Planar microwave biased RF-SQUIDs in niobium technology

R. Mezzena, Stefano Vitale

Michael Mück, D. Diehl, Christoph Heiden

Michael Mück, Christoph Heiden

Two new SQUID variants have been made and tested using nanobridges as Josephson contacts. The first is a dc SQUID whose hysteretic weak links are inductively shunted such that it operates as a relaxation oscillator with a frequency depending on the applied magnetic flux. When standard ICs are used for FM detection, the readout electronics are cheap and compact, of the size of a matchbox. The second variant is a microwave-biased rf SQUID incorporated into a λ/2 stripline resonator, which is capacitively coupled to a coaxial cable leading to the room-temperature electronics. Peak-to-peak modulation voltages of several hundred μV were obtained in the investigated frequency range of 3 to 4.2 GHz. For both SQUID variants, white flux noise values of some 10−5Φ0/Hz1/2 have been found.

Michael Mück, G. Hallmanns, Christoph Heiden +4 more

We report on the design and performance of a planar microwave rf-SQUID readout by a cryogenic preamplifier. The SQUID-sensor consists of a planar half-wavelength microstrip resonator, into which the SQUID-loop is integrated. The pump frequency is 1.7 GHz. The cryogenic preamplifier employs a microwave high electron mobility transistor (HEMT), located close to the SQUID sensor. Measurements of the flux noise and the fractional step rise parameter were carried out in the temperature range between 4.2 and 1.5 K. In an open loop white flux noise values of 2×10−6 Φ0/√Hz were obtained at 4.2 K.

Thomas W. Bradshaw, William P. Pratt

Satoshi Kohjiro, Fuminori Hirayama, Daiji Fukuda +3 more

Dissipationless microwave radio frequency superconducting interference device (RF-SQUID) multiplexers are attractive for readouts of large format arrays of transition edge sensors (TES). This multiplexer has the potential for readout bandwidth of several GHz and power consumption of ≈0.1 pW per pixel, which are ≈103 and ≈10-4 times as large as those of DC-SQUID-based conventional multiplexers. To evaluate their readout performance for TES photon counters ranging from near infrared to gamma ray, white noise of this multiplexer was experimentally studied at 4 K. The reason for this was, firstly, to avoid the low-frequency fluctuation obvious at around 0.1 K but trivial for the energy resolution of photon counters, and secondly for a feasibility study of readout operation at 4 K for extended applications. To increase resonant Q at 4 K and maintain low-noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1×104≤QI≤2×104 and the square root of spectral density of current noise referred to the SQUID input √SI =31 pA/√Hz, where QI is the internal Q of resonators. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Systematic noise measurement with various microwave readout power PMR and two-directional readout on the complex plane of transmission component of scattering matrix S21 makes the contribution of each noise source obvious. By decreasing these noises to the degree achievable by current technology, we predicted the microwave RF-SQUID multiplexer would exhibit √SI ≤5 pA/√Hz; i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.