Session J01: Quantum Information Science and Engineering

Reinforcement Learning for Hamiltonian Engineering of Dipolar Coupled Spin Systems

In systems of electronic and nuclear spins, spin-spin interactions and onsite disorder can lead to a decay of the spin coherence. However, by applying a sequence of resonant pulses to the system, the effective Hamiltonian for the system can be engineered to suppress these effects and extend the coherence times of the spins. Methods such as Average Hamiltonian Theory and Floquet theory have provided a framework to generate effective pulse sequences, both analytically and using numerical methods. However, the performance of these sequences depends on the relative strengths of the disorder and interaction strength. For example, sequences that work well for nuclear spins where interactions typically dominate do not work as well for electronic spins where disorder often dominates. Additionally, different experimental errors influence sequence performance in different ways. Here we show that the reinforcement learning assisted sequence design can be tuned to the specific degree of disorder and interactions present in the experimental system of interest, while also allowing us to compensate for a broad range of experimental errors.   

Simulating Trapped Ions with (py)LIon

The (py)LIon package was written by Bentine et al. to simulate the classical trajectories of ion ensembles in electrodynamic traps. This package outsources simulations to LAMMPS, an established classical molecular dynamics program. This research project attempts to recreate past experiments in ion trapping by using (py)LIon, as well as to debug the package. (py)LIon demonstrates success in some simulations with linear Paul traps: it accurately predicts the locations of each ion in a trapped ion chain; it accurately predicts the change in temperature of an ion system subjected to Langevin baths of different temperatures and damping times. Laser cooling can be simulated by using an effective damping force, but the (py)LIon package fails to simulate laser cooling experiments due to erroneous coding. After some debugging, laser cooling simulations become consistent with preexisting theory and experiments. With this update, (py)LIon successfully reproduces expected results for experiments involving sympathetic cooling between a Ca$^+$ ion and a NH3$^+$ ion. We hope to use this package in the future to simulate sympathetic cooling between Ytterbium isotopes.   

Accurate Prediction of Charge-State-Decay Timescales for NV- centers in Diamond

The NV^- color center in diamond has broad potential for applications in quantum sensing, computation, and communication. Nonetheless, the tendency of the NV^- to revert to its neutral counterpart the NV⁰, which detrimentally affects its useful characteristics, remains to be fully overcome. We provide an ab initio formalism for accurately estimating the timescales for charge-state decay of the NV^- in diamond and, more generally, of color centers in wide-bandgap semiconductors. Working in the context of thermal equilibrium, our formalism employs hybrid density functional theory calculations to achieve its accuracy. The transition of NV⁻ to NV⁰ in the presence of substitutional N is used to illustrate the method [see Z. Yuan et al., PRR 2, 033263 (2020)].   

Postselection and Energy Conservation

Measurements, of observables that do not commute with energy, can shift the expected energy of a measured qubit. Owing to energy conservation, such a shift should be accompanied by an equal-and-opposite energy shift of the apparatus, on average. However, it is less obvious how the energy of an apparatus shifts on a case-by-case basis, i.e., for particular qubit measurement outcomes (postselections). We compute this shift using two measurement models that respect energy conservation explicitly: a toy quantum clock model, and a more realistic model utilizing the Jaynes-Cummings interaction. Our main findings are that the postselected energy shift of the apparatus does not, in general, balance that of the measured qubit (and may even be anomalous, exceeding the level spacing of the qubit), and that the results depend on the particular measurement implementation (as opposed to the targeted measurement). The clock energy shift also explicitly contains the qubit's energy weak value, despite a lack of deliberate weak measurements. We compare our Jaynes-Cummings model results with the experimental findings of [J. Stevens, et al, Phys. Rev. Lett. 129, 110601 (2022)], which analyzed the energetics of single qubit gates, and find agreement in the appropriate regime.   

Progress towards Quantum Fisher Information of the Mach-Zehnder interferometer with losses

We present progress toward determining the Quantum Fisher Information (QFI) for the Mach-Zehnder interferometer with losses. A process for calculating the QFI for high-rank density operators with general non-orthogonal states is introduced. This method is dependent on inverting a matrix of the form (A+I), for which a novel finite series solution is introduced.