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Advancing Quantum Frontiers in Molecular and Materials Science

We are a research group in theoretical and computational physical chemistry advancing frontiers of molecular and materials science where novel quantum mechanical effects play an important role. We tackle emerging problems at the interface between Chemistry and Physics, Optics and Nanoscience. We develop new theories, methods and computational algorithms. We strive to identify new phenomenology and new fundamental principles. We work closely with experimentalists, but we also like to be a few years ahead of experiments.

We are currently working on four exciting areas of research at the cutting edge of quantum science, nanoscience and physical chemistry:

Quantum Control

Strong light-matter interactions can endow matter with unique physico-chemical properties with fundamental and technological implications. Our goal is to develop the theory and simulation needed to understand the emergent electronic properties of matter when driven far from equilibrium by lasers and investigate the limits in the quantum control of matter at the level of electrons. The vision is to go beyond usual efforts to establish structure-function relations based on equilibrium properties and actively seek to establish “structure-drive-function” relations that apply far from thermodynamic equilibrium. We aim at catalyzing the development of a novel class of laser-dressed dynamical electronic materials with “on-demand” effective properties that are tunable on an ultrafast timescale.

Our group advances general strategies for the ultrafast laser control of matter at the level of electrons. In particular, we have demonstrated that the Stark effect induced by strong ultrashort non-resonant laser pulses provides a general route to control electron dynamics that has the advantage of being robust to decoherence . We have proposed schemes based on the Stark effect to drive ultrafast electronic currents in nanojunctions (see 1), transiently convert transparent material into absorbers (see 2), and insulators into conductors (see 3, 4, 5).

We are currently working on understanding the effective physical and chemical properties of molecules and materials when dressed by non-resonant light (see, e.g., 2) and in developing schemes to control optically driven currents along nanojunctions (see, e.g., 1)

Quantum Coherence in Matter: Tackling the Decoherence Challenge

One of the greatest challenges for science and engineering in the 21st century is to harness the quantum features of matter to fuel the next technological revolution. The challenge in using molecules for quantum technologies is that the molecular quantum coherence –that enables its desirable quantum features such as its ability to interfere, be controlled or entangle– is very sensitive to the unavoidable interactions of the molecule with its surrounding environment. Such interactions introduce decoherence (or quantum noise) processes that corrupt the desired time-evolution of the molecule and thus its quantum controllability. In response to this challenge, our goal is to advance our fundamental understanding and our ability to model, control and preserve quantum coherence in molecules. Using state-of-the-art methods, we are tackling the basic questions in molecular decoherence research: How fast is the decoherence? What are the main mechanisms for coherence loss? How to quantify and model the decoherence? How can the decoherence be mitigated or exploited?

For example, we developed a useful theory of early-times decoherence time scales (see 1), and then used it to generalize the theory of molecular electronic decoherence (see 2). This theory has now led to important efforts to control electronic coherence loss via lasers (see 3). We clarified the extent to which classical nuclei and noise models of the bath are useful to model quantum decoherence (see 1, 4 and 5), providing a solid basis for the development of mixed quantum-classical schemes to model decoherence in matter. We have investigated many-body aspects of electronic coherence loss (see 6) and, in doing so, opened a path to use approximate electronic structure theories to model exact decoherence dynamics in molecules. At a computational level, we have emphasized approaches were the quantum dynamics of the bath is considered explicitly (see 4), thus providing detailed insights into the system-bath entanglement that leads to coherence loss.

Most recently, we have been working on developing a theory of dissipation pathways that now enable us to understand how the structure of the environment leads to coherence loss (see 7).

Quantum Simulation

Computing the quantum dynamics of molecules in condensed phases with high precision is a central challenge in chemistry. Tackling this problem exactly with conventional computers using state of the art methods is a formidable task as it involves solving the Liouville von Neumann (LvN) equation for a molecule interacting with a macroscopic environment that can operate at disparate timescales with varying interaction strengths to the system (thus highly structured), remember the dynamical history of the system (thus non-Markovian), and lead to both energy relaxation, loss of quantum coherence and environment-mediated interactions (thus quantum many-body). In addition to developing new methods in quantum dynamics, we are developing analog quantum simulators of condensed phase chemical dynamics. In this approach, instead of trying to solve the LvN using digital computation, the problem of physical interest is mapped onto a highly controllable experimental setup and Nature is allowed to do the computations.

Molecular Electronics and Quantum Transport

Molecular electronics experiments have emerged as a powerful and versatile platform where voltages, force, and light can simultaneously be applied and used to investigate chemistry and physics at the single-molecule limit. In particular, this class of experiments enable us to interrogate how molecular structure determines charge transport. Our group is developing useful atomistic modeling strategies to capture the two main experimental techniques in this field: ultra-high vacuum STM experiments and room-temperature break junction experiments. We are very interested in modeling and proposing experiments that simultaneously measure force and conductance on single-molecules, as we believe this can become the basis of a useful multidimensional spectroscopy that operates at the single-molecule limit. These are our advances:

Modeling STM Experiments: We developed an accurate force field for metal-molecule interactions that now enable us to model STM experiments on experimentally relevant system sizes and time scales (see 1 and 2). Once we are able to establish contact with experiments, the simulations offer an atomically sharp microscope of the molecular dynamics!

Modeling break-junction experiments: Reproducibility in break-junction experiments relies on measuring the conductance on thousands of freshly formed junctions and performing statistics. The resulting conductance histograms are highly reproducible, and their width is believed to arise because of the uncontrollable variations in junction configurations between experiments. While most simulation efforts focus on a few representative conformations, our group considers all possible conformations and the statistics of junction formation and rupture (see 3 and 4). Contrary to conventional wisdom, these atomistic simulations have revealed that conductance changes during mechanical manipulation is the key physical aspect that needs to be controlled to narrow the width of the conductance histograms (see 4).

Transport beyond Landauer: The central formula in molecular electronics is the Landauer formula which supposes that transport is in steady state and due to quantum tunneling. We are investigating the limits of this formula in thermal environments where the decohering interactions can lead to transport paths that go beyond Landauer (see 5).

Chemical Reactivity:  A nascent frontier in molecular electronics is the possibility to investigate chemical reactions.  We are studying how how reactivity changes when molecules are nanoconfined in the context of molecular junctions and investigate the fundamental limits of this platform to investigate chemical reactivity at the single-entity limit (see 6 and 7).

Force-Conductance Spectroscopies: Our hypothesis is that force and conductance measurements provide complementary information about the behavior of single-molecules. We have studied this correlation to investigate conformational dynamics (see 1, 2 and 8), hydrogen-bonding (see 9), and propose mechanically-activated molecular switches (see 10 and 11).

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