T. Tanaka, K. Karatsu, H.Kutsuma, A.H.M.Coppens, Akira Endo, M.Hattori, S.Honda, C. Otani, Yoshinori Sueno, O. Tajima, David Johannes Thoen, Junya Suzuki, R. Genova-Santos, M. Peel, A. Fasano, Y.Jo, Eunil Won, Miku Tsujii, J. J. A. Baselmans
Context. Microwave kinetic inductance detectors (MKIDs) are superconducting detectors that have the potential to contribute to the construction of over 10 000 pixel cameras required for mm and submm astronomical observations over the next decade. One of the issues related to its application is two-level-system (TLS) noise that creates a 1/ f term in the power spectrum density (PSD) of the detectors. A hybrid-type MKID has been proposed for TLS noise suppression without affecting the MKID performance. In this paper, we present a systematic method to model a hybrid-type MKID for mitigating TLS noise while achieving high sensitivity. Aims. Previous studies have shown that TLS noise in MKIDs depends on the sampling frequency, internal power, device geometry, and temperature. The strength of the electrical field distribution along the resonator line also affects the TLS noise properties. Based on this knowledge, we constructed a noise model of a hybrid-type MKID. Methods. Using this model, we designed MKIDs for the cosmic microwave background radiation experiment, GroundBIRD. We evaluated the 1/ f noise properties and NET of the fabricated devices using the on-site measurements. Based on the NET measurements, we can confirm that the white noise of fabricated detectors is limited by atmospheric photon noise. From the 1/ f noise measurements, we also confirm that it is possible to reach an atmospheric photon noise limit down to a low sampling frequency (i.e., of a few Hz) at the typical observation conditions from Teide observatory. Results. We confirm the design methodology we propose for hybrid-type MKID works well in optimizing the TLS model to the hybrid-type MKID, as evidenced by performance verification measurements carried out in the laboratory and on site.
Context. Microwave kinetic inductance detectors (MKIDs) are superconducting detectors that have the potential to contribute to the construction of over 10 000 pixel cameras required for mm and submm astronomical observations over the next decade. One of the issues related to its application is two-level-system (TLS) noise that creates a 1/ f term in the power spectrum density (PSD) of the detectors. A hybrid-type MKID has been proposed for TLS noise suppression without affecting the MKID performance. In this paper, we present a systematic method to model a hybrid-type MKID for mitigating TLS noise while achieving high sensitivity. Aims. Previous studies have shown that TLS noise in MKIDs depends on the sampling frequency, internal power, device geometry, and temperature. The strength of the electrical field distribution along the resonator line also affects the TLS noise properties. Based on this knowledge, we constructed a noise model of a hybrid-type MKID. Methods. Using this model, we designed MKIDs for the cosmic microwave background radiation experiment, GroundBIRD. We evaluated the 1/ f noise properties and NET of the fabricated devices using the on-site measurements. Based on the NET measurements, we can confirm that the white noise of fabricated detectors is limited by atmospheric photon noise. From the 1/ f noise measurements, we also confirm that it is possible to reach an atmospheric photon noise limit down to a low sampling frequency (i.e., of a few Hz) at the typical observation conditions from Teide observatory. Results. We confirm the design methodology we propose for hybrid-type MKID works well in optimizing the TLS model to the hybrid-type MKID, as evidenced by performance verification measurements carried out in the laboratory and on site.
Microwave Kinetic Inductance Detectors (MKIDs) are extremely sensitive radiation detectors based on superconducting resonators that can be combined in large arrays on a single readout line within a limited frequency bandwidth. This makes MKIDs ideal detectors for the ultimate far-infrared observatory: a future space-based actively cooled telescope with its performance solely limited by the low universe background radiation. However, to reach these detector requirements, state-of-the-art MKIDs still need a order of magnitude improvement in device sensitivity. In this work, the MKID sensitivity is improved by reducing the aluminium volume that absorbs pair-breaking radiation into quasiparticle excitations, while making sure all radiation is still absorbed. Furthermore, a key requirement is sufficient reduction of excess noise as to keep the device intrinsically limited by thermally driven random fluctuations in the number of quasiparticles in absence of radiation, or Generation-Recombination (G-R) noise. To this end, a model is developed that describes the noise contributions as function of device geometry, readout power, material properties and radiation power. Subsequently, a realistic MKID design is presented and tested that reduces excess noise and maximises the sensitivity, expressed as Noise Equivalent Power (NEP). At high temperatures, good overall agreement is found between the measured noise spectra and the model. At low temperature T = 120 mK, the measurement results give an optical NEP similar to current state-of-the-art MKIDs. The NEP is not as low as expected due to short quasiparticle lifetimes, an unexpected decrease in the G-R noise level and a very high excess noise attributed to Two-Level Systems (TLS) noise that starts to dominate the already low G-R noise spectrum at low temperatures. Possibly, the quick quasiparticle lifetime saturation and noise level drop are caused by a strong readout power effect, as the readout power is known to create excess quasiparticles and to cause a strongly non-thermal electron energy distribution in the aluminium strip of the MKID. However, the exact microscopic details of these effects are unknown and not studied in this project. Based on the current chip design, a straightforward way to improve device performance and study the readout power effect in more detail is a reduction of the high TLS noise levels, which is possibly fabrication related. This would allow an unobstructed view of the G-R noise spectrum at low temperatures, thereby allowing both a study of the readout power effect on the quasiparticle system, and ultimately achieving the improvement in NEP needed reach the detector requirements for the ultimate space-based far-infrared observatory.
