The electron has dominated technology, measurement, communications and information processing for around one century. Hard limits may restrict its future dominance. One promising disruptive technology that may grow in future is based on NEMS (nano-scale electromechanical system).

Resonators based on NEMS (so called NMRs) are expected to have a range of applications, from ultra-sensitive sensors for mass, force, charge, spin and chemical specificity, through single-molecule bio-sensing, information storage and processing technologies, to nanoscale refrigerators.

They are sufficiently small that mesoscopic quantum mechanical behaviour is expected to appear, at low temperatures or even at room temperature, with all of the quantum metrology capabilities that have hitherto been found in atomic and condensed matter physics.

There is a key requirement to extend quantum metrology to the nanoscale and to achieve measurements that are limited only by counting statistics or, going further, by the Heisenberg uncertainty principle limit.

This work focuses on metrological aspects of NMRs as they approach the quantum regime, where the resonator’s state is not significantly ‘mixed’ by thermal noise; in other words, one requires

ħω>k_BT

where ω is one of the resonator’s fundamental eigenfrequencies, T is its effective operating temperature, and ħ and k_B are fundamental constants.

The ultimate metrological target, towards which this work is aiming, is the provision of robust, convenient quantum-NEMS-based devices for the generation and counting of individual phonons. If this can be achieved we will have the mechanical analogue of ‘quantum optics’ and a whole new area of device engineering.

To avoid the need for ultra-low temperatures an NMR should operate at the highest possible frequency. It should also be capable of fabrication on the smallest possible length scale. We are researching a number of  nanofabricated structures composed of single-crystal materials such as Si, Si₃N₄ and particularly carbon nanotubes.

Paddle resonator with SQUID

Characterisation of the resonators’ properties requires a range of techniques such as electrical and thermal transport methods in which we already have considerable experience^[1].

A disruptive technology such as NMRs brings also new requirements: as NEMS devices become smaller the oscillation amplitude scales down in proportion to size, thus ultrasensitive transducer techniques and low-dissipation excitation schemes are needed.

We have two novel methods for excitation and readout of NMRs:

• A parametric microwave technique evolved from our previous work on scanned microwave microscopy^[2]: movement of the resonator placed close to an open circuit transmission line modulates the line impedance and this can be detected by a variety of standard methods, with estimated sensitivity of <0.1 nm.
• A nano-SQUID^[3,4] (Superconducting Quantum Interference Device) readout method: the vibration of an electrically conducting NMR close to the nanoSQUID sensing loop (~ 80 nm diameter) will modify its self inductance, the SQUID readout will then carry sideband information which can be detected in the conventional way.

References

1. L. Hao and J. Gallop, 'Spatially resolved measurements of HTS microwave surface impedance', IEEE Trans, Appl. Supercond.,v 9, p1944-46, 1999.
2. L. Hao, J.C. Gallop and J. C. Macfarlane, 'Coupled Microwave Resonators For Sensitive Bolometric Detection', IEEE Trans. Instrum. &amp; Meas. Vol. 54, No. 2, pp886-889, 2005.
3. 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.
4. 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’,  J. Appl. Phys. 99, 123916, 2006.
5. L. Hao, J.C. Macfarlane, J.C. Gallop, D. Cox, D. Hutson, P. Josephs-Franks, J. Chen and S.K. H.  Lam, ‘'ovel methods of fabrication and metrology of superconducting nano-structures', IEEE Trans. Instrum. &amp; Meas. Vol. 56, No. 2, p392-395, 2007.
6. L. Hao, J.C. Macfarlane, Ed Romans, J.C. Gallop, D. Cox, D. Hutson, S.K. H.  Lam and J. Chen, 'Spatial resolution of Variable-Inductance LTS SQUIDs incorporating Nano-bridge Junctions made by Focussed Ion Beam milling',  IEEE Trans. Appl. Supercond., Vol. 17, No. 2 pp.742-745 June 2007.
7. L. Hao, J. C. Macfarlane, J. C. Gallop, D. Cox, J. Beyer, D. Drung, and T. Schurig,  "Measurement and noise performance of nano-superconducting quantum interference devices fabricated by focused-ion-beam", Appl. Phys. Lett. Vol. 92, 192507 (2008).