Understanding and Engineering the Timing Precision of Superconducting Nanowire Single Photon Detectors

Superconducting electronics and radiation sensors are exceptional for their speed of operation and precision of timing. As a result, they find application in critical niches such as space communications, metrology, sensing, and computation. The performance of these devices thus sets the limit of what can be achieved in these domains. One type of superconducting detector in particular has demonstrated high speed and timing precision: the superconducting nanowire single photon detector. This type of detector is able to detect the arrival of the smallest amounts of light possible, a single photon. As a result of its excellent speed and precision characteristics, it has found application in a wide variety of areas. For example, quantum key distribution, the secure communications method of the future, crucially relies on timing precision of photon detection in order to guarantee security. In a related field, emerging quantum computing thrusts such as those taking place on photonic integrated circuits rely on the precise detection of single photons. Unfortunately, although the speed limitations of the superconducting nanowire single photodetector are well understood, we do not yet understand what limits timing precision (typically referred to as "jitter"), and thus cannot yet engineer improvement. Many theories have been developed that can explain how these superconducting nanowires function. However, none of these theories can justify the jitter seen in these detectors. In this work, we will investigate the fundamental limits of jitter in superconducting nanowire single-photon detectors, and thus enable improvements in a wide array of application areas. For example, communication data rates depend directly on the jitter because the standard low- power digital communication protocol, pulse-position-modulation, uses timing precision to enhance the data rate. By investigating and characterizing possible sources of timing jitter in these detectors, this work will directly increase the impact of the relevant applications in industry, space, and defense.

Although superconducting nanowires have been studied since the 1970s and have been used as radiation sensors for over 13 years, their picosecond-time-scale dynamics are still not fully understood. Early attempts to explain the timing dynamics in superconducting nanowire single photon detectors focused on possible microscopic origins. In the field of radiation sensors based on superconducting nanowires, some theories related these picosecond-time-scale effects to environmental causes and others to processes intrinsic to the physics of the superconducting nanowires. For example, the hotspot model of the detection mechanism was suggested to explain the time delay between the photon arrival and voltage response as a function of number of incident photons at two different bias currents, but fitting to a theoretical model of gap suppression time was poor and no mention of jitter was made. Later, phase slip centers were purported as the mechanism for the initial hotspot creation but again, no substantive connection to jitter came about from those analyses. In this project, we will probe commonly accepted theories in the field as well as unexplored sources of jitter using both numerical and experimental approaches. We have identified several key components of the nanowire operation that we consider likely sources of jitter: (1) nanowire self-resonance; (2) trapping of vortices; and (3) stochastic elements in the microscopic physics of the hotspot. We intend to characterize the jitter contributions of each of these possible sources, and design modified devices that can reduce these contributions to jitter.