M. Podt

Sensitivity of the spherical gravitational wave detector MiniGRAIL operating at 5 K

We present the performances and the strain sensitivity of the first spherical gravitational wave detector equipped with a capacitive transducer and readout by a low noise two-stage SQUID amplifier and operated at a temperature of 5 K. We characterized the detector performance in terms of thermal and electrical noise in the system output signal. We measured a peak strain sensitivity of 1.5x10{sup -20} Hz{sup -1/2} at 2942.9 Hz. A strain sensitivity of better than 5x10{sup -20} Hz{sup -1/2} has been obtained over a bandwidth of 30 Hz. We expect an improvement of more than 1 order of magnitude when the detector will operate at 50 mK. Our results represent the first step towards the development of an ultracryogenic omnidirectional detector sensitive to gravitational radiation in the 3 kHz range.

Two-stage amplifier based on a double relaxation oscillation superconducting quantum interference device

A low-noise single-chip two-stage superconducting quantum interference device (SQUID) system with a double relaxation oscillation SQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was 1.3 μΦ0/√Hz, corresponding to an energy sensitivity of e≈27h. Owing to the large flux-to-voltage transfer of up to 3.6 mV/Φ0, the room-temperature preamplifier noise did not dominate the overall flux noise.

Coded SQUID arrays

We report on a superconducting quantum interference device (SQUID) system to read out large arrays of cryogenic detectors. In order to reduce the number of SQUIDs required for an array of these detectors, we used code-division multiplexing. This simplifies the electronics because of a significantly reduced number of wires from the cryogenic detectors to the SQUIDs and the room temperature electronics. Several prototype chips based on SQUIDs with multiple inputs coils have been developed and direct and indirect crosstalk properties are discussed.

Two-stage SQUID systems and transducers development for MiniGRAIL

We present measurements on a two-stage SQUID system based on a dc-SQUID as a sensor and a DROS as an amplifier. We measured the intrinsic noise of the dc-SQUID at 4.2 K. A new dc-SQUID has been fabricated. It was specially designed to be used with MiniGRAIL transducers. Cooling fins have been added in order to improve the cooling of the SQUID and the design is optimized to achieve the quantum limit of the sensor SQUID at temperatures above 100 mK. In this paper we also report the effect of the deposition of a Nb film on the quality factor of a small mass Al5056 resonator. Finally, the results of Q-factor measurements on a capacitive transducer for the current MiniGRAIL run are presented.

Development of a SQUID readout system for the MiniGRAIL

The MiniGRAIL is one of the three similar spherical gravitational wave detectors that are currently being developed. The detector has a resonant frequency of about 3 kHz and will be operated at 20 mK. The ultimate goal is to use a readout system consisting of six transducers coupled to nearly quantum limited two-stage SQUIDs. The two-stage SQUIDs are based on double relaxation oscillation SQUIDs, which enables a direct voltage readout scheme. We have developed nonintegrated two-stage SQUIDs and experimentally verified the proper operation of the system coupled to a capacitive transducer. Based on the results that were achieved, integrated two-stage SQUIDs were designed. Special attention has been paid to the sensor SQUID, the back action of the SQUID and the feedback scheme that is used for linearizing the SQUID output.

Cooling down MiniGRAIL to milli-Kelvin temperatures

The latest developments in the construction of the ultra-cryogenic spherical detector MiniGRAIL are presented. The room temperature part of the vibration isolation system was improved and provided with an attenuation of about 60 dB around 3 kHz. The transfer function of the cryogenic stages gave about 20 dB per stage, at the resonant frequency of the sphere. The latest results of three cryogenic tests at ultra-low temperature of the spherical detector MiniGRAIL, using several thermal anchorings, are presented. Minimum temperatures of 20 mK on the mixing chamber of the dilution refrigerator and 79 mK on the surface of the sphere were reached. During the last cool down, two capacitive transducers were mounted on the sphere. The first was coupled to a room temperature FET amplifier and the second to a transformer and a double stage SQUID amplifier. Unfortunately the SQUID did not work, so only the first resonator could be used. An equivalent temperature of about 20 K was measured during an acquisition run of 7 h, using the first transducer corresponding to the FET white noise.

A spherical gravitational wave detector readout by nearly quantum limited SQUIDs

We are developing nearly quantum limited two-stage SQUID systems for the readout of the MiniGRAIL, the first spherical gravitational wave detector with a diameter of 65 cm and a mass of 1150 kg. The two-stage SQUID systems are based on a conventional dc SQUID as the sensor and a double relaxation oscillation SQUID (DROS) as the second stage. The SQUID systems will be coupled to the detector via a multi-mode inductive transducer. The large flux-to-voltage transfer of the two-stage SQUID system allows a direct voltage readout scheme, i.e. without (ac flux) modulation. At T = 4.2 K, the energy resolution in flux-locked loop was measured to be e = 27 h, and at a temperature around 20 mK, the energy resolution is expected to reach the quantum limit.

Low-noise SQUIDs with large transfer: two-stage SQUIDs based on DROSs

We have realized a two-stage integrated superconducting quantum interference device (SQUID) system with a closed loop bandwidth of 2.5 MHz, operated in a direct voltage readout mode. The corresponding flux slew rate was 1.3×105 Φ0/s and the measured white flux noise was 1.3 μΦ0/√Hz at 4.2 K. The system is based on a conventional dc SQUID with a double relaxation oscillation SQUID (DROS) as the second stage. Because of the large flux-to-voltage transfer, the sensitivity of the system is completely determined by the sensor SQUID and not by the DROS or the room-temperature preamplifier. Decreasing the Josephson junction area enables a further improvement of the sensitivity of the two-stage SQUID systems.

Results on a fast digital DROS

A digital double relaxation oscillation (DROS) superconducting quantum interference device (SQUID) has the potential for a very large dynamic range and a slew rate that is orders of magnitude larger than that of conventional SQUID systems. This is, for example, advantageous when using the SQUID in an unshielded environment. An important characteristic of the digital DROS is that the feedback flux is quantized since this flux is supplied by a superconducting up-down counter. Together with the clock frequency, the quantization unit of the feedback flux determines the maximum slew rate. In an optimized design, the quantization unit is adapted to the broadband flux noise of the DROS, so that the slew rate is maximized without compromising the sensitivity. Simulations on our optimized digital DROS showed proper operation for slew rates up to 5 x 10 Φ 0 s -1 . The relaxation oscillations generated at a frequency of 100 MHz deliver an on-chip clock signal, so that no external clock is required. Slew rates up to 10 8 Φ 0 s -1 can be achieved by increasing the relaxation oscillation frequency to a few GHz. This means that sub-μm 2 Josephson junctions with high critical current densities are required. The first experiments using low-T c ramp-type Josephson junctions are presented in this paper.

Numerical analysis of the smart DROS

A digital superconducting quantum interference device (SQUID) has the potential for a very large dynamic range and a very high slew rate. Our concept of the digital SQUID consists of a double relaxation oscillation SQUID (DROS) with the complete flux locked loop (FLL) electronics on one single chip. The key element of the FLL circuitry is a superconducting up-down counter, which supplies the feedback flux to the DROS. In this paper we will concentrate on the numerical simulations of our new 100 MHz smart DROS. In this new design, the quantization unit of the feedback flux, δΦfb=52 mΦ0, was optimized with respect to the flux noise of the DROS. By doing so, the flux slew rate was maximized without compromising the sensitivity. The optimization resulted in a maximum flux slew rate of δΦsig/δt=5×106Φ0/s.