Silicon-vacancy (SiV
−) centers in diamond are promising candidates as sources of single-photons in quantum networks due to their minimal phonon coupling and narrow optical linewidths. Correlating SiV
− emission with the defect's atomic-scale structure is important for controlling and optimizing quantum emission, but remains an outstanding challenge. Here, we use cathodoluminescence imaging in a scanning transmission electron microscope (STEM) to elucidate the structural sources of non-ideality in the SiV
− emission from nanodiamonds with sub-nanometer-scale resolution. We show that different crystalline domains of a nanodiamond exhibit distinct zero-phonon line (ZPL) energies and differences in brightness, while near-surface SiV
− emitters remain bright. We correlate these changes with local lattice expansion using 4D STEM and diffraction, and show that associated blue shifts from the ZPL are due to defect density heterogeneity, while red shifts are due to lattice distortions.
Single-photon sources play a pivotal role in quantum information technology and quantum sensing [1]. Quantum dots and color centers allow the creation of single photons on demand, are capable of generating higher Fock states and can be electrically driven [1], [2]. Spontaneous parametric down-conversion and spontaneous four-wave mixing, in turn, serve as widely tunable sources of heralded single photons and complex quantum states [3]. The generation of photons radiated from swift free electrons makes it possible to obtain a broad spectrum, amplification, and electron-photon entanglement. Using flatband resonances and microcavities may even enhance the processes [4], [5]. Modulation of free electrons enables the construction of photonic cat and GKP states, which are crucial for fault-tolerant quantum computing [6]. Recently, the effect of generating 1550 nm photons from free electrons directly into a nanophotonic Si3N4 ring resonator in a transmission electron microscope (TEM) at 120 keV has been experimentally shown [5].
We employ ptychography, a phase-retrieval imaging technique, to show experimentally that a partially coherent high-energy matter (electron) wave emanating from an extended source can be decomposed into a set of mutually independent modes of minimal rank. Partial coherence significantly determines the optical transfer properties of an electron microscope and so there has been much work on this subject. However, previous studies have employed forms of interferometry to determine spatial coherence between discrete points in the wave field. Here we use the density matrix to derive a formal quantum mechanical description of electron ptychography and use it to measure a full description of the spatial coherence of a propagating matter wave field, at least within the fundamental uncertainties of the measurements we can obtain.