We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at 50 mHz signal frequencies. The sub-Hz part of the detection spectrum is important for millimeter-wave instruments, yet photon noise below 1 Hz in MKIDs has not previously been demonstrated unambiguously. Devices are feedhorn-coupled, direct-absorber-type and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN, Al, and an amorphous-Si passivation layer. Viewing a 7K thermal load and a passband centered on 2mm, the device noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. We will present these measurements and discuss the multiple design choices that lead to the result.
Abstract A microwave kinetic inductance detector (MKID) is a cutting-edge superconducting detector. It comprises a resonator circuit constructed with a superconducting film on a dielectric substrate. To expand its field of application, it is important to establish a method to suppress the two-level system (TLS) noise that is caused by the electric fluctuations between the two energy states at the surface of the substrate. The electric field density can be decreased by expanding the strip width (S) and gap width from the ground plane (W) in the MKID circuit, allowing the suppression of TLS noise. However, this effect has not yet been confirmed for MKIDs made with niobium films on silicon substrates. In this study, we demonstrate its effectiveness for such MKIDs. We expanded the dimension of the circuit from (S, W) = (3.00 μm, 4.00 μm) to (S, W) = (5.00 μm, 23.7 μm), and achieved an increased suppression of 5.5 dB in TLS noise.
Due to their multiplexing capability and their good sensitivity to radiation from submillimeter to X-ray wavelengths, microwave kinetic inductance detectors (MKIDs) are increasingly used in the field of astrophysics. The Advanced Technology Center of the National Astronomical Observatory of Japan is developing MKIDs for astronomical observations such as CMB B-mode search with LiteBIRD. MKIDs are made of superconductors whose energy gap determines the detector frequency range. The energy gap depends on Tc, the critical temperature of the superconductor. It is thus important to be able to adjust Tc in order to choose the suitable frequency range. When using a single-layer MKID, the Tc is fixed by the superconducting gap energy of the unique component and cannot be changed. One possibility is to make a bilayer MKID using the proximity effect to adjust its critical temperature. This paper presents our new study on MKIDs made of superconductor/metal bilayers. We investigated niobium and copper bilayers (Nb/Cu) and fabricated different bilayers in our clean room. The critical temperature of each of them has been measured. We show that the Tc depends on the ratio between Nb and Cu thicknesses and that we are able to control it. Then, we characterized one of these Nb/Cu bilayers (Nb = 8 nm and Cu = 22 nm) once integrated in a MKID. We measured the temperature dependence of the resonant frequency, and we achieved quality factors as high as 2 × 104. The measurement of the noise spectrum provided a lower limit equal to -85 dBc/Hz, and the calculation of the noise equivalent power has shown that the sensitivity of the Nb/Cu bilayer MKID is not very far from that of an Al monolayer MKID.
The separation and optimization of noise components is critical to microwave-kinetic inductance detector (MKID) development. We analyze the effect of several changes to the lumped-element inductor and interdigitated capacitor geometry on the noise performance of a series of MKIDs intended for millimeter-wavelength experiments. We extract the contributions from two-level system noise in the dielectric layer, the generation-recombination noise intrinsic to the superconducting thin-film, and system white noise from each detector noise power spectrum and characterize how these noise components depend on detector geometry, material, and measurement conditions such as driving power and temperature. We observe a reduction in the amplitude of two-level system noise with both an elevated sample temperature and an increased gap between the fingers within the interdigitated capacitors for both aluminum and niobium detectors. We also verify the expected reduction of the generation-recombination noise and associated quasiparticle lifetime with reduced inductor volume. This study also iterates over different materials, including aluminum, niobium, and aluminum manganese, and compares the results with an underlying physical model.
Over the past two decades, microwave kinetic inductance detectors (MKIDs) have gained exceptional importance in millimeter and sub-millimeter astronomy. MKIDs consist of thin strip resonators capable of detecting changes in the surface impedance of superconductor strips, which result from variations in resonance circuit properties. The principal noise in MKIDs comprises excess frequency noise and two-level system noise. In this paper, we propose a technique to mitigate the effect of two-level system (TLS) noise in MKIDs using a parallel plate capacitor with three layers of high ε dielectrics. To achieve this, we employ three layers of Al2O3, HfO2, and TiO2 with equal thickness between the capacitor plates. The experimental results demonstrate a nearly 30% reduction in TLS power spectral density.