Michael Mück

Superconducting quantum interference device as a near-quantum-limited amplifier at 0.5 GHz

A dc superconducting quantum interference device (SQUID) with a resonant microstrip input is operated as an amplifier at temperatures down to 20 mK. A second SQUID is used as a postamplifier. Below about 100 mK, the noise temperature is 52±20 mK at 538 MHz, estimated from measurements of signal-to-noise ratio, and 47±10 mK at 519 MHz, estimated from the noise generated by a resonant circuit coupled to the input. The quantum-limited noise temperatures are 26 and 25 mK, respectively. The measured noise temperature is limited by hot electrons generated by the bias current.

Radio-frequency amplifier based on a niobium dc superconducting quantum interference device with microstrip input coupling

A dc superconducting quantum interference device (SQUID) was used to amplify radio-frequency signals that were coupled to one end of the microstrip formed by the input coil and the SQUID washer. For one device, the resonant frequency of the microstrip was increased from about 200 to 620 MHz by progressively shortening the length of the coil. At an operating temperature of 4.2 K, the gain was typically 18 dB, and the system noise temperature ranged from 0.5±0.3 K at 80 MHz to 3.0±0.7 K at 500 MHz.

YBa2Cu3O7 thin film SQUID gradiometer for biomagnetic measurements

Low‐noise rf SQUID washers fabricated from YBa2Cu3O7 epitaxial thin films have been used to construct a first‐order electronic gradiometer operating at 77 K and suitable for biomagnetic measurements. Mechanical adjustment of the two‐SQUID gradiometric setup made it possible to attenuate signals due to far magnetic field sources by three orders of magnitude. A magnetic field resolution of ≤280 fT/Hz1/2 above 2 Hz was attained through the use of large flux focusers. The fine structure of human heart magnetocardiograms was recorded in unshielded space. In a shielded room, magnetoencephalograms were obtained. The system was used to obtain new data on auditory evoked cortical response.

Magnetism in SQUIDs at Millikelvin Temperatures

We have characterized the temperature dependence of the flux threading dc SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data are compatible with the thermal polarization of surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest an explanation for the universal 1/f flux noise in SQUIDs and superconducting qubits.

Measurements of T1‐relaxation in ex vivo prostate tissue at 132 μT

The proton T1 was measured at 132 μT in ex vivo prostate tissue specimens from radical prostatectomies of 35 patients with prostate cancer. Each patient provided two specimens. The NMR and MRI measurements involved proton repolarization, a field of typically 150 mT and detection of the 5.6‐kHz signal with a superconducting quantum interference device. Values of T1 varied from 41 to 86 ms. Subsequently, the percentages of tissue types were determined histologically. The theoretical image contrast is quantified for each case by δ = [1 – T1(more cancer)/T1(less cancer)]. A linear fit of δ versus difference in percentage cancer yields T1 (100% cancer)/T1 (0% cancer) = 0.70 ± 0.05 with correlation coefficient R2 = 0.30. Two‐dimensional T1 maps for four specimens demonstrate variation within a single specimen. These results suggest that MR images with T1 contrast established at ultra‐low fields may discriminate prostate cancer from normal prostate tissue in vivo without a contrast agent. Magn Reson Med, 2012.

Microwave rf SQUID integrated into a planar YBa2Cu3O7 resonator

We fabricated and characterized microwave rf SQUIDs integrated into a planar, S‐shaped λ/2 microstrip resonator. This 3 GHz resonator was fabricated from a pulsed‐laser‐deposited YBa2Cu3O7 epitaxial film. The SQUID structures incorporated double step‐edge junctions and had a loop inductance of 120 pH. Such unoptimized SQUIDs operated between 4.2 and 85 K with dV/dΦ=18–20 μV/Φ0 at 77 K. At that temperature, the energy resolution of (8±2)×10−29 J/Hz above 0.1 Hz (in the best samples) was limited by the white noise, SΦ1/2=(7±1)×10−5 Φ0/Hz1/2. Optimization may increase dV/dΦ and improve the energy resolution by up to an order of magnitude.

Superconducting quantum interference device amplifiers at gigahertz frequencies

A series of five dc superconducting quantum interference devices (SQUIDs) have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12±1 to 6±1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2–4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor–insulator–superconductor and hot-electron bolometer mixers.

A voltage biased superconducting quantum interference device bootstrap circuit

We present a dc superconducting quantum interference device (SQUID) readout circuit operating in the voltage bias mode and called a SQUID bootstrap circuit (SBC). The SBC is an alternative implementation of two existing methods for suppression of room-temperature amplifier noise: additional voltage feedback and current feedback. Two circuit branches are connected in parallel. In the dc SQUID branch, an inductively coupled coil connected in series provides the bias current feedback for enhancing the flux-to-current coefficient. The circuit branch parallel to the dc SQUID branch contains an inductively coupled voltage feedback coil with a shunt resistor in series for suppressing the preamplifier noise current by increasing the dynamic resistance. We show that the SBC effectively reduces the preamplifier noise to below the SQUID intrinsic noise. For a helium-cooled planar SQUID magnetometer with a SQUID inductance of 350 pH, a flux noise of about 3 μΦ0 Hz − 1/2 and a magnetic field resolution of less than 3 fT Hz − 1/2 were obtained. The SBC leads to a convenient direct readout electronics for a dc SQUID with a wider adjustment tolerance than other feedback schemes.

Radio-frequency amplifiers based on dc SQUIDs

SQUIDs are an attractive candidate for the amplification of low-level rf and microwave signals. Compared to semiconductor amplifiers, they offer lower noise and much lower power dissipation. Especially at frequencies below 1 GHz, the improvement in noise temperature compared to the best cold semiconductor amplifiers can be as high as 50; noise temperatures only slightly above the quantum limit have been achieved in this frequency range. This article will review the current status of radio-frequency amplifiers based on dc SQUIDs and provide detailed discussions of amplifier noise temperature, input and output impedance, and nonlinearities.

Radio-frequency amplifier with tenth-kelvin noise temperature based on a microstrip direct current superconducting quantum interference device

A dc superconducting quantum interference device (SQUID) with a resonant microstrip input and a cooled heterostructure field-effect transistor as a postamplifier is used as a radio-frequency amplifier in the frequency range 90–500 MHz. At liquid 3He temperatures, gains of 24 and 20 dB and intrinsic noise temperatures of 0.06±0.02 and 0.12±0.10 K were achieved at 89.6 and 438 MHz, respectively. The system noise temperature at 438 MHz was also estimated from the Nyquist noise produced by a resonant circuit coupled to the input of the microstrip SQUID.