as seen through x-ray) vs the MIT.  The structural transition temperature is higher, and coincides with max anisotropy in the imaged patterns.  They also see pieces of persistent metallic state at low T, suggesting that some other frustration is going on to stabilize this.

Anatole von Lilienfeld of Basel gave an interesting talk about using machine learning techniques to get quantum chemistry information about small molecules faster and allegedly with better accuracy than full density functional theory calculations.  Basically you train the software on molecules that have been solved to some high degree of accuracy, parametrizing the molecules by their structure (a "Coulomb matrix" that takes into account the relative coordinates and effective charges of the ions) and/or bonding (a "bag of bonds" that takes into account two-body bonds).  Then the software can do a really good job interpolating quantum properties (HOMO-LUMO gaps, ionization potentials) of related molecules faster than you could calculate them in detail.  Impressive, but it seems like a powerful look-up table rather than providing much physical insight.

Melissa Eblen-Zayas gave a fun talk about trying to upgrade the typical advanced junior lab to include real elements of experimental design.  Best line:  "At times student frustration was palpable."

Dan Ralph gave a very compelling talk about the origins of spin-orbit torques in thin-film heterostructures.  I've written in the past about related work.   This was a particularly clear exposition, and went to new territory.  Traditionally, if you have a thin film of a heavy metal (tantalum, say), and you pass current through that film, at the upper (and lower) film surface you will accumulate spin density oriented in the plane and perpendicular to the charge current.  He made a clear argument that this is required because of the mirror symmetry properties of typical polycrystalline metal films.  However, if instead you work with a thin material with much lower symmetry (WTe2, for example) instead of the heavy metal, you can exert spin torques on adjacent magnetic overlayers as if the accumulated spin was out of the plane (which could be useful for certain device approaches).

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