Superconducting sensor helps detecting gravitation waves

To be able to detect gravitation waves in space, physicist have to measure truly minimal displacements: ten billion times smaller than the size of an atom. An improved superconducting sensor is a suitable candidate for this job, Martin Podt of the University of Twente now states in his PhD thesis. He has improved the sensitivity of a so-called ‘SQUID’ in way that it can be combined with a large ball-shaped gravitation detector. Podt succeeds in this by improving the sensitivity. He integrates the sensor with electronics and lowers the operating temperature. He is defending his Phd thesis on January 17, within the Faculty of Science of the University of Twente, The Netherlands.

Gravitation waves, ‘ripples in space’, are very interesting because they provide information about collisions in space. Physicist around the world are working on ways to detect them. Leiden University in The Netherlands currently develops a ball-shaped detector of 65 centimeters in diameter. This grows over a distance of no more than 10 exp –20 meter. To compare it with the size of the earth (and therefore multiplying the size by 20 million), you would like to detect a growth of one fifth of a picometer (one picometer is a millionth of a millionth of a meter).

The superconducting sensor Martin Podt of the University of Twente has designed and developed, gets to the desired sensitivity and can be combined with the MiniGrail system. It is a so-called Superconducting Qantum Interference Devices (SQUID). Podt has improved it by lowering the temperature to a value close to zero Kelvin, and by integrating sensor and electronics. “Our current SQUID did not reach the extreme demands of this application. We would then measure too much noise, and you simply cannot distinguish the noise from the parameter you want to measure,” says Podt. He lowers the temperature to about 20 milliKelvin -the MiniGrail is also cooled down to that temperature. The noise of the ‘conventional’ SQUID is introduced when the signal is amplified using an amplifier operating at room temperature. Podt therefore chooses to put the amplifier on the chip as well, so that both are operating at very low temperatures. The result is that it works substantially faster and introduces no noise.

A SQUID, however, measures a magnetic field or flux, and no distance. Therefore the displacement of the MiniGrail will be converted into an electric current. This gives a magnetic flux, and that is what Podt’s system will detect. Together with the scientists from Leiden, he will further develop this principle the coming year.

Already now, the SQUID is one of the most successfull applications of superconductivity. In their current form, they are already fit for detecting the very small magnetic activity of the brain or the heart, even of a foetus. Unlike the system Podt proposes, it is also possible to produce SQUIDs working on a higher temperature and still . These can be cooled down in an easier way and show superconductivity at a higher temperature, but they don’t reach the requirements Podt wants for his application.