High-speed And Spatially-resolved Superconducting Single Photon Detection System And Its Applications

Superconducting nanowire single-photon detectors (SNSPDs) are novel detectors for detecting single-photons efficiently, rapidly and precisely. The sensitive area is meandered nanowire patterned from superconducting ultra-thin film. The detector operates at a bias current slightly lower than its critical current. Once a photon is absorbed by the nanowire, superconductivity in the registered area is broken, resulting in a hotspot. With the assistance of Joule heating, the resistive domain grows up to a maximum. Then it returns back to superconducting state as the heat is cooled along the nanowire and down to the substrate. The dynamic process of photon absorption gives an electrical pulse with fast rising edge and exponentially decayed falling edge. By detecting the amplified pulses, the arrival of photons can be known.With years of fast development, the latest SNSPDs show greater properties than other existing single-photon detectors, such as avalanche photon diodes and photo multiplier tubes. The detection efficiency of niobium nitride (NbN) SNSPDs goes up to80%, and it is over90%for tungsten silicide (WSi) SNSPDs. Dark count rate in such SNSPDs is lower than10Hz at a high efficiency. For NbN SNSPDs, the timing jitter is typically less than50ps, and the maximum counting rate is theoretically higher than100MHz. Moreover, SNSPDs operate in free-running mode. Attributed from SNSPD’s outstanding performances, they have been applied in quantum key distribution, integrated circuit testing, biomedical fluorescent imaging, characterization of quantum emitters, fiber distributed sensing, and time of flight depth imaging.The thesis started with a brief introduction of SNSPDs with the latest progresses. Then, we discussed approaches for improving detector performance, presented novel architectures, and implemented SNSPDs. The primary results are given in brief as follows.(1).Fabricating and systematically characterizing detector performances, and giving approaches for further optimizing detector designs. Especially in improving the high-speed performance of SNSPDs, we investigated sources that gave additional timing jitter and derived the intrinsic timing jitter of a detector; measured the kinetic inductance of detectors and discussed the nonlinear relationship between current-reset time and count rate; and reported an novel room-temperature readout circuit to increase the count rate by removing the capacitor coupling effects that occurred in conventional readout circuits(2).Modeling electrical-thermal dynamics of a SNSPD pulse generation. Using this model, we gave approaches to reduce current reset time and increase count rate. We also discussed latch effect that happed as current reset too fast. Through combining the thermal model and the circuit model, we simulated outputs from SNSPD arrays and multi-stage cascading nanowire detectors, which were both hybrid architectures consisted of nanowires and electric components.(3).Designing a novel architecture for SNSPD arrays:nanowires were distributed in spatial, and their output pulses went to the two outputs through a current-splitting network made of nanowire inductors. Through reading the amplitude of the differential pulses, position information of the fired pixel can be known. We successfully fabricated4-pixel SNSPD arrays, and resolved pixel positions without crosstalk in a124ps temporal resolution. All pixels show uniform detection efficiencies.(4).Designing a novel architecture for multi-stage cascading nanowire detectors (m-n-SNAP):nanowires were connected through nanowire-inductors in a binary tree format; by assigning sequential avalanches, the avalanche current was reduced. We successfully fabricated a3-2-SNAP made of8parallel nanowires. The detector was fully characterized. Compared to a SNSPD with single nanowire, output of the3-2-SNAP was increased by8times; compared to an equivalent detector with8parallel nanowires arranged in a conventional way, in the3-2-SNAP, the avalanche current was reduced by13%.(5).Implementing SNSPDs into optical time domain reflectrometies (OTDRs). Benefiting from our outstanding SNSPDs, and the improved circuit, we successfully realized a long-haul OTDR with a maximum sensing distance up to247km, and a high-spatial-resolution OTDR with a minimum spatial resolution down to4mm. Meanwhile, we investigated dependences of OTDR’s performances on SNSPD’s properties.As focusing in improving SNSPD’s performances and expanding the scope of its applications, we also studied problems about physics in SNSPDs, such as microscopic explanations for photon absorptions and dark counts, transport properties of superconducting nanowires and other non-equilibrium dynamics. With understanding SNSPDs by such theoretical efforts, we presented some approaches for upgrading SNSPDs.