P. J. de Visser

Number Fluctuations of Sparse Quasiparticles in a Superconductor

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium. The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles. We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to 25-55/μm(3). We show that this saturation is consistent with the measured saturation in the quasiparticle lifetime, which also explains similar observations in qubit decoherence times.

Strongly Disordered TiN and NbTiN s-Wave Superconductors Probed by Microwave Electrodynamics

We probe the effects of strong disorder (2.4<k(F)l<8.6) on superconductivity in thin films of niobium titanium nitride and titanium nitride by measuring the microwave electrodynamics in coplanar waveguide resonators. We find a gradual evolution of the electromagnetic response with disorder, deviating from BCS theory, for both materials. This result can be understood as due to changes in the quasiparticle density of states, induced by the short elastic scattering length. The observations are consistent with a model using an effective pair breaker, dependent on the level of disorder.

Reduced frequency noise in superconducting resonators

We report a reduction in the frequency noise in coplanar waveguide superconducting resonators. The reduction of 7 dB is achieved by removing the exposed dielectric substrate surface from the region with high electric fields and by using NbTiN. In a model-analysis the surface of NbTiN is found to be a negligible source of noise, experimentally supported by a comparison with NbTiN on SiOx resonators. The reduction is additive to decreasing the noise by widening the resonators.

Design of an Integrated Filterbank for DESHIMA: On-Chip Submillimeter Imaging Spectrograph Based on Superconducting Resonators

An integrated filterbank (IFB) in combination with microwave kinetic inductance detectors (MKIDs), both based on superconducting resonators, could be used to make broadband submillimeter imaging spectrographs that are compact and flexible. In order to investigate the possibility of adopting an IFB configuration for DESHIMA (Delft SRON High-redshift Mapper), we study the basic properties of an IFB circuit using electromagnetic simulation. In an idealized situation, one could integrate onto a 4-inch wafer a 9 pixel × 920 color 3 dimensional imaging device, which instantaneously covers multiple submillimeter telluric windows with a resolving power of f/df=1000. We also simulate a slightly more realistic situation, in which the dimensions of the filters differ from their designed values, to see how fabrication errors introduce scattering in the transmission of the channels.

Development of DESHIMA : A redshift machine based on a superconducting on-chip filterbank

Distant, dusty and extremely luminous galaxies form a key component of the high redshift universe, tracing the period of intense cosmic activity that ultimately gave rise to the present-day universe. These highly luminous galaxies, first detected in the ground-based submillimeter region, are however optically very faint, which hampers identification of the optical counterpart and the measurement of a redshift. We are developing a new direct-detection submm spectrograph DESHIMA. By taking advantage of the rapidly advancing technology of superconducting microresonators, DESHIMA will revolutionize the appearance and capabilities of a submm spectrograph. There will no longer be large grating optics; instead DESHIMA will be equipped with a single chip, onto which the entire system of a dispersive filterbank and MKID sensor array is integrated. This chip will host 5000-10000 MKID sensors to instantaneously cover the entire submillimeter wave band (320-950 GHz) with a resolution of f/Δf = 1000, further multiplied by 6-9 spatial pixels. With the broader bandwidth and higher detector sensitivity, DESHIMA will be very efficient compared to ALMA in picking up THz lines from submm galaxies with unknown redshifts. The expected outcome of this project is; 1) a record of the properties and evolution of distant luminous galaxies, 2) a powerful and compact multi-purpose spectrometer suitable for future ground base telescopes as well as satellite missions, and 3) the emergence of a new branch of observational astronomy based on flexible on-chip submillimeter optics.

Microwave-induced excess quasiparticles in superconducting resonators measured through correlated conductivity fluctuations

We have measured the number of quasiparticles and their lifetime in aluminium superconducting microwave resonators. The number of excess quasiparticles below 160 mK decreases from 72 to 17 μm−3 with a 6 dB decrease of the microwave power. The quasiparticle lifetime increases accordingly from 1.4 to 3.5 ms. These properties of the superconductor were measured through the spectrum of correlated fluctuations in the quasiparticle system and condensate of the superconductor, which show up in the resonator amplitude and phase, respectively. Because uncorrelated noise sources vanish, fluctuations in the superconductor can be studied with a sensitivity close to the vacuum noise.

Anomalous response of superconducting titanium nitride resonators to terahertz radiation

We present an experimental study of kinetic inductance detectors (KIDs) fabricated of atomic layer deposited TiN films and characterized at radiation frequencies of 350 GHz. The responsivity to radiation is measured and found to increase with the increase in radiation powers, opposite to what is expected from theory and observed for hybrid niobium titanium nitride/aluminium (NbTiN/Al) and all-aluminium (all-Al) KIDs. The noise is found to be independent of the level of the radiation power. The noise equivalent power improves with higher radiation powers, also opposite to what is observed and well understood for hybrid NbTiN/Al and all-Al KIDs. We suggest that an inhomogeneous state of these disordered superconductors should be used to explain these observations.

Microwave Properties of Superconducting Atomic-Layer Deposited TiN Films

We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 μs. The high normal state resistivity of the films (>; 100 μΩ · cm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modeled using a quasiparticle density of states modified with an effective pair-breaker, consistently describing the measured temperature dependence of the quality factor and the resonant frequency.

Coherent Excited States in Superconductors due to a Microwave Field.

We describe theoretically the depairing effect of a microwave field on diffusive s-wave superconductors. The ground state of the superconductor is altered qualitatively in analogy to the depairing due to a dc current. In contrast to dc depairing, the density of states acquires, for microwaves with frequency ω_{0}, steps at multiples of the photon energy Δ±nℏω_{0} and shows an exponential-like tail in the subgap regime. We show that this ac depairing explains the measured frequency shift of a superconducting resonator with microwave power at low temperatures.

The non-equilibrium response of a superconductor to pair-breaking radiation measured over a broad frequency band

We have measured the absorption of terahertz radiation in a BCS superconductor over a broad range of frequencies from 200 GHz to 1.1 THz, using a broadband antenna-lens system and a tantalum microwave resonator. From low frequencies, the response of the resonator rises rapidly to a maximum at the gap edge of the superconductor. From there on, the response drops to half the maximum response at twice the pair-breaking energy. At higher frequencies, the response rises again due to trapping of pair-breaking phonons in the superconductor. In practice, this is a measurement of the frequency dependence of the quasiparticle creation efficiency due to pair-breaking in a superconductor. The efficiency, calculated from the different non-equilibrium quasiparticle distribution functions at each frequency, is in agreement with the measurements.