Photon-Number-Resolving Based On A Series Superconducting Nanowire Detector

Superconducting nanowire single photon detector (SNSPD) is a novel and excellent single photon detector for its advantages of ultra-high sensitivity and ultra-low dark count rates in visible and near-infrared wavelengths, and it is widely used in many rising weak light detection fields such as quantum communication. However, the traditional SNSPD usually works in nonlinear mode and has no ability of photon number resolving. Nevertheless many fields, such as linear optics and quantum computation, require detectors can distinguish incident photon number.At present, there are three projects to realize photon number resolving:SNSPDs array, parallel superconducting nanowire detector (PND) and series superconducting nanowire detector (SND). However, the readout circuit of SNSPDs array is extremely complicated, and the current redistribution of parallel superconducting nanowire detector (PND) limits the maximum number of photons and the system efficiency, whereas series superconducting nanowire detector (SND) has a simple readout circuit and no current redistribution. SND can resolve a large number of photons and obtain a high system efficiency in theory. Therefore, the core content of our research is SND detector.In this thesis, in order to achieve the goal of photon number resolving, we modeled and analyzed the details of electrical-thermal dynamics of the SND detector, and we also designed, fabricated and measured 6 pixel SND detector, which has a series connection of 6 nanowire units, each connected in parallel to a resistor Rp. The primary results are summarized in brief as follows.Firstly, we optimized the fabricating process of the SND detector. The fabricating process includes six steps, such as depositing NbN films by magnetron sputtering, fabricating superconducting nanowire by electron beam lithography, fabricating Au electrical contact pads by optical lithography, removing extra NbN films by reactive ion etching, fabricating Ti resistances by optical lithography and integrating optical cavities by optical lithography. Compared with the present fabricating process of the SND detector, ours only need one step of electron beam lithography to fabricate superconducting nanowire and several steps of optical lithography to fabricate the other structures. Hence, our fabricating process is in favor of protecting the uniformity of nanowires. At the same time, reducing frequency and time of electron beam lithography could reduce the fabricating cost of the SND detector.Secondly, we fabricated a 6 pixel SND detector with the ability of photon number resolving. Regulating the incident light power, six different amplitudes of light pulse responses could be observed, and six different threshold voltages of light pulse responses could be distinguished. In addition, during experimental measurement, the SND detector could have been working at 0.971c without current redistribution. Adjusting the incident light power, we could also observe a phenomenon that the SQE was compressed, while the incident light power increased.Thirdly, we designed a 6 pixel SND detector which can realize position resolution and number resolution at the same time. Through changing the values of 6 resistances Rp, SND detector could realize position resolution and number resolution at the same time. While changing the values of 6 resistances Rp, the thickness of Ti film is uniform, and the length to width ratio of the parallel resistor meets the proportional relationship of 1/4:3/8:1/2:1:2:4. Therefore, the SND detector can realize 6 incident photons resolution and 26-1= 63 spatial positions resolution in theory. During the process of micro fabrication, we only need to modify the optical lithography masks of NbN etching mask and Ti resistance, so we could obtain a 6 pixel SND detector which can realize position resolution and number resolution at the same time.