They sound fishy, but SQUIDs — superconducting quantum interference devices — are ultra-sensitive gadgets used for measuring the strength of magnetic fields. These 'magnetometers' have widespread applications ranging from materials science to medicine. Writing in Nature Nanotechnology, Cleuziou, Wernsdorfer and colleagues describe a minuscule version of this useful device (J.-P. Cleuziou et al. Nature Nanotech. 1, 53–59; 2006).
The 'nano-SQUID' is made of a square loop of 50-nm-thick aluminium wire, which becomes superconducting at temperatures below 1.2 K. Insulating junctions disrupt the loop in two places, with each junction consisting of a single-walled carbon nanotube. The maximum supercurrent flowing through the loop varies with changes in the magnetic flux (the magnetic field multiplied by the area) passing through the loop. This response is what makes SQUIDs such sensitive magnetometers.
The nanotube junctions behave like 'quantum dots', which have discrete electronic energy levels. The alignment of these energy levels with those of the superconducting wire can be controlled by applying a voltage near to either — or both — of the nanotube junctions. In this way, the voltage effectively turns the current on (energy levels aligned) and off (misaligned). The two-dimensional colour plot shown on the right indicates the peaks (red) and valleys (blue) in the conductance of the device as the junctions are tuned through their on and off states.
So why make a SQUID that has carbon-nanotube junctions? The ultimate goal is to detect changes in direction of the magnetic moment of a single molecule or nanoparticle. This could be achieved by placing the nanoparticle directly on one of the nanotubes, optimizing the magnetic flux passing through the loop and so maximizing the device's sensitivity. Although such a measurement has not yet been made, it is well within the sensitivity of this tiniest of SQUIDs.