02 November 2012

Photonic Chip Using Quantum Nonlocality To Explain Wave Particle Duality

Scientists have developed a new experiment based on a quantum photonic chip to explain the paradox of wave-particle duality.

In 1803, Thomas Young performed the now famous double slit experiment. Decades after, the double slit experiment has demonstrated that energy and matter behave with properties inherent to both waves and particles.

Young conducted the experiment to prove that light is emitted like a series of waves (Wave theory of light). This was controversial at the time since the accepted theory was that of Isaac Newton where light is made up of particles (Corpuscular Theory of Light).

The argument at the time whether light is made up of wave or particles persisted. In 1905, Albert Einstein showed that having light as particles would explain the photoelectric effect. This effect explained that shining a light on certain metals would lead to an electric current in a circuit. He observed that a dim light caused a current but a bright light does not. This behavior only would happen if photons were made of particles and not of waves.

But in quantum mechanics as seen in Einstein's photoelectric effect and Young's double slit experiment, photons exhibit properties of both particles and waves.

The Double Slit Experiment

The modern double slit experiment is done by having shooting photons one at a time to a plate with two parallel slits. The passing photons are observed on a screen behind the plate. The corresponding pattern displayed shows an interference pattern that can only be formed by waves.

Video: Explaining Quantum Mechanics

Since only single photons were being shot off, it was as if it entered both slits and collided with itself to form the pattern. To find an explanation to this phenomenon, they tried to observe the photon while it passes through the slit. Surprisingly, they saw that the electron passed through the slit as if it were a particle.

Furthermore, during observation, the photon behaved either as a particle or as a wave but are never observed as both simultaneously. In fact, which behavior it exhibits depends on the type of measurement it is presented with. These astonishing phenomena have been experimentally investigated in the last few years, using measurement devices that can be switched between wave-like and particle-like measurements.

This behavior confused scientists since there are no existing laws in classical mechanics that can explain this behavior. They called this paradox as wave-particle duality. It simply means that in quantum mechanics, all particles exhibit both wave and particle properties.

Measuring A Wave and A Particle Simultaneously

In a paper published today in Science, physicists from the University of Bristol give a new twist on these ideas. Dr Alberto Peruzzo, Peter Shadbolt and Professor Jeremy O'Brien from the Centre for Quantum Photonics teamed up with quantum theorists Dr Nicolas Brunner and Professor Sandu Popescu to devise a novel type of measurement apparatus that can measure both particle and wave-like behaviour simultaneously. This new device is powered by quantum nonlocality