The controlled manipulation of individual quantum objects, such as atoms, ions or photons, has developed considerably in recent years. A ‘billiard ball’ approach to individual atoms has been popularized by the fantastic landscapes created and explored using atomic force microscopy techniques, which exploit the repulsive force between microscope tip and atomic sample. But many experiments about the wave nature of matter, or about the particle nature of light, remind us that the fundamental nature of microscopic objects obeys the laws of quantum mechanics. A single photon provides the most striking evidence that a quantum particle is something much more elusive than a billiard ball. A new experiment reported on page 239 of this issue1 demonstrates how to store a single photon, and, more importantly, how to watch it repeatedly.
As far as the position and momentum of particles are concerned, Heisenberg's well-known ‘uncertainty relation’ claims that both cannot be known simultaneously with perfect accuracy. A related idea is that a precise measurement in the microscopic world is not possible without introducing a perturbation, or ‘back action’, inherent in all measurements. For example, a very precise measurement of a particle's position will greatly disturb the particle's momentum and vice versa. This can be generalized to any pair of ‘non-compatible’ physical quantities, represented in quantum language as operators A and B. Although the precision in a single measurement of A is not restricted, the large fluctuations induced in B when measuring A may eventually couple back to A, which will then also be perturbed, making it difficult to perform repeated or continuous measurements. In response to this problem the concept of ‘quantum non-demolition’ (QND) measurements was introduced in the 1970s, in which a measurement strategy is chosen that evades the undesirable back action2. The key issue is to devise a measurement scheme so that the back-action ‘noise’ is kept entirely within unwanted observables. The quantity of interest then remains uncontaminated by the measurement process, allowing repeated measurements to be performed with arbitrarily high accuracy.
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