A version of Schroedinger’s cat: if the hammer falls, the poison is released and the cat is in trouble. Assume that the hammer is exquisitely sensitive and will fall if any form of radiation or matter whatsoever touches it. Can you find out (measure) whether the hammer is still in place without making it fall? Yes, you can. The interferometer is set up so that, in the absence of the hammer, light or a photon entering on path a leaves on path e with probability equal to one due to interference between the paths c and d. If the hammer is in place, the interference cannot take place and so a photon entering on path a can leave on path f. Therefore detection of a photon on path f, means the hammer is in place but the photon has not touched the hammer (otherwise it could not have reached f)! Therefore, you can achieve the aim, at least in some runs of the experiment.

Quantum physics is very successful in predicting the measured properties of quantum systems (e.g. electrons, atoms, molecules) but does not concern itself with unmeasured properties. In one view, quantum physics is incomplete because physics should be concerned with describing an objective physical reality for both measured and unmeasured properties. The effort to ‘complete’ quantum mechanics is rewarding not only because of its fundamental interest but also because it leads to practical innovations. To a significant extent, the exciting developments in quantum information theory, quantum cryptography and quantum computing over the last decade or so can be seen to flow from research on the foundations of the subject in the last third of the last century, especially in the area of the Bell inequalities after 1964.

I am interested in a time-symmetric formulation of quantum physics in which the future and the past determine both the measured and unmeasured properties of a quantum system. This idea is counterintuitive, because of our subjective experience of a clear distinction between the future and the past, but there does not appear to be any objection to the concept within physics itself.

One practical consequence of the research is the possibility of using the phenomena of interaction-free measurement to improve the sensitivity of experimental techniques. In interaction-free measurement it is possible to detect the presence of an object without interacting with it in the sense of changing the energy, momentum or any other property of the object. For example, we could find out whether Schroedinger’s cat was alive or dead without disturbing in any way the contents of the box containing the cat. There are more practical consequences, for example the possibility of measuring the spectrum of a sample without causing any transitions between the energy levels that are involved in producing the spectrum.

David Miller