Time Travel Impossible ?

Shengwang Du and His Team Measured the Speed of a Photon

Hong Kong scientists claim to have proved that a single photon cannot travel faster than the speed of light in a vacuum, making time travel seemingly impossible.

These findings are consistent with Einstein’s claim that the speed of light is the “traffic law of the universe” or, otherwise put, that nothing can travel faster than light. Of course, humanity’s dream of time travel has long relied on the hope that Einstein was wrong. We got a glimmer of hopeback in 1999 when scientists found superluminal (faster-than-light) propagation of optical pulses in some specific medium, the team said. Unfortunately, this was later revealed to be a visual effect, and the photons involved couldn’t actually transmit information.

Since then, there has been a debate surrounding whether or not a single photon might actually be able to travel faster than the speed of light. This experiment, conducted by a team at the Hong Kong University of Science and Technology and led by Professor Shengwang Du, measured the ultimate speed of a photon using controllable waveforms. An explanation, from the press release:

"HKUST's team used a demonstration which required not only producing single photons, but separating the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon wave packet. To do so, Prof Du's team generated a pair of photons, and then passed one of them through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT). For the first time, they successfully observed optical precursors of a single photon.

The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum. The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow light medium. Even in a superluminal medium where the group velocity (of an optical pulse peak) is faster than the speed of light in vacuum, the main part of the single photon has no possibility to travel faster than its precursor."

What that means for us: probably no time travel. Du says their results "confirm the upper bound on how fast information travels with light. By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon." That indicates, in turn, that time travel is not possible, and that decades of beloved science fiction may not be entirely based on fact.

Source: Popsci via Daily Mail, Hong Kong University of Science and Technology

Researchers Succeed in Quantum Teleportation of Light Waves

This is a Teleportation Device The setup Noriyuki Lee and colleagues used to teleport quantum light.

In a real-life use of Schrödinger's theoretical paradoxical cat, researchers report that they were able to quickly transfer a complex set of quantum information while preserving its integrity. The information, in the form of light, was manipulated in such a way that it existed in two states at the same time, and it was destroyed in one spot and recreated in another. The new breakthrough is a major step toward building safe, effective quantum computers.

No felines were harmed in the making of this experiment, which actually studied wave packets of light that existed in a state of quantum superposition, meaning they existed in two different phases simultaneously. This phenomenon is described in Erwin Schrodinger’s quantum mechanics thought experiment, in which a cat is simultaneously dead and alive, depending on the state of a subatomic particle.

In this experiment, researchers in Australia and Japan were able to transfer quantum information from one place to another without having to physically move it. It was destroyed in one place and instantly resurrected in another, “alive” again and unchanged. This is a major advance, as previous teleportation experiments were either very slow or caused some information to be lost.

The team employed a mind-boggling set of quantum manipulation techniques to achieve this, including squeezing, photon subtraction, entanglement and homodyne detection. The photo above depicts their device, nicknamed the Teleporter, in the lab of Akira Furusawa at the University of Tokyo.

The results pave the way for high-speed, high-fidelity transmission of information, according to Elanor Huntington, a professor at the University of New South Wales in Australia who was part of the study.

“If we can do this, we can do just about any form of communication needed for any quantum technology,” she said in a news release.

Instead of using ones and zeroes, quantum computers store data as qubits, which can represent one and zero simultaneously. This superposition enables the computers to solve multiple problems at once. The new, faster teleportation process means scientists can move blocks of this quantum information around within a computer or across a network, Huntington said.

Optics researcher Philippe Grangier at the Institut d’Optique in Palaiseau, France, said it was a major breakthrough.

“It shows that the controlled manipulation of quantum objects has progressed steadily and achieved objectives that seemed impossible just a few years ago,” he wrote in an editorial that accompanies the study.

The results were published today in the journal Science.

Source: Popsci via Science