Compact microwave kinetic inductance nanowire galvanometer for cryogenic detectors at 4.2 K

Wepresent a compact current sensor based on a superconductingmicrowave lumped-element resonator with a nanowire kinetic inductor, operating at 4.2 K. The sensor is suitable formultiplexed readout inGHz range for large-format arrays of cryogenic detectors. The device consists of a lumpedelement resonant circuit, fabricated from a single 4 nm-thick superconducting layer of niobium nitride. Thus, the fabrication and operation is significantly simplified in comparison to state-of-theart current readout approaches. Because the resonant circuit is inductively coupled to the feed line the current to bemeasured can directly be injectedwithout having the need of an impedancematching circuit, reducing the system complexity.With the proof-of-concept device wemeasured a current noisefloor δImin of 10 pA/Hz 1/2 at 10 kHz. Furthermore, we demonstrate the ability of our sensor to amplify a pulsed response of a superconducting nanowire single-photon detector using aGHz-range carrier for effective frequency-divisionmultiplexing. Superconducting transition-edge sensors (TES) or superconducting nanowire single-photon detectors (SNSPD) are able to detect light on the single photon level over awide spectral range. Thismakes them suitable formany researchfields, e.g. the astronomy [1, 2], particle physics [3] ormaterial science [4, 5]. Since their response after photon absorption is veryweak, a sensitive preamplifier is essential. In case of voltage biasedTES very small current changes needs to be carefully amplified, which is realized in state-of-the-art systems using superconducting quantum interference devices (SQUIDs). SQUIDs allow the most sensitive readout possible, down to 4 fA Hz at 4.2 K [6]. However, an SQUIDbased amplifier increases the system complexity and costs significantly. For next generationmulti-pixel TES arrays also an array-scalable readout technique is required. This technique should reduce the number of cables between the low-temperature stage and the ambient-temperature back-end electronics, without compromising the sensitivity of the detectors. This is best realized using frequency-divisionmultiplexing (FDM) approaches. So far, frequency-based multiplexing of TES arrays is realized by coupling the SQUIDswith additional resonant circuits [7, 8] further increasing the complexity. FDM is also verywell suited for SNSPD arrays. However, to date there are very limited demonstrations of FDM in SNSPD arrays [9].Mostmultiplexing approaches are based on singleflux quantum (SFQ) logic schemes [10] or current splitting techniques [11]which aswell increase the system complexity and reduce the filling factor of the sensing area. An alternative approach tomeasure small currents ormagnetic fields is based on nonlinear kinetic inductance Lk in superconductors [12, 13]. The nonlinearity could be considered quadratic on current Lk/Lk(0)≈1+ (I/I ) for small current variations, where I is the characteristic current, which depends on the critical current IC of the device [14]. Nowadays, this effect is being actively used for novel superconducting devices, e.g., tunable RFfilters [15], low-noise wide-band parametric amplifiers [16]. Despite rather small current-driven changes of the kinetic inductance (15%) it is relatively easy to employ the effect. In this case it is beneficial to use superconductors with high kinetic inductance and geometries with a large fraction of kinetic inductance, for example nanowiresmade of ultra-thinNbN. The effect of the nonlinear Lk inNbNnanowires OPEN ACCESS