We demonstrate photon noise limited performance in both phase and amplitude readout in microwave kinetic inductance detectors (MKIDs) consisting of NbTiN and Al, down to 100 fW of optical power. We simulate the far field beam pattern of the lens-antenna system used to couple radiation into the MKID and derive an aperture efficiency of 75%. This is close to the theoretical maximum of 80% for a single-moded detector. The beam patterns are verified by a detailed analysis of the optical coupling within our measurement setup.
We demonstrate photon noise limited performance in both phase and amplitude readout in microwave kinetic inductance detectors (MKIDs) consisting of NbTiN and Al, down to 100 fW of optical power. We simulate the far field beam pattern of the lens-antenna system used to couple radiation into the MKID and derive an aperture efficiency of 75%. This is close to the theoretical maximum of 80% for a single-moded detector. The beam patterns are verified by a detailed analysis of the optical coupling within our measurement setup.
In the next decades millimeter and sub-mm astronomy requires large format imaging arrays and broad-band spectrometers to complement the high spatial and spectral resolution of the Atacama Large Millimeter/submillimeter Array. The desired sensors for these instruments should have a background limited sensitivity and a high optical efficiency and enable arrays thousands of pixels in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and Al have shown to satisfy these requirements. We present the second generation hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and amplitude readout for loading levels P850GHz < 10 fW. Thanks to the increased responsivity, the photon noise level achieved in phase allows us to simultaneously read out approximately 8000 pixels using state-of-the-art electronics. In addition, the choice of superconducting materials and the use of a Si lens in combination with a planar antenna gives these resonators the flexibility to operate within the frequency range 0:09 < v < 1:1 THz. Given these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.
Microwave kinetic inductance detectors (MKIDs) promise high sensitivity, combined with device simplicity and intrinsic multiplexibility. We have measured the response and noise equivalent power (NEP) of an all-aluminium, antenna coupled MKID at 1.54 THz, both as a function of radiation power and as a function of readout power. Eight optical filters and a box-in-box configuration at the bath temperature, allow a large range of optical powers by varying the temperature of a black body source. In the range of 80 aW-700 fW the detector is photon-noise limited, with an optical efficiency of 48±8% for one polarisation, which is limited by the antenna design. At lower optical powers we reveal a saturation level in the NEP of 3.8×10-19W/Hz1/2, competitive with other detectors for this purpose and the lowest optical NEP so far for MKIDs. The noise over the entire power range is limited by photon- and generation-recombination noise, the fundamental noise sources for these detectors.
Excess phase noise has been observed in microwave kinetic inductance detectors (MKIDs) which prevents the noise-equivalent power (NEP) of current detectors from reaching theoretical limits. One characteristic of this excess noise is its dependence on the power of the readout signal: the phase noise decreases as the readout power increases. We investigated this power dependence in a variety of devices, varying the substrate (silicon and sapphire), superconductor (aluminum and niobium) and resonator parameters (resonant frequency, quality factor and resonator geometry). We find that the phase noise has a power law dependence on the readout power, and that the exponent is -1/2 in all our devices. We suggest that this phase noise is caused by coupling between the high-Q microwave resonator that forms the sensitive element of the MKID and two-level systems associated with disorder in the dielectric material of the resonator. The physical situation is analogous to the resonance fluorescence in quantum optics, and we are investigating the application of resonance fluorescence theory to MKID phase noise.
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.
Microwave kinetic inductance detectors (MKIDs) are generally read out with microwave readout tones of high enough amplitude to adequately suppress the noise contribution of the first-stage amplifier. At high readout power, the detector's resonant frequency is altered as a result of the dependence of the kinetic inductance on the internally circulating microwave current. With the tone placed below the resonant frequency, the nonlinear frequency shift results in a positive feedback effect that can significantly enhance the responsivity of the detector, to both optical and microwave power. We report a factor of 10 enhancement in optical response by tuning the readout power and frequency to close to the resonator's bifurcation point. A corresponding decrease in the bandwidth of the resonator is observed under these conditions. We show that the strength of the feedback effect can be easily selected by adjusting the excitation, and provide a map of possible operational states to do so. Operation of MKIDs in this mode could be used to improve sensitivity when non-intrinsic noise sources are significant.
We present results from completely lithographic antenna-coupled Microwave Kinetic Inductance detectors (MKIDs). MKIDs are superconducting resonators with resonant frequency and quality-factor which are highly sensitive to changes in the density of the quasiparticle population which occurs when photons above the superconducting gap energy are absorbed. The resonators are coupled to submm light through on-chip phased-array slot antennas. Each planar antenna consists of an array of N long slots which are fed along their length at M points. The resulting N*M feed points are combined in-phase using a binary summing tree made of low-loss superconducting microstrip lines. Due to its large size, the resulting planar antenna produces a narrow beam pattern and can therefore be used without additional optical coupling elements such as feedhorns or substrate lenses. The output of the antenna is a single superconducting thin-film microstrip line which can be efficiently coupled to one (or more) kinetic inductance coplanar waveguide resonators to produce a single (or multi-color) pixel in an imaging focal plane array, using in-line lumped element lithographic band-pass filters. Such highly integrated architectures can be easily fabricated on a single substrate, and many detectors can be frequency multiplexed through coupling to a single feedline. Microwave readout provides a lot of bandwidth per detector, allowing a large number of pixels to be read using a single cryogenic microwave amplifier and warm readout electronics.