Day 1 – OSA Incubator Defects by Design: Quantum Nanophotonics in Emerging Materials
By 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)
The OSA Incubator: Defects by Design kicked off this morning at the OSA Headquarters in Washington, D.C. Top researchers in the field of optically-active material defects gathered to discuss efforts to predict and identify potentially useful quantum defects in new materials, lending the incubator its subtitle “Quantum Nanophotonics in Emerging Materials.” Lee Bassett, (University of Pennsylvania - pictured here), one of the organizers, began the scientific discussion by reviewing advances in the field and introducing some of the key questions for the incubator - such as what makes a good defect system and how do we predict materials which will host these beneficial defects?
The nitrogen-vacancy (NV) center in diamond is the prototypical defect for this field, but it’s certainly not the only one. Silicon carbide (SiC) was highlighted by David Awschalom (University of Chicago) as a potential non-diamond defect system with advantageous quantum control properties. Much like diamond, SiC is a very stable material that hosts single emitting defects. Advances such as nuclear spin decoupling, a property unique to SiC because of its heteronuclear spin environment, have allowed early control fidelities upwards of 99%.
One of the leading applications of spin defects is sensing. The small scale and fast dynamics of defects allow them to sense microscopic signals with good localization in space and time. Beyond allowing good resolution, defect-based sensors are part of a new class called “Quantum Sensors.” According to Fedor Jelezko (Ulm University), quantum sensors are distinguished from their classical counterparts because their coherence times are longer than those of the system they are studying. Furthermore, defects exist in even nanometer-sized particles, so they can be dispersed into biological tissue and detected in-vivo.
Rounding out the morning talks, Dirk Englund (MIT) discussed applications of quantum defects to quantum repeater networks. In the search to build a quantum network, it is known that classical repetition schemes are impossible, as quantum information cannot be copied the same way that classical information can. However, to beat out the exponential fiber-loss in long-distance entanglement schemes, it is still possible to build a so-called quantum repeater (QR). NV-based QRs have been shown on kilometer-scale distances at TU Delft, and longer networks based on telecom fibers are being built at MIT, Delft, and elsewhere.
To operate a network based on NV QRs, it is important they all have identical emission and absorption spectra - as the efficiency of entanglement protocols depends on the two-photon overlap. As brought up in the discussion period after the talks, resolving the challenge of how to make identical NV centers is still an open question. Previous experiments have used the electrical Stark shift to spectrally align NV centers, but it is difficult to imagine that will be feasible for large-scale arrays. The discussion panel proposed that a more fundamental technique, such as chemically synthesizing equivalent defects, would be required.
Another open question is exactly how advantageous it is to have a truly quantum sensor, as opposed to a classical defect-based sensor. Existing classical sensors are extremely sensitive, and it’s likely the advantages of defect sensors will rely mostly on their small size and stability to probe systems not accessible by traditional techniques, rather than their quantum nature. On the other hand, it was pointed out during the discussion that it’s possible the quantum nature of these defects will let quantum sensors beat classical ones, using purification to improve sensitivities beyond the classical single-shot limit.
While the discussion occasionally veered into technical specifics, it also touched on generic properties in successful defect species (such as the symmetry of diamond color centers and spin decoupling in SiC) and how to translate these into new materials, leading naturally into the next session after lunch.
The second session focused on efforts to identify and create defect species in new materials, looking particularly at hosts that display similar advantageous properties to diamond. One of the principal challenges in defect discovery is determining exactly what a particular defect in a new material is. Two important case studies are the defects in zinc oxide (ZnO) and hexagonal boron nitride (h-BN). In the case of ZnO, which displays anomalous n-type conductivity as grown, oxygen vacancies were originally theorized to be responsible based on an increase in conductivity in oxygen-poor environments. Eventually, ab-initio techniques calculated that oxygen vacancies are highly energetically unfavorable and a slew of various experimental techniques (EPR, Raman scattering, etc.) ultimately ruled out the oxygen vacancy. This highlighted the importance of using many different techniques to identify defects. This is clearly the case for h-BN, which displays polarized single emission, but in which the responsible defect species has yet to be identified. Spectroscopic measurements show the point-source emitters have zero-phonon lines (ZPLs) distributed across a broad range of energies, and though polarization measurements have opened an interesting window into the underlying physics, no definitive identification has yet been reported.
In cases where defects are intentionally added to materials, there can be problems with unwanted interactions between the defects and the host materials. Rare earth ions have been deterministically implanted into yttrium aluminum garnet (YAG), and benefit from long coherence times, and strong ZPLs with promise for spin-arrays. Unfortunately, the ions are vulnerable to the nuclear spin environment of YAG. Silicon-vacancies in SiC are promising emitters, however, it is hard to anneal out carbon-vacancies without affecting Silicon-vacancies.
The Incubator continues later today and tomorrow, with three more sessions scheduled. Further sessions focus on electron microscopy in imaging and creating defects, theoretical techniques for understanding and predicting defect properties, and how to apply these defects to emerging technologies and devices. The field of quantum defects is rapidly developing, and the discussion sessions have highlighted many new avenues which have yet to be widely recognized. Many of the comments have pointed to preliminary work as well as planned future work, bringing the full assembled expertise in the field to bear on exciting new directions for defect systems.
David Awschalom, University of Chicago, provides an overview of the state of the art of the technology.
Posted: 29 October 2018 by Alex Breitweiser, Christian Pederson & Lukas Razinkovas | with 0 comments
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