The objective of this program is to engineer nanostructures to control the thermal properties of superconducting nanowire single-photon detectors. Specific objectives are: engineer the electro-thermal response of the detectors to improve the speed of these devices; and, model the microscopic mechanism of photodetection to shed light on the physical origin of the detector jitter and minimize it. To achieve the first objective, an improved electro-thermal model of the superconducting nanowire will be developed. Nanostructures will be designed to directly control the thermal behaviour of the nanowires at the nanoscale, which will in turn allow validating the predictions the theoretical model. Achieving the second objective will require the development of a detailed model of the excitation and expansion of the hot-spot. The intellectual merit of this program is to advance the speed and timing precision of superconducting nanowire single-photon detectors by first clarifying their fundamental physics of operation, and then engineering a new device architecture based on this understanding. The broader impacts of this program are: (1) the introduction of a new generation of diverse undergraduate and graduate students to the fields of superconductivity, thermal processes in condensed matter, and nano- and quantum-optics; (2) the integration of the scientific results as case studies into the curriculum of two courses (one for graduate students and advanced undergraduates, and another targeted at post-graduate professionals), which focus on the practical role of nanofabrication in advanced research; and (3) the development of technological applications in industrial and scientific areas ranging from communications to astrophysical observations.
A superconductor is a material that allows electrical current to pass through it without any energy loss, unlike the normal conductors which pervade our daily lives, inside of our computers and our homes. Nanostructures are objects designed and created to have dimensions that are on the order of a nanometer, approximately one ten-thousandth the width of a human hair. Our grant from the NSF helped us understand, design and fabricate nanostructures made out of superconducting material that we used to detect tiny amounts of infrared light and to create new superconducting devices. What happens when you make a nanostructure out of a superconductor? If you make it small enough, then the heat from a tiny amount of light is able to stop it from superconducting. This transition is detectable and is the basis for creating some of the most sensitive detectors of light ever made. By making a nanoscale wire out of superconducting material, we can create a detector that is sensitive to the smallest amount of light possible. This tiny and discrete amount of light is known as a photon, and our devices are single photon detectors. Currently, there are a few different types of single photon detectors, but not all are created equal. Each type of photon detector has a varying ability to detect a photon when it is present, they all suffer from some degree of false detections, they all have different maximum speeds and they all have some randomness in the time that elapses between photon arrival and detection. Superconducting nanowire single photon detectors beat the competition in all of these areas, detecting photons more efficiently, faster, with fewer false positives. Additionally, they can determine the time of a photon’s arrival very precisely, which is an important factor in secure communications applications.However, the theoretical limitations of these detectors are still not known and their microscopic behavior is still being understood. These detectors are enabling advances in low power communication, quantum cryptography and computation, LIDAR (RADAR using infrared light instead of radio waves), and other areas requiring ultrasensitivite detection of light. Previously, we used a simplified model to predict some aspects of our detector’s performance. It made a lot of assumptions, but we were able to make some predictions about how to make sure the devices would always be able to reset themselves to be ready to detect again after firing. However, the model did not allow us to take into account many important effects, such as the geometry of the device. Additionally it made a lot of simplifying assumptions. We needed to expand it to try to incorporate more phenomenon so that we could try to tackle some of the remaining questions about these detectors. Our NSF funding allowed us to create and test a more complex model that allows us to predict the behavior of superconducting nanostructures with complex geometries. It also includes some of the subtle issues related to whether the heating goes into moving electrons or sound waves in the material.. Our improved simulation can run on a desktop computer and allowed us predict how our detectors respond to various influences that can suppress superconductivity in our nanocircuits. In particular, modeling the heating in the detection event lead to a better understanding of how localized heating could suppress superconductivity in larger sections. Our improved understanding of local heating’s effect allowed us to predict and then create a new superconducting three terminal device, which functions similarly to a transistor but is capable of operation at cryogenic temperatures. We’ve used this new device to implement a variety of superconducting circuits, similar to many circuits that are implemented with standard semiconductors but with the promise of combining speed and lower power dissipation. Our simulation is also helping us improve standard SNSPDs, and further explore how they work. There are still phenomena that occur in superconducting nanowires and nanocircuits that are not well-understood. The fundamental limits for what frequency of photon can be detected is still being explored. Structures similar to our detectors are being used as platforms to better understand quantum mechanics.
