Quantum metrology with PTB-made SQUIDs

PTB plays a worldwide leading role in developing SQUIDs. These superconducting quantum interference devices are sensors used to measure extremely small changes in magnetic flux with great accuracy. PTB's SQUIDs are employed for the most various types of measurements. Whereas they have been used in biomagnetic experiments for two decades in order to detect, for instance, the very weak magnetic fields of the human heart or brain, they are continuously involved in new metrological developments. SQUIDs are used, e.g., as sensitive current sensors in the most diverse configurations or as full, integrated susceptometers. PTB supplies not only the SQUID chips themselves, but also the electronics and the metrological know-how to implement the sensors in the respective cryogenic setup and experimental periphery. This was also the case in the two international cooperation projects.

One of these projects was a so-called “Bell experiment” of Anton Zeilinger's group from the Austrian Academy of Sciences (Österreichische Akademie der Wissenschaften – ÖAW). In order to detect quantum-mechanically entangled photons with high efficiency and – what is decisive – a sufficient number of them, PTB's SQUIDs were used to amplify the output currents of single-photon detectors based on superconducting transition edge sensor microcalorimeters. The highly efficient single-photon counters have been developed by the quantum detector group of the National Institute of Standards and Technology (NIST, USA). The TES/SQUID detector modules used were configured and tested at PTB. In addition, for the experiments which took place at ÖAW, cryogenic technology and electronics of the companies Entropy and Magnicon (development partners of PTB) were used. The experiments allowed the most complete detection of the quantummechanical entanglement of photons so far. This makes photons the first quantum particles for which the so-called “loopholes” in Bell experiments have been closed.

The second experiment is part of a close cooperation which has existed since the mid-1990s between PTB's “Cryosensors” Working Group and John Saunders' group of the Royal Holloway University, London, in which particularly sensitive NMR spectrometers are developed for experiments at ultra-low temperatures. This experiment is, among other things, aimed at investigating the isotope helium- 3 which, at very low temperatures, becomes superfluid – which means that it can flow without friction. Together with colleagues from Cornell University, Ithaca, NY, USA, the cooperation partners from London locked the fluid under pressure and at extremely low temperatures below a millikelvin in thin traps which were only a few hundreds of nanometers thick. The properties of the helium-3 fluid confined in a nanofluidic cavity, in which the interactions of the magnetic moments of the atomic nuclei of helium-3 play a decisive role, were investigated by means of a sensitive SQUID-NMR spectrometer. The measurements showed that the complex phase pattern of helium-3 is strongly modified by the confinement and by the properties of its surface towards its environment.