Day 2: Defects by Design Incubator
Alex Breitweiser, Christian Pederson & Lukas Razinkovas
Alex Breitweiser, University of Pennsylvania; Christian Pederson, University of Washington; Lukas Razinkovas, Center for Physical Sciences and Technology (FTMC)
Closing out the first day of the OSA Incubator: Defects by Design was a panel of talks on electron microscopy techniques. Electron microscopy offers incredible resolution, giving the ability to measure and manipulate individual atoms, a powerful use for defect identification and formation. However, difficulties in imaging host materials and co-aligning electron microscopes with their optical counterparts have preempted some of these uses. New machines and capabilities address these challenges - recent advancements in STEM (Scanning Transmission Electron Microscopy), presented by Eric Stach (University of Pennsylvania), have allowed imaging down at 30kV - which is low enough in energy to avoid affecting defects in materials. Many imaging techniques are possible via STEM such as differentiating between heavy and light elements in materials, mapping plasmons, 3D imaging of crystal structure and much more. Jay Gupta (Ohio State University) demonstrated how STM (Scanning Tunneling Microscopy) could be used to very precisely measure and manipulate defects and adatoms. This can be used for a whole host of experiments, including looking at how the spectra of a sample are affected by doping concentrations or to observe charge state conversion initiated by either probe tip or light. Surface passivation techniques and clever use of the probe tip can even be used to create single-atom devices, such as transistors with great precision.
Computational techniques were the focus of the first session on the second day of the Incubator. Density Functional Theory (DFT) is the typical tool to calculate level structures for defect systems, but traditional codes have difficulty capturing effects such as phonon coupling and spin-orbit interactions, which can have important consequences in intersystem crossings - even allowing forbidden transitions! Capturing these corrections requires further computational developments to capture the observed phenomena. Furthermore, it was pointed out that group-theory based techniques can capture a wide range of phenomenological behavior, which is complementary to new spectroscopic techniques that extract multipole electromagnetic symmetries of defect emission.
The discussion following these talks revolved around how to determine the credibility of new computational techniques, and the relative merits of simpler (but more general) phenomenological models as opposed to exact numerical results that are specific to given systems. The wide accessibility and low barrier to entry of exact numerical codes has led to a plethora of results, not all of which receive the same level of care and scrutiny. On the other hand, these methods have been very successful in particular systems, such as the SiV and GeV centers in diamond, and it’s possible that purely phenomenological models miss corrections that can change behaviors, such as the spin-phonon coupling which modifies inter-system crossings mentioned above. Still, phenomenological models can capture universal behavior of a wide range of defects, roughly predicting the properties of a large family of defects with only a few calibration measurements.
Device applications are the final goal of these defects, and were thus the focus of the final session of the incubator. Applications to quantum communication, quantum memories, and quantum LEDs were discussed by the final panel of speakers. NV centers are a natural way to generate entangled GHZ and cluster photon states, due their successfully demonstrated optical entanglement and access to adjacent carbon-13 nuclear spins for auxiliary qubits. Rare-earth ions benefit from electronic shielding of outer shells and serve as natural candidates for memory based qubits - displaying coherence times of up to several hours, with a rich level structures offering shelving states and high tunability. Two-dimensional material defects are another area of recent interest, with heterostructures built from materials like Transition Metal Dichalcogenides (TMDs) and hBN offering the potential to build devices such as quantum LEDs. Recent strain-engineering methods have allowed near-deterministic emitter placement in these materials, opening an avenue for large-scale photonic circuits.
The final discussion focused on what properties defects need to be usable in devices. Room temperature coherence, indistinguishability, tunability, and scalability were all discussed as ideal characteristics of defects but were found to not be a necessary requirement in all applications. Even as the last session closed, lively discussions continued around the room as people waited for their flights. It’s clear this Incubator has brought about a great amount of collaborative effort to tackle the challenges in this field, as well as highlighting new directions for further study.
