By Edwin Cartlidge of Nature magazine

An international group of physicists has found a way of measuring both the position and the momentum of photons passing through the double-slit experiment, upending the idea that it is impossible to measure both properties in the lab at the same time.

In the classic double-slit experiment, first done more than 200 years ago, light waves passing through two parallel slits create a characteristic pattern of light and dark patches on a screen positioned behind the slits. The patches correspond to the points on the screen where the peaks and troughs of the waves diffracting out from the two slits combine with one another either constructively or destructively.

In the early twentieth century, physicists showed that this interference pattern was evident even when the intensity of the light was so low that photons pass through the apparatus one at a time. In other words, individual photons seem to interfere with themselves, so light exhibits both particle-like and wave-like properties.

However, placing detectors at the slits to determine which one a particle is passing through destroys the interference pattern on the screen behind. This is a manifestation of Werner Heisenberg's uncertainty principle, which states that it is not possible to precisely measure both the position (which of the two slits has been traversed) and the momentum (represented by the interference pattern) of a photon.

What quantum physicist Aephraim Steinberg of the University of Toronto in Canada and his colleagues have now shown, however, is that it is possible to precisely measure photons' position and obtain approximate information about their momentum, in an approach known as 'weak measurement'.

Steinberg's group sent photons one by one through a double slit by using a beam splitter and two lengths of fibre-optic cable. Then they used an electronic detector to measure the positions of photons at some distance away from the slits, and a calcite crystal in front of the detector to change the polarization of the photon, and allow them to make a very rough estimate of each photon's momentum from that change.

Average trajectory

By measuring the momentum of many photons, the researchers were able to work out the average momentum of the photons at each detector. They then moved the crystal progressively further away from the slits, and so by "connecting the dots" were able to trace out the average trajectories of the photons. They did this while still recording an interference pattern at each detector position.

Intriguingly, the trajectories closely match those predicted by an unconventional interpretation of quantum mechanics known as pilot-wave theory, in which each particle has a well-defined trajectory that takes it through one slit while the associated wave passes through both slits. The traditional interpretation of quantum mechanics, known as the Copenhagen interpretation, dismisses the notion of trajectories, and maintains that it is meaningless to ask what value a variable, such as momentum, has if that's not what is being measured.

Steinberg stresses that his group's work does not challenge the uncertainty principle, pointing out that the results could, in principle, be predicted with standard quantum mechanics. But, he says, "it is not necessary to interpret the uncertainty principle as rigidly as we are often taught to do", arguing that other interpretations of quantum mechanics, such as the pilot-wave theory, might "help us to think in new ways".

David Deutsch of the University of Oxford, UK, is not convinced that the experiment has told us anything new about how the universe works. He says that although "it's quite cool to see strange predictions verified", the results could have been obtained simply by "calculating them using a computer and the equations of quantum mechanics".

"Experiments are only relevant in science when they are crucial tests between at least two good explanatory theories," Deutsch says. "Here, there was only one, namely that the equations of quantum mechanics really do describe reality."

But Steinberg thinks his work could have prectical applications. He believes it could help to improve logic gates for quantum computers, by allowing the gates to repeat an operation deemed to have failed previously. "Under the normal interpretation of quantum mechanics we can't pose the question of what happened at an earlier time," he says. "We need something like weak measurement to even pose this question."

This article is reproduced with permission from the magazine Nature. The article was first published on June 3, 2011.