Weak Light Detection Based On Low Temperature Superconducting Technology

In the past several-decade, low temperature superconducting single-photon detection technology had got extensively attentions in the fields including:dark-matter detection, cosmic background radiation detection, optical quantum information processing and linear optics quantum computation etc.. Comparing with the traditional semi-conductor single photon detectors (e.g. the photomultipliers and avalanche photodiodes), the superconducting single photon detectors (SSPD) have more fabulous advantages like the ultra high detection sensitivity, extremely low background noise influence, low dark counting rate and fast responding speed due to this kind of detectors normally working below 1K temperature range. In this case, the SSPD have become a new kind of promising detector in those fields. Moreover, the SSPD also exhibit the ability in responding to long band range of incident light signals from submillimeter to X-ray wavelength through adjusting the various superconducting film materials. Currently, the detector of single pixel can successfully respond to a single photon signal, and also is able to count the 0-,1-,2-to 8-maximum single photons simultaneously incident events. Consequently, it is not hard to imagine what magnificent effects can be brought to explore the universe origin, the quantum secure communication and the quantum key distribution with the large array of this kind of SSPDs in the future.In this thesis, we will mainly discuss two kinds of low temperature single photon detection technologies based on superconducting thin film:one is the superconducting transition edge sensor (TES) detection technology, which basic principle is thermal fluctuation (photon incident event) will cause a sharp vibration in TES’s resistance, and this change can be read as the change of weak current signal passing through the entire measurement circuit; the other one is the superconducting coplanar waveguide resonator (CPW) technology, in which the measured signal is caused by the change of resonant characteristics due to absorb the incident weak light energy. Following, we also experimentally present and discuss about the weak light detection results through a quarter-wavelength niobium CPW designed and fabricated by ourselves.In chapter 1, we briefly review the elementary background theory knowledge of low temperature superconducting, including the origin of superconductivity, the BCS theory and the Ginzburg-Landau theory. Additionally, we also discuss the scientific motivation and the real application of the SSPD in astronomy filed.Then in chapter 2, we introduce some of the most essential characteristics when the superconducting thin film devices are served as the SSPD. For instance, we consider the concepts of the photon detection efficiency, the dead time, dark counting rate, responding speed etc, and comparing the SSPD with the traditional semi-conductor photon detector to list their advantages and disadvantages, the result obviously present that SSPDs are much proper than the semiconductor when serving as single photon detectors.Chapter 3 specifically talk about the first category of the SSPD, the TES system. We mainly discuss about the detectors’basic physical principle, the operating limit, the measurement system, the following readout circuit and the noise effects in detail, then briefly introduce the I-V curves measurements with some imperfect tungsten and titanium films in our lab through the four-point method.In chapter 4, we investigate the other kind of the SSPD, the CPW photon detector. We also starting from the elementary theory of superconducting resonators, the principle of regarding CPW as SSPD and the following readout circuit. Then, we further discuss about two substantially significant theories which can well explain the CPW behavior in extremely low temperatures:the Marttis-Bardeen Theory (MBT) introduced form the BCS theory, and the Two-Level-System (TLS) believed existence in dielectric layer on top of the resonant mental film.Considering the entire measurements need a stable and extremely low temperature environment, we then describe the helium free dilution refrigerator (DR) produced by Leiden corporation. In chapter 5 we present the working principle and the main structure of the DR, then specifically discuss about the two different measurement circuits based on the DR to fit our experiments:one is the low-frequency circuit serving for I-V curve measurement of TES, and the other one is high-frequency circuit transmission the microwave signal through CPW to measure the resonant characteristics of resonator. Moreover, we also present the inletting optical fiber method to assure the 1550nm and 635nm weak light signal can directly illuminate on top of the CPW surface. At the end of this chapter, we also shortly demonstrate the fabrication processes of a quarter-wavelength niobium superconducting resonator.In chapter 6, we have shown several experiment results based on the CPW and the DR system. We mainly focus on the operating temperature dependence of the transmission curves, specifically on the central frequency (f0) of the resonant dip and the quality factor (Q) of the resonator. While some anomalous effects have appeared in these experiments. Following the MBT prediction, the increasing temperature will bring the central frequency of resonant peak to lower area (i.e. the blue shift), while the opposite dependence has been found clearly in our results, in other words, we find that the increasing temperature makes the central frequency of our resonator getting larger (i.e. the red shift). In fact, this effects can be well explained using the TLS theory mentioned before. Besides this result, we also find similar explanations about the TLS existence in our CPW through the experiments about temperature dependence of quality factor (Q) of this resonator.After the elementary measurements of the CPW’s characteristics, we further investigate the CPW responding to weak light signals in extremely low temperature with the fixing fiber in chapter 7. We present the experiments of 635nm and 1550nm weak light radiating on the CPW, where one can clearly find that the magnitude of weak light signal will make strong effects on the central frequency of resonant dip, and the results apparently show the evidence of that CPW can reach its ultra energy resolution at 26pW and 35pW with 635nm and 1550nm incident weak light signals respectively.