NPL nanoSQUID fabricated from a thin film
of niobium using Focussed Ion Beam milling.
The Josephson junction width is around 70 nm 

The phenomenon of superconductivity has the continuing ability to fascinate. One reason for this is that it is the archetypal example of quantum mechanics on a macroscopic scale. An even more fascinating concept is the SQUID (Superconducting Quantum Interference Device) which combines two counter-intuitive aspects of superconductivity (magnetic flux quantisation and the Josephson effects) to produce devices which have been called "…the most sensitive instruments for measuring anything".

SQUIDs are capable of demonstrating quantum behaviour themselves but are also potentially capable of quantum-limited measurement of a vast range of physical parameters. Their exquisitely sensitive capabilities have already been demonstrated for magnetic field, voltage, magnetic moment, displacement, temperature and so on, an almost endless list. SQUIDs have long played important roles in the development of quantum metrology at NMIs, including quantum standards for voltage and resistance. They will have a new role to play in future as the size of the SQUID is reduced to the nanometre scale.

It has been well known for years that thermal noise limitations on SQUID sensitivity can be reduced by lowering the inductance and capacitance of the SQUID device. What has not been attempted until recently is to achieve this by minimising both the size of the SQUID superconducting loop and the capacitance of the Josephson junctions by using microbridge technology rather than tunnel-effect junctions. Our group at NPL, in collaboration with the Universities of Surrey and Strathclyde, was able to demonstrate that nanoSQUIDs fabricated by a combination of conventional photolithography and focussed ion beam (FIB) milling have probably the best magnetic flux resolution achieved with a SQUID at normal liquid helium operating temperatures. These high performance devices and the fabrication technologies on which they are based are expected to find uses in a number of projects.

Quantum limited measurement by Nano-scale SQUIDs

NanoSQUIDs will continue to contribute to the development and accuracy of quantum metrology. Further improvements arising from novel SQUID readout techniques should enable faster quantum-limited detectors. A better understanding of back-action of the detectors onto the quantum state of a system under measurement, along with reduction in other external and internal sources of decoherence, will enable widespread use of SQUID measurement schemes for quantum non-demolition readout.

ISTED Nano-bolometers

Another use of the nanoSQUID technology has led to an energy resolving (0.1 eV) non-dissipative bolometer. We have continued the development of the NPL-invented superconductor based ISTED (Inductive Superconductive Transition Edge Detector) which is a non-dissipative nano-bolometer combining extremely small heat capacity (~ 10^-15 JK^-1) with an energy resolution approaching ~1 yoctojoule (10^-24 J)  in unit bandwidth. This already allows for a single particle measurement of photons with 0.1 eV spectral resolution while operating at relatively high temperatures (~8 K) and in future may be used for detection of single macromolecules and even phonons.  Advanced SQUID readout methods (e.g. absorber coupled Double Relaxation Oscillator SQUIDs) and our proposed ‘magnetothermal feedback’  should also contribute greatly to the performance of these detectors.

References

1. J.C. Gallop, 'SQUIDs: some limits to measurement', Supercond. Sci. Technol. Vol. 16 pp 1575-82 (2003).
2. L. Hao, J.C. Macfarlane, S.K.H. Lam, C.P Foley, P. Josephs-Franks and J.C. Gallop 'Inductive Sensor Based on Nano-scale SQUIDs', IEEE Trans. Appl. Supercond.,  Vol. 15 No. 2 pp.514-517 2005.
3. L. Hao, J.C. Macfarlane, J. C. Gallop and S.K.H. Lam, 'Direct measurement of penetration length in ultra-thin and or mesoscopic superconducting structures', Jnl. Appl. Phys.  Vol. 99, 123916, 2006.