Putting Mechanics into Quantum Mechanics

verything moves! In a world dominated by electronic devices and instruments it is easy to forget that all measurements involve motion, whether it be the motion of electrons through a transistor, Cooper pairs or quasiparti-cles through a superconducting quantum interference device (SQUID), photons through an optical interferome-ter—or the simple displacement of a mechanical element. Nanoscience today is driving a resurgence of interest in mechanical devices, which have long been used as front ends for sensitive force detectors. Among prominent historical examples are Coulomb's mechanical torsion balance , which allowed him in 1785 to establish the inverse-square force law of electric charges, and Cavendish's mechanical instrument that allowed him in 1798 to measure the gravitational force between two lead spheres. Today, micro-and nanoelectromechanical systems (MEMS and NEMS) are widely employed in ways similar to those early force detectors, yet with vastly greater force and mass sensitivity—now pushing into the realm of zeptonewtons (10 ⊗21 N) and zeptograms (10 ⊗21 g). These ul-traminiature sensors also can provide spatial resolution at the atomic scale and vibrate at frequencies in the giga-hertz range. 1 Among the breadth of applications that have become possible are measurements of forces between individual biomolecules, 2 forces arising from magnetic resonance of single spins, 3 and perturbations that arise from mass fluctuations involving single atoms and molecules. 4 The patterning of mechanical structures with nanometer-scale features is now commonplace; figure 1 and the cover display examples of current devices. The technological future for small mechanical devices clearly seems bright, yet some of the most intriguing applications of NEMS remain squarely within the realm of fundamental research. Although the sensors for the applications mentioned above are governed by classical physics, the imprint of quantum phenomena upon them can now be readily seen in the laboratory. For example, the Casimir effect, arising from the zero-point fluctuations of the electromagnetic vacuum, can drive certain small mechanical devices with a force of hundreds of piconewtons and produce discernible motion in the devices. 5 But it is now possible , and perhaps even more intriguing, to consider the intrinsic quantum fluctuations—those that belong to the mechanical device itself. The continual progress in shrinking devices, and the profound increases in sensitivity achieved to read out those devices, now bring us to the realm of quantum mechanical systems. What conditions are required to observe the quantum properties of a mechanical structure, and what can we learn …