Titanium nitride (TiN x ) films are ideal for use in superconducting microresonator detectors because: a) the critical temperature varies with composition (0 < T c < 5 K); b) the normal-state resistivity is large, ρ n ∼ 100 µΩ cm, facilitating efficient photon absorption and providing a large kinetic inductance and detector responsivity; and c) TiN films are very hard and mechanically robust. Resonators using reactively sputtered TiN films show remarkably low loss (Q i > 10 7 ) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10−200 µs. TiN microresonators should therefore reach sensitivities well below 10 −19 W Hz −1/2 .

If driven sufficiently strongly, superconducting microresonators exhibit nonlinear behavior including response bifurcation. This behavior can arise from a variety of physical mechanisms including heating effects, grain boundaries or weak links, vortex penetration, or through the intrinsic nonlinearity of the kinetic inductance. Although microresonators used for photon detection are usually driven fairly hard in order to optimize their sensitivity, most experiments to date have not explored detector performance beyond the onset of bifurcation. Here, we present measurements of a lumped-element superconducting microresonator designed for use as a farinfrared detector and operated deep into the nonlinear regime. The 1 GHz resonator was fabricated from a 22 nm thick titanium nitride film with a critical temperature of 2 K and a normal-state resistivity of 100 lX cm. We measured the response of the device when illuminated with 6.4 pW optical loading using microwave readout powers that ranged from the low-power, linear regime to 18 dB beyond the onset of bifurcation. Over this entire range, the nonlinear behavior is well described by a nonlinear kinetic inductance. The best noise-equivalent power of 2 Â 10 À16 W=Hz 1=2 at 10 Hz was measured at the highest readout power, and represents a $10 fold improvement compared with operating below the onset of bifurcation. V C 2013 American Institute of Physics. [http://dx

We demonstrate very high resolution photon spectroscopy with a microwave-multiplexed two-pixel transitionedge sensor (TES) array. We measured a 153 Gd photon source and achieved an energy resolution of 63 eV full-width-at-half-maximum at 97 keV and an equivalent readout system noise of 86 pA/ √ Hz at the TES. The readout circuit consists of superconducting microwave resonators coupled to radio-frequency superconductingquantum-interference-devices (SQUID) and transduces changes in input current to changes in phase of a microwave signal. We use flux-ramp modulation to linearize the response and evade low-frequency noise. This demonstration establishes one path for the readout of cryogenic X-ray and gamma-ray sensor arrays with more than 10 3 elements and spectral resolving powers R = λ/∆λ > 10 3 .Multiplexed readout of sub-Kelvin cryogenic detectors is an essential requirement for large focal plane arrays. Next-generation instruments for the detection of electromagnetic radiation from gamma-ray to far-infrared wavelengths will have pixel counts in the 10 3 -10 6 range and require readout techniques that do not compromise their sensitivity. To date, many instruments have used time-, frequency-, or code-domain SQUID multiplexing schemes 1-3 . One such instrument, the TES bolometer camera SCUBA2, has achieved background-limited sensitivity in 10 4 pixels using time-domain multiplexing (TDM) 4 . Similarly, calorimetric gamma-ray/X-ray spectrometers that use TDM have reached excellent energy resolutions of δE ≈ 50 eV at 100 keV in a 256-pixel array 5 . However, the scalability of these readout approaches is limited by the finite measurement bandwidth (∼ 10 MHz) achievable in a flux-locked loop.Kinetic Inductance Detectors (KIDs) 6,7 , on the other hand, provide a possible path to higher multiplexing factors. These devices are naturally frequency-multiplexed and the ultimate limit on the available bandwidth is many gigahertz, which is set by the readout cryogenic amplifier. Present limits in room-temperature electronics impose a 550 MHz bandwidth limit 8 , but this figure will improve steadily. However, the sensing element is part of a thin-film superconducting resonator, so readout and signal generation can be difficult to simultaneously optimize. This challenge is particularly severe for spectroscopic X-ray and gamma-ray detectors, which must stop high-energy photons and where spatial variation in the device response must be smaller than 0.1%. X-ray and gamma-ray spectroscopy results achieved to date with KIDs are not yet compellingly better than conventional semiconducting detectors 9,10 . a) Electronic mail: omid.noroozian@nist.govMicrowave SQUID multiplexing 11,12 (µMux) is a readout technique that potentially combines the proven sensitivity of TESs and the scalable multiplexing power found in KIDs. Microwave SQUID multiplexing uses radiofrequency (rf) SQUIDs coupled to high quality-factor (Q) microwave resonators and has sufficiently low noise to read out the most sensitive cryogenic detectors. Additionally, it al...