Publications

We describe a robust method for determining Pipek–Mezey (PM) Wannier functions (WF), recently introduced by Jónsson et al. (J. Chem. Theor. Chem. 2017, 13, 460), which provide some formal advantages over the more common Boys (also known as maximally-localized) Wannier functions. The Broyden–Fletcher–Goldfarb–Shanno-based PMWF solver is demonstrated to yield dramatically faster convergence compared to the alternatives (steepest ascent and conjugate gradient) in a variety of one-, two-, and three-dimensional solids (including some with vanishing gaps) and can be used to obtain Wannier functions robustly in supercells with thousands of atoms. Evaluation of the PM functional and its gradient in periodic linear combination of atomic orbital representation used a particularly simple definition of atomic charges obtained by Moore–Penrose pseudoinverse projection onto the minimal atomic orbital basis. An automated “canonicalize phase then randomize” method for generating the initial guess for WFs contributes significantly to the robustness of the solver.

The use of electric fields to modify chemical reactions is a promising, emerging technique in catalysis. However, there exist few guiding principles, and rational design requires assumptions about the transition state or explicit atomistic calculations. Here, we present a linear free energy relationship, familiar in other areas of physical organic chemistry, that microscopically relates field-induced changes in the activation energy to those in the reaction energy, connecting kinetic and thermodynamic behaviors. We verify our theory using first-principles electronic structure calculations of a symmetric S

In the field of polaritonic chemistry, strong light-matter interactions are used to alter chemical reactions inside optical cavities. To understand these processes, the development of reliable theoretical models is essential. While traditional methods have to balance accuracy and system size, new developments in quantum computing offer a path for accurate calculations on currently available quantum devices. Here, we introduce the quantum electrodynamics unitary coupled cluster (QED-UCC) method combined with the Variational Quantum Eigensolver algorithm, as well as the quantum electrodynamics equation-of-motion (QED-EOM) method formulated in the qubit basis that allow accurate calculations of ground-state and excited-state properties of strongly coupled light-matter systems suitable for quantum computers. These methods show excellent agreement with the exact reference results and can outperform their traditional counterparts when strong electronic correlations become significant. This work sets the stage for future developments of polaritonic quantum chemistry methods suitable for both classical and quantum computers.

Proton transfer is ubiquitous in many fundamental chemical and biological processes, and the ability to modulate and control the proton transfer rate would have a major impact on numerous quantum technological advances. One possibility to modulate the reaction rate of proton transfer processes is given by exploiting the strong light-matter coupling of chemical systems inside optical or nanoplasmonic cavities. In this work, we investigate the proton transfer reactions in the prototype malonaldehyde and Z-3-amino-propenal (aminopropenal) molecules using different quantum electrodynamics methods, in particular quantum electrodynamics coupled cluster theory (QED-CC) and quantum electrodynamical density functional theory (QEDFT). Depending on the cavity mode polarization direction, we show that the optical cavity can increase the reaction energy barrier by 10--20% or decrease the reaction barrier by ∼5%. By using first principles methods, this work establishes strong light-matter coupling as a viable and practical route to alter and catalyze proton transfer reactions.

Recent experimental advances in strongly coupled light-matter systems has sparked the development of general ab-initio methods capable of describing interacting light-matter systems from first principles. One of these methods, quantum-electrodynamical density-functional theory (QEDFT), promises computationally efficient calculations for large correlated light-matter systems with the quality of the calculation depending on the underlying approximation for the exchange-correlation functional. So far no true density-functional approximation has been introduced limiting the efficient application of the theory. In this paper, we introduce the first gradient-based density functional for the QEDFT exchange-correlation energy derived from the adiabatic-connection fluctuation-dissipation theorem. We benchmark this simple-to-implement approximation on small systems in optical cavities and demonstrate its relatively low computational costs for fullerene molecules up to C

We use density functional theory (DFT) to explore the physical properties of an ErW point defect in monolayer WS2. Our calculations indicate that electrons localize at the dangling bonds associated with a tungsten vacancy (VW) and at the Er3+ ion site, even in the presence of a net negative charge in the supercell. The system features a set of intra-gap defect states, some of which are reminiscent of those present in isolated Er3+ ions. In both instances, the level of hybridization is low, i.e., orbitals show either strong Er or W character. Through the calculation of the absorption spectrum as a function of wavelength, we identify a broad set of transitions, including one possibly consistent with the Er3+ 4I15/2→4I13/2 observed in other hosts. Combined with the low native concentration of spin-active nuclei as well as the two-dimensional nature of the host, these properties reveal Er:WS2 as a potential platform for realizing spin qubits that can be subsequently integrated with other nanoscale optoelectronic devices.

The utility of near-term quantum computers and simulators is likely to rely upon software-hardware co-design, with error-aware algorithms and protocols optimized for the platforms they are run on. Here, we show how knowledge of noise in a system can be exploited to improve the design of gate-based quantum simulation algorithms. We concretely demonstrate this co-design in the context of a trapped ion quantum simulation of the dynamics of a Heisenberg spin model. Specifically, we derive a theoretical noise model describing unitary gate errors due to heating of the ions' collective motion, finding that the temporal correlations in the noise induce an optimal gate depth. We then illustrate how tailored feedforward control can be used to mitigate unitary gate errors and improve the simulation outcome. Our results provide a practical guide to the co-design of gate-based quantum simulation algorithms.

Computational simulations of nuclear magnetic resonance (NMR) experiments are essential for extracting information about molecular structure and dynamics, but are often intractable on classical computers for large molecules such as proteins and protocols such as zero-field NMR. We demonstrate the first quantum simulation of a NMR spectrum, computing the zero-field spectrum of the methyl group of acetonitrile on a trapped-ion quantum computer. We reduce the sampling cost of the quantum simulation by an order of magnitude using compressed sensing techniques. Our work opens a new practical application for quantum computation, and we show how the inherent decoherence of NMR systems may enable the simulation of classically hard molecules on near-term quantum hardware.

In this work I will discuss some numerical results on the stability of the many-body localized phase to thermal inclusions. The work simplifies a recent proposal by Morningstar et al. [arXiv:2107.05642] and studies small disordered spin chains which are perturbatively coupled to a Markovian bath. The critical disorder for avalanche stability of the canonical disordered Heisenberg chain is shown to exceed W>20. In stark contrast to the Anderson insulator, the avalanche threshold drifts considerably with system size, with no evidence of saturation in the studied regime. I will argue that the results are most easily explained by the absence of a many-body localized phase.

We analyze a crossover between ergodic and non-ergodic regimes in an interacting spin chain with a dilute density of impurities, defined as spins with a strong local potential. The dilute limit allows us to greatly suppress finite size effects and understand the mechanism of delocalization of these impurities in the thermodynamic limit. In particular, we show that at any finite impurity potential, impurities can always relax by exchanging energy with the rest of the chain. The relaxation rate only weakly depends on the impurity density and decays exponentially, up to logarithmic corrections, with the impurity potential. We show that the same mechanism, which leads to the finite decay rate, also destabilizes the finite-size local integrals of motion at any finite disorder strength. At finite impurity density the system will appear to be localized over a wide range of system sizes. However, this is a transient effect and in the thermodynamic limit the system will always eventually relax to equilibrium.