Photonics Trick Finesses Heisenberg's Uncertainty Principle

2 June 2011—An unconventional alternative to quantum mechanics gets a boost this week with a new experiment that tracks the path of individual photons. And the alternative theory, Bohmian mechanics, and the novel photonics technology behind the experiment find possible applications in everything from simulating complex molecules to quantum tunneling.

In the traditional view of quantum phenomena, photons are both waves and particles—the latter manifesting as clouds of probability that can’t be precisely pinpointed. The canonical Heisenberg uncertainty principle restricts measuring both a particle’s position and velocity: Knowledge of one characteristic makes it difficult to measure the other. If you try to find out too much about the particle, the impact of your observation will alter the property of that particle.

Yet a team of American, Australian, Canadian, and French physicists report in this week’s Science that they have exploited a clever workaround to achieve what had seemed impossible. Like detectives tracking an elusive subject, they managed to compile a complete history of the position and velocity of individual photons in their experiment. The trick they used still adheres to the uncertainty principle but demonstrates how Heisenberg’s shackles aren’t nearly as restricting as was sometimes thought.

Sacha Kocsis and Boris Braverman at the University of Toronto and Sylvain Ravets at the Université Paris-Sud Campus Polytechnique used a supercooled quantum dot made of the semiconductor indium gallium arsenide to spit out single visible photons. The group found, as in the classic 1909 experiment using X-rays, that their single photons, when sent through a double slit, still interfere with themselves, forming interference patterns on a photo plate or CCD detector as if they were a whole wave front of particles.

Naively, one might simply want to map out the path of each photon’s every step through space, as it travels from the double slit to the CCD wall where it’s detected. But here is where Heisenberg’s eponymous principle comes into play.

If you try to glean too much information about the photons’ position and direction as they fly between the double slit and the detector, the system "collapses." The act of measurement destroys the previously simple interference patterns.

To discover the photons’ trajectory without collapsing the system, Kocsis and his colleagues first sent the single photons through thin crystals made of calcite, a mineral that has the optical property of "birefringence," the ability to rotate a photon’s polarization differently depending on the direction the photon travels through the crystal.

At various points along the detector wall, the group focused the detector’s lens to bring various regions of space inside the interferometer into view. Then, because of the focus, they knew how far away each photon coming into view was (its position). And the photon’s polarization provided information about the original direction in which the photons had been traveling (a proxy for velocity).

If this experiment is performed thousands of times—examining all the points in space between the double slit and the detector and all the possible photon directions at each of those points—some sophisticated statistical analysis yields a map of the trajectories of each photon’s path in the interferometer.

Although such a complete map seems at odds with quantum mechanics as it is generally understood, objective, knowable particle trajectories are in fact a mainstay of an alternative interpretation of quantum physics called Bohmian mechanics (after its creator, David Bohm). Bohmian mechanics has fallen out of favor with the mainstream physics community, in part because of the misconception that it has been refuted.

Yet, says physics professor Aephraim Steinberg, also at the University of Toronto (and a professed agnostic regarding such alternative quantum interpretations), the Bohmian view "makes the same predictions as quantum mechanics."

On the other hand, for all their compatibility with orthodox physics, alternative quantum interpretations do matter. "The Bohmian interpretation does have practical applications, too," says Howard Wiseman, professor of physics at Griffith University in Australia, who was not involved in the quantum-dot-and calcite experiment. "It gives a way to develop approximate treatments of molecular dynamics, for example, which is too hard to treat without approximations."

Wiseman first developed the proxy-measurement trick used in the quantum-dot-and-calcite experiment—in which a photon’s polarization becomes a placeholder for its original direction in space. These "weak measurements," as Wiseman calls them, enable new ways to finesse Heisenberg limits that offer great potential for further applications, he says.

One such possibility, Wiseman says, may come in the precision timing of quantum tunneling—the physics behind flash memory, among other technologies.

But perhaps the most important outcome of the experiment, says Sheldon Goldstein, professor of mathematics at Rutgers University, who was not involved in the research, is that it brings a too frequently scorned alternative to quantum physics more into the mainstream. Bohmian mechanics, Goldstein says, "doesn’t conflict with any experimentally established facts. But it does conflict with certain prejudices."

Says Wiseman: "The Bohmian interpretation has said all along that these trajectories exist, so to some people this [research] will merely be a pleasant surprise. But for many people, I think it will be shocking."