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 .

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance. 1 . Arranged in a centered tetragonal lattice, the spin of the Mn 12 clusters is subject to strong magnetic anisotropy along the symmetry axis (the c-axis of the crystal). Below the blocking temperature of ≈ 3.5 K, the crystal exhibits remarkable staircase magnetic hysteresis due to resonant quantum spin tunneling between energy levels on opposite sides of the anisotropy barrier corresponding to different spin projections, as illustrated in Fig. 1(a).2 This and other interesting properties of Mn 12 -ac have been intensively studied in the last decade (see Refs. 3,4,5,6 for reviews).It has been known for some time 7 that Mn 12 -ac crystals exhibit an abrupt reversal of their magnetic moment under certain conditions. This phenomenon, also observed in other molecular magnets, has been attributed to a thermal runaway (avalanche) in which the initial relaxation of the magnetization toward the direction of the field results in the release of heat that further accelerates the magnetic relaxation. Direct measurements of the heat emitted by Mn 12 -ac crystals, 8 as well as measurements of the magnetization reversal in pulsed magnetic fields, 9 have confirmed the thermal nature of the avalanches. More recently, the electromagnetic signal associated with avalanches was detected 10,11 and it was argued that if the radiation is of thermal origin it would indicate a significant increase in the temperature of the crystal. This has not been confirmed by direct bulk measurements of the temperature using a thermometer. Evidence has been obtained 12 that the avalanche may not be a uniform process throughout the sample. No clear understanding of the avalanche process has emerged to date.In this Letter we report local time-resolved measurements of fast magnetization reversal (avalanches) in mmsize single crystals of Mn 12 -ac. We show that a magnetic avalanche takes the form of a thin interface between regions of opposite magnetization which propHall Sensors (a) (b)Spin-up

Abstract:Microwave Kinetic Inductance Detectors, or MKIDs, have proven to be a powerful cryogenic detector technology due to their sensitivity and the ease with which they can be multiplexed into large arrays. A MKID is an energy sensor based on a photon-variable superconducting inductance in a lithographed microresonator, and is capable of functioning as a photon detector across the electromagnetic spectrum as well as a particle detector. Here we describe the first successful effort to create a photon-counting, energy-resolving ultraviolet, optical, and near infrared MKID focal plane array. These new Optical Lumped Element (OLE) MKID arrays have significant advantages over semiconductor detectors like charge coupled devices (CCDs). They can count individual photons with essentially no false counts and determine the energy and arrival time of every photon with good quantum efficiency. Their physical pixel size and maximum count rate is well matched with large telescopes. These capabilities enable powerful new astrophysical instruments usable from the ground and space. MKIDs could eventually supplant semiconductor detectors for most astronomical instrumentation, and will be useful for other disciplines such as quantum optics and biological imaging.

Design and analysis of multi-color confocal microscopy with a wavelength scanning detector Rev. Sci. Instrum. 83, 053704 (2012) Short wavelength thermography: Theoretical and experimental estimation of the optimal working wavelength J. Appl. Phys. 111, 084903 (2012) Hole shape effect induced optical response to permittivity change in palladium sub-wavelength hole arrays upon hydrogen exposure J. Appl. Phys. 111, 084502 (2012) Few-photon-level two-dimensional infrared imaging by coincidence frequency upconversion Appl. Phys. Lett. 100, 151102 (2012) Channelling optics for high quality imaging of sensory hair Rev. Sci. Instrum. 83, 045001 (2012) Additional information on Rev. Sci. Instrum. Microwave kinetic inductance detectors (MKIDs) are superconducting detectors capable of counting single photons and measuring their energy in the UV, optical, and near-IR. MKIDs feature intrinsic frequency domain multiplexing (FDM) at microwave frequencies, allowing the construction and readout of large arrays. Due to the microwave FDM, MKIDs do not require the complex cryogenic multiplexing electronics used for similar detectors, such as transition edge sensors, but instead transfer this complexity to room temperature electronics where they present a formidable signal processing challenge. In this paper, we describe the first successful effort to build a readout for a photon counting optical/near-IR astronomical instrument, the ARray Camera for Optical to Nearinfrared Spectrophotometry. This readout is based on open source hardware developed by the Collaboration for Astronomy Signal Processing and Electronics Research. Designed principally for radio telescope backends, it is flexible enough to be used for a variety of signal processing applications.

Using a wire heater to ignite magnetic avalanches in fixed magnetic field applied along the easy axis of single crystals of the molecular magnet Mn 12 acetate, we report fast local measurements of the temperature and time-resolved measurements of the local magnetization as a function of magnetic field. In addition to confirming maxima in the velocity of propagation, we find that avalanches trigger at a threshold temperature which exhibits pronounced minima at resonant magnetic fields, demonstrating that thermally assisted quantum tunneling plays an important role in the ignition as well as the propagation of magnetic avalanches in molecular magnets.