論文の公開元へDevelopment of novel calibration methods and performance forecaster of cutting-edge superconducting detector MKIDs for CMB experiments
概要
The Big Bang theory has recognized widely as the standard model describing the evolution of the universe. However, the theory is inherent by the fundamental prob- lems, e.g., the horizontal problem and the flatness problem. In 1980s, Alan Guth and Katsuhiko Sato proposed the inflationary cosmology. Assuming the universe had an exponentially expanding period at the very early universe, they showed that these problems are naturally solved. According to the standard inflation theory, the tensor fluctuation was generated due to the quantum fluctuation of the space-time during the inflation period and it drifts in our universes as the primordial gravita- tional wave. The cross mode and plus mode primordial gravitational waves imprint the B-mode and E-mode polarizations in the CMB, respectively. Since the scalar mode fluctuation generates only the E-mode polarization, the detection of the B- mode CMB polarization provides smoking gun evidence of the inflation theory.
Many observation efforts have been done aiming for the first detection of the primordial B-mode CMB polarization. The power spectrum of the CMB B-mode po- larization has two bumps. One is called recombination bump appeared at around small angular scale of 2 degree (l 100), and the other is called reionization bump appeared at around large angular scale of 20 degree (l < 10). Many conventional ground-based CMB experiments target to detect the recombination bump. How- ever, the expected amplitude of the primordial B-mode CMB polarization is less than the B-mode polarization caused by the disturbance on the E-mode CMB po- larization due to the gravitational lensing effect of the large scale structure. On the other hand, the detection of the reionization bump from the ground-based observa- tion is limited by 1/f atmospheric fluctuation. The atmospheric fluctuation becomes significant below 0.1 Hz. It is hard to detect reionization bump by conventional ground-based observations since it is impossible to cover a few tenth degree of sky within a few second. To access the reionization bump by the ground-based CMB polarization experiments, invention for observational strategy to mitigate the atmo- spheric fluctuation is required.
The sum of neutrino masses is one of the important parameters in describing the evolution of the early universe. It is experimentally proposed that the neutrinos have mass. Since the non-zero neutrino mass can not be explained by the standard model of the particle physics, the neutrinos are the only particles beyond the stan- dard model currently known. We can evaluate the sum of the neutrino masses from the observation of the B-modes polarization due to the gravitational lensing effect of the large scale structure. However, to limit the sum of neutrino masses from the B-mode polarization due to the gravitational lensing effect of the large scale struc- ture we need to know the precise optical depth at the reionization epoch τ, since the influence of the gravitational lensing effect of the large scale structure and Thom- son scattering by the free electrons in the reionization are strongly degenerate. To evaluate the optical depth at the reionization epoch, the CMB E-mode polarization below l 10 is useful since the scalar perturbation below l 10 entered inside of the Hubble horizon after the reionization epoch. There is a systematic difference in the estimated τ between WMAP and Planck satellites results. The independent measurement of the optical depth at the reionization epoch by the CMB polarization experiment which is able to perform the secure measurement of the large angular scale signal is an important.
In order to observe the faint signal like the CMB polarization, various types of large format detector arrays toward astronomical observations, including CMB po- larization observations are proposed. Recently, majority of CMB polarization experi- ments use a superconducting detector as a focal plane detector, because it is sensitive enough to reach the noise level of the photon noise of the atmosphere for the ground- based observations. At present, many millimeter and submillimter telescope includ- ing CMB observation use a large format Transition Edge Sensor (TES) array as a focal plane detector. The TES is a superconducting detector. In next decade, over mega pixel focal plane detector is going to be required in order to increase the precision of the observations. However, the development of the mega pixel TES camera is hard with the current readout multiplexer system. The Microwave Kinetic Induc- tance Detector (MKID) is the cutting-edge superconducting detector which enable to break the mega pixel wall. The advantage of the MKID is that it has a potential to read over thousands pixels per single readout line. Moreover, the time response of the MKID (< 100 µs) is significantly faster than the TES.
Although the MKID is the detector technology which is supposed to explore the mega pixel era, it has several fundamental problems which have to be overcome. The one is that there is significant systematic uncertainty involved in the calibration of the detector performance since there is no novel method for the responsivity cali- bration. The MKID for millimeter and submillimter astronomical observations is op- erated at 250 300 mK. Every day or a few day, the MKID is once warmed up above the transition temperature and cooled down below the transition temperature again. Since the performance of the MKID changes every cooling cycle, we have to perform calibration of the performance of the MKID, especially its responsivity, every cool- ing cycle. Conventionally, the calibration of the responsivity of the MKID has been performed by measuring the change of the response when the temperature of the detector mount plate is heated up by controlling the heater attached to the mount plate. This method is inevitable from following systematic error. It always accom- panies uncertainties whether the plate temperature measured by the thermometer coincides with the detector temperature. This method is also time consuming. It takes several hours for every calibration. Therefore, a few 10% of the observational time is consumed by the responsivity calibration. The other problem is that the 1/f type noise always appears and it limits the performance in low sampling frequency. This noise is supposed to be attributed to the two level system (TLS) formed in the interface of the supercoducting material and substrate. To realize the photon noise limit high sensitivity MKID down to low sampling frequency, we have to mitigate the TLS noise in someway. The third problem is that there is no method to measure the superconducting transition temperature, Tc, of the hybrid type MKID which is widely used for the recent astronomical observations. The superconducting transi- tion temperature of the MKID is one of the crucially important parameters to fix the design of MKID and evaluate performance.
The GroundBIRD is a ground-based CMB polarization experiment to probe the inflationary cosmology. For enabling to attack the reionization bump of the primor- dial B-mode CMB polarization and to observe the precise optical depth to reioniza- tion from the ground by mitigating the 1/f atmospheric fluctuation, the Ground- BIRD performs a rapid rotation scan around the zenith direction with inclining the telescope 30 degree from zenith at rotation speed of 20 rotations per minute, which corresponds to 3 seconds for one rotation. Because of the earth rotation 44% of the full sky area is covered in a day. Since the time response of MKID is significantly faster than TES and satisfies the requirements from the rapid rotation scan strategy, MKID is installed on the focal plane of the GroundBIRD. We show in this thesis that the performance of the prototype MKID is far from the GroundBIRD observation requirements based on the results of our performance verification experiments as shown in Chapter.3. The 1/f type TLS noise dominates over the generation and re- combination noise below 100Hz. Further research and development is required to
optimize performance of the MKID to the GroundBIRD observation. However, the one cycle from the design to evaluation is about three months. We have to iterate this cycle several times to feed back the results to new design. Dramatic reduction of the consumption for this research and development cycle is desired.
We propose new method for the responsivity calibration in Chapter 4. The method uses the change of the number of the excess quasiparticles while changing the mi- crowave readout power. By changing microwave readout power from high power to low power abruptly, the number of the excess quasiparticles transit to a new steady state with time constant. This time constant is called quasiparticle lifetime and the time has an relation between the number of quasiparticles in the MKID. We eval- uate the number of quasiparticles from the quasiparticle lifetime using theoretical formula. As a result, the responsivity is extracted. We apply this method for the real measurement using the MKID maintained at 285 mK. We confirm the consis- tency between the results obtained using this method and conventional calibration methods. Since our method is free from the above mentioned systematic accom- panying in the conventional method, the our method provides much more secure results compared with the conventional method. Furthermore, the time duration consumed for the calibration dramatically shortened, down to 10 minutes, by our proposed method.
We propose a new method to measure the Tc of MKID by abrupt change of the applied readout microwave power. The number of quasiparticles in the MKID de- crease with the quasiparticle lifetime during abrupt change of the applied readout microwave power. Therefore, we can measure the relation between the quasiparticle lifetime and the detector phase response by abrupt change of the readout microwave power. As a results, we can estimate the intrinsic quasiparticle lifetime. The intrin- sic quasiparticle lifetime is theoretically modeled by Tc, the physical temperature of the device, and other known parameters. We can extract Tc by comparing the measured lifetime with theoretical model. Using an MKID made of aluminium, we demonstrate this method at a 0.3 K operation. The results are consistent with those obtained by Tc measured by monitoring the transmittance of the readout microwave power for various device temperature. The proposed method opens a possibility to measure Tc of the hybrid type MKID directly. Since there was no method to mea- sure Tc, the speculated value of Tc has been adopted. The speculated values vary largely from author to author in the range from 1.1 K to 1.5 K. This introduces ten- fold difference in the estimated noise level of the MKID under dark condition. Our method fixes this large uncertainty and dramatically improves precision of design- ing the MKID. Since the photon noise of the atmosphere dominates over the intrinsic noise of the MKID for the GroundBIRD application, the uncertainty of the noise level introduced by the uncertainty of Tc in the range of 1.1 K to 1.5 K is about 20%.
We develop the forecaster which evaluate the performance of MKID quantita- tively by setting environmental variables and design parameters as shown in Chap- ter 6. By inputting the design parameters of the prototype MKID into the forecaster, we confirmed that the TLS noise dominates over the BLIP noise below 100 Hz and that the main problem of the prototype MKID is its design. We show that this bad performance is attributed to the design. Since the total width of the coplanar waveg- uide (CPW) line made from Nb of the prototype MKID is too narrow, the contri- bution of the TLS noise became prominent. A new design of MKID with widening the total width of CPW line made from Nb is proposed. We evaluate the expected performance of the new design MKID using the forecaster in Chapter 7. We showed that the TLS noise is significantly reduced from that of the prototype MKID and is suppressed below the BLIP noise down to the GroundBIRD rotation frequency (0.3 Hz).
この論文で使われている画像
