Exploring finite temperature properties of materials with quantum computers.

Thermal properties of nanomaterials are crucial to not only improving our fundamental understanding of condensed matter systems, but also to developing novel materials for applications spanning research and industry. Since quantum effects arise at the nano-scale, these systems are difficult to simulate on classical computers. Quantum computers can efficiently simulate quantum many-body systems, yet current quantum algorithms for calculating thermal properties of these systems incur significant computational costs in that they either prepare the full thermal state on the quantum computer, or they must sample a number of pure states from a distribution that grows with system size. Canonical thermal pure quantum (TPQ) states provide a promising path to estimating thermal properties of quantum materials as they neither require preparation of the full thermal state nor require a growing number of samples with system size. Here, we present an algorithm for preparing canonical TPQ states on quantum computers. We compare three different circuit implementations for the algorithm and demonstrate their capabilities in estimating thermal properties of quantum materials. Due to its increasing accuracy with system size and flexibility in implementation, we anticipate that this method will enable finite temperature explorations of relevant quantum materials on near-term quantum computers.

Computing Free Energies with Fluctuation Relations on Quantum Computers.

As a central thermodynamic property, free energy enables the calculation of virtually any equilibrium property of a physical system, allowing for the construction of phase diagrams and predictions about transport, chemical reactions, and biological processes. Thus, methods for efficiently computing free energies, which in general is a difficult problem, are of great interest to broad areas of physics and the natural sciences. The majority of techniques for computing free energies target classical systems, leaving the computation of free energies in quantum systems less explored. Recently developed fluctuation relations enable the computation of free energy differences in quantum systems from an ensemble of dynamic simulations. While performing such simulations is exponentially hard on classical computers, quantum computers can efficiently simulate the dynamics of quantum systems. Here, we present an algorithm utilizing a fluctuation relation known as the Jarzynski equality to approximate free energy differences of quantum systems on a quantum computer. We discuss under which conditions our approximation becomes exact, and under which conditions it serves as a strict upper bound. Furthermore, we successfully demonstrate a proof of concept of our algorithm using the transverse field Ising model on a real quantum processor. As quantum hardware continues to improve, we anticipate that our algorithm will enable computation of free energy differences for a wide range of quantum systems useful across the natural sciences.

Semiconductor-metal structural phase transformation in MoTe2 monolayers by electronic excitation.

Optical modulation of the crystal structure and materials properties is an increasingly important technique for functionalization of two-dimensional and layered semiconductors, where traditional methods like chemical doping are ineffective. Controllable transformation between the semiconducting (H) and semimetallic (T') polytypes of transition metal chalcogenide monolayers is of central importance to two-dimensional electronics, and thermally-driven and strain-driven examples of this phase transformation have been previously reported. However, the possibility of a H-T' phase transformation driven by electronic or optical excitation is less explored and little is known about the potential energy surface and the magnitude of activation barriers or the mechanism of the phase transformation in the excited state. Here, we model the electronic and ionic structure of excited MoTe2 crystals and demonstrate how electronic excitation leads to a Fermi-surface-nesting driven softening of phonon modes at the Brillouin zone boundary and the subsequent stabilization of a low-energy intermediate crystal structure along the semiconductor-metal phase transition pathway. The significantly reduced barriers for this transformation upon electronic excitation suggest that optical excitation may enable rapid and controllable synthesis of lateral semiconductor-metal heterophase homojunctions in monolayer materials for use in next-generation two-dimensional nano-electronics applications.

Ultrafast non-radiative dynamics of atomically thin MoSe2.

Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.