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Showing 135 posts tagged Light


Researchers from Cornell University in Ithaca, N.Y., have demonstrated for the first time that it’s possible to cloak a singular event in time, creating what has been described as a “history editor.” In a feat of Einstein-inspired physics, Moti Fridman and his colleagues sent a beam of light traveling down an optical fiber and through a pair of so-called “time lenses.” Between these two lenses, the researchers were able to briefly create a small bubble, or gap, in the flow of light. During that fleetingly brief moment, lasting only the tiniest fraction of a second, the gap functioned like a temporal hole, concealing the fact that a brief burst of light ever occurred.

Their ingenious system, which is the first physical demonstration of a phenomenon originally described theoretically a year ago by Martin McCall and his colleagues at Imperial College London in the Journal of Optics, relies on the ability to use short intense pulses of light to alter the speed of light as it travels through optical materials, in this case an optical fiber. (In a vacuum, light maintains its predetermined speed limit of 180,000 miles per second.) As the beam passes through a split-time lens (a silicon device originally designed to speed up data transfer), it accelerates near the center and slows down along the edges, causing it to balloon out toward the edges, leaving a dead zone around which the light waves curve. A similar lens a little farther along the path produces the exact but opposite velocity adjustments, resetting the speeds and reproducing the original shape and appearance of the light rays.

To test the performance of their temporal cloak, the researchers created pulses of light directly between the two lenses. The pulses repeated like clockwork at a rate of 41 kilohertz. When the cloak was off, the researchers were able to detect a steady beat. By switching on the temporal cloak, which was synchronized with the light pulses, all signs that these events ever took place were erased from the data stream.

Unlike spatial optical cloaking, which typically requires the use of metamaterials (specially created materials engineered to have specific optical properties), the temporal cloak designed by the researchers relies more on the fundamental properties of light and how it behaves under highly constrained space and time conditions. The area affected by the temporal cloak is a mere 6 millimeters long and can last only 20 trillionths of a second. The length of the cloaked area and the length of time it is able to function are tightly constrained — primarily by the extreme velocity of light. Cloaking for a longer duration would create turbulence in the system, essentially pulling back the curtain and hinting that an event had occurred. Also, to achieve any measurable macroscopic effects, an experiment of planetary and even interplanetary scales would be necessary.

Love waking up to news like this.

Uncertainty isn’t what it used to be

One of the central planks of quantum mechanics was called into question in a new take on the classic two-slit experiment.

One of the central notions in quantum mechanics is that light and matter can behave as both particle and wave. The principle of “complementarity” has always been understood to prevent the observation of both behaviours simultaneously. However, new research published in Science on 2 June, suggests that physicists at the University of Toronto and Griffith University in Brisbane have for the first time observed both behaviours at the same time.

In Thomas Young’s 19th century “two-slit experiment”, light is passed through two tiny holes and is then viewed on a screen. The two beams interfere with each other, forming a diffraction pattern, as if the light were made of waves. If one of the slits is blocked, the light can be seen as a single beam on the screen, as if light were made of particles. The two-slit experiment shows that, depending on how it’s measured, a photon will act like either a particle or a wave, but never both.

Aephraim Steinberg of the University of Toronto and Sacha Kocsis of Griffith recreated this experiment, easily observing the interference pattern indicative of the wave nature of light. But significantly, they were also able measure the path of the particles of light.

Science reporter, Adrian Cho elaborates on the importance of their new research:
"For decades, [the] experiment has served as physicists’ canonical example of the uncertainty principle: the law of nature that says you can’t know both where a subatomic particle is and how fast it is moving, and thus can’t trace its trajectory. But now physicists have tweaked that classic experiment to show that they can follow the average path taken by many particles."

Steinberg and his team allowed photons to pass through a calcite crystal which gave each photon a small deviation in its path. By measuring the light patterns on a camera, the team was able to deduce what paths the photons had taken. They clearly saw the interference pattern which infers the wave nature of light, but surprisingly they also could see from which slits the photons had come from, a telltale sign of the particle nature of light.

Marlan Scully, a quantum physicist at Texas University, commented:

"It’s a beautiful series of measurements by an excellent group, the likes of which I’ve not seen before.",

"This paper is probably the first that has really put this weak measurement idea into a real experimental realisation." He said that the work would - inevitably - raise philosophical issues as well. "The exact way to think about what they’re doing will be researched for some time, and the weak measurement concept itself will be a matter of controversy"

Professor Steinberg commented, “I feel like we’re starting to pull back a veil on what nature really is”.

(via Particle Decelerator)