Quantum materials are driving a technology revolution
in sensing,
communication, and computing, while simultaneously testing many core
theories of the past century. Materials such as topological insulators,
complex oxides, superconductors, quantum dots, color center-hosting
semiconductors, and other types of strongly correlated materials can
exhibit exotic properties such as edge conductivity, multiferroicity,
magnetoresistance, superconductivity, single photon emission, and
optical-spin locking. These emergent properties arise and depend strongly
on the material’s detailed atomic-scale structure, including
atomic defects, dopants, and lattice stacking. In this review, we
describe how progress in the field of electron microscopy (EM), including
in situ and in operando EM, can accelerate advances in quantum materials
and quantum excitations. We begin by describing fundamental EM principles
and operation modes. We then discuss various EM methods such as (i)
EM spectroscopies, including electron energy loss spectroscopy (EELS),
cathodoluminescence (CL), and electron energy gain spectroscopy (EEGS);
(ii) four-dimensional scanning transmission electron microscopy (4D-STEM);
(iii) dynamic and ultrafast EM (UEM); (iv) complementary ultrafast
spectroscopies (UED, XFEL); and (v) atomic electron tomography (AET).
We describe how these methods could inform structure–function
relations in quantum materials down to the picometer scale and femtosecond
time resolution, and how they enable precision positioning of atomic
defects and high-resolution manipulation of quantum materials. For
each method, we also describe existing limitations to solve open quantum
mechanical questions, and how they might be addressed to accelerate
progress. Among numerous notable results, our review highlights how
EM is enabling identification of the 3D structure of quantum defects;
measuring reversible and metastable dynamics of quantum excitations;
mapping exciton states and single photon emission; measuring nanoscale
thermal transport and coupled excitation dynamics; and measuring the
internal electric field and charge density distribution of quantum
heterointerfaces- all at the quantum materials’ intrinsic atomic
and near atomic-length scale. We conclude by describing open challenges
for the future, including achieving stable sample holders for ultralow
temperature (below 10K) atomic-scale spatial resolution, stable spectrometers
that enable meV energy resolution, and high-resolution, dynamic mapping
of magnetic and spin fields. With atomic manipulation and ultrafast
characterization enabled by EM, quantum materials will be poised to
integrate into many of the sustainable and energy-efficient technologies
needed for the 21st century.