Shortcuts to dynamic polarization08 February 2021
Our paper on Shortcuts to dynamic polarization has now also been published in Physical Review B. Even better, it has been selected as Editors' Suggestion and is currently advertised on the Physical Review B homepage!
Dynamic polarization protocols aim to hyperpolarize a spin bath by transferring spin polarization from a well-controlled qubit such as a quantum dot or a color defect. Building on techniques from shortcuts to adiabaticity, we design fast and efficient dynamic polarization protocols in central spin models that apply to dipolarly interacting systems. The protocols maximize the transfer of polarization via bright states at a nearby integrable point, exploit the integrability-breaking terms to reduce the statistical weight on dark states that do not transfer polarization, and realize experimentally accessible local counterdiabatic driving through Floquet engineering. A master equation treatment suggests that the protocol duration scales linearly with the number of bath spins with a prefactor that can be orders of magnitude smaller than that of unassisted protocols. This work opens pathways to cool spin baths and extend qubit coherence times for applications in quantum information processing and metrology.
Adiabatic landscape and optimal paths in ergodic systems01 February 2021
Whether one is interested in quantum state preparation or in the design of efficient heat engines, adiabatic (reversible) transformations play a pivotal role in minimizing computational complexity and energy losses. Understanding the structure of these transformations and identifying the systems for which such transformations can be performed efficiently and quickly is therefore of primary importance. In this paper we focus on finding optimal paths in the space of couplings controlling the system's Hamiltonian. More specifically, starting from a local Hamiltonian we analyze directions in the space of couplings along which adiabatic transformations can be accurately generated by local operators, which are both realizable in experiments and easy to simulate numerically. We consider a nonintegrable 1D Ising model parametrized by two independent couplings, corresponding to longitudinal and transverse magnetic fields. We find regions in the space of couplings characterized by a very strong anisotropy of the variational adiabatic gauge potential (AGP), generating the adiabatic transformations, which allows us to define optimal adiabatic paths. We find that these paths generally terminate at singular points characterized by extensive degeneracies in the energy spectrum, splitting the parameter space into adiabatically disconnected regions. The anisotropy follows from singularities in the AGP, and we identify special robust weakly thermalizing and nonabsorbing many-body “dark” states which are annihilated by the singular part of the AGP and show that their existence extends deep into the ergodic regime.
Quantum eigenstates from classical Gibbs distributions22 January 2021
Our paper on Quantum eigenstates from classical Gibbs distributions has been published in SciPost Physics! Even better, it has been awarded the SciPost Select label, with inclusion of an extended abstract in SciPost Selections, "the highest mark of recognition given by the Editorial College". Various new results have been added to the published version compared to the preprint, including a numerical illustration of the Berry-Tabor and BGS conjectures in classical systems and extensive results for Hamiltonians including a vector potential, where we also found a way of obtaining Landau levels starting from the classical Gibbs distribution.
I'm pretty happy with how this paper has been received so far, including its inclusion in the Journal Club for Condensed Matter Physics. It also continues my positive experience with SciPost's model — where if you're interested, you can check out the paper's revision history here.