Experiment confirms quantum theory weirdness

27 MAY 2015

Associate Professor Andrew Truscott (L) with PhD student Roman Khakimov.

The bizarre nature of reality as laid out by quantum theory has survived another test, with scientists performing a famous experiment and proving that reality does not exist until it is measured.

Physicists at The Australian National University (ANU) have conducted John Wheeler’s delayed-choice thought experiment, which involves a moving object that is given the choice to act like a particle or a wave. Wheeler’s experiment then asks – at which point does the object decide?

Common sense says the object is either wave-like or particle-like, independent of how we measure it. But quantum physics predicts that whether you observe wave like behavior (interference) or particle behavior (no interference) depends only on how it is actually measured at the end of its journey. This is exactly what the ANU team found.

“It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it,” said Associate Professor Andrew Truscott from the ANU Research School of Physics and Engineering.

Despite the apparent weirdness, the results confirm the validity of quantum theory, which governs the world of the very small, and has enabled the development of many technologies such as LEDs, lasers and computer chips.

The ANU team not only succeeded in building the experiment, which seemed nearly impossible when it was proposed in 1978, but reversed Wheeler’s original concept of light beams being bounced by mirrors, and instead used atoms scattered by laser light.

“Quantum physics’ predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness,” said Roman Khakimov, PhD student at the Research School of Physics and Engineering.

Professor Truscott’s team first trapped a collection of helium atoms in a suspended state known as a Bose-Einstein condensate, and then ejected them until there was only a single atom left.

The single atom was then dropped through a pair of counter-propagating laser beams, which formed a grating pattern that acted as crossroads in the same way a solid grating would scatter light.

A second light grating to recombine the paths was randomly added, which led to constructive or destructive interference as if the atom had travelled both paths. When the second light grating was not added, no interference was observed as if the atom chose only one path.

However, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads.

If one chooses to believe that the atom really did take a particular path or paths then one has to accept that a future measurement is affecting the atom’s past, said Truscott.

“The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence,” he said.

The research is published in Nature Physics.



A Monolithic White Laser

Valerie C. Coffey

The tunable white laser created at ASU comprises a novel nanosheet that lases in three elementary colors. The device is tunable to any visible color as well as white. [Image: ASU/Nat. Nanotech.]
A white-light laser has the potential to replace white LEDs in lighting, displays, sensing and telecom, with higher energy conversion efficiencies and higher output powers. But multicolor lasing from a small, monolithic semiconductor-based laser has remained elusive due to fundamental challenges in materials and structures.

Researchers at the Arizona State University (ASU; Tempe, Arizona, USA) have reportedly overcome these challenges for the first time, to create a multi-segment monolithic white laser that emits at one or all of the visible colors of the spectrum at once (Nat. Nanotech., doi:10.1038/nnano.2015.149). ASU professor Zheng Ning and colleagues worked with various nanomaterials in multiple configurations for more than ten years before successfully finding the solution. Lasing in green and red from a monolithic semiconductor sheet came first; but growing blue-emitting materials on the same sheet took two years to perfect.

The winning technique involved dynamically manipulating a ZnCdSSe alloy nanosheet along an axial temperature gradient during chemical vapor deposition growth. The team achieved the desired segmented nanosheet morphology separately from the desired material composition, a novel approach requiring multiple steps and simultaneous cation-anion exchange in a careful sequence. The resulting structures measure 60 by 45 µm and range in thickness from 60 to 350 nm.

The multi-segment monolithic nanosheets lase at red, green and blue wavelengths, and every color in between, when excited by a 355-nm pulsed laser at room temperature. The emission reaches across 190 nm of the visible wavelength range, the largest ever reported for such a structure.

The team’s experimental proof of concept will require further efforts to operate via battery instead of optical pumping. Still, the researchers believe that demonstration of the required growth process marks a significant step toward the goal of electrically operated white lasers.

[Correction, 2015/08/07: We have changed the title of this story; the previous title incorrectly suggested that this development represented the first white laser rather than the first monolithic white laser.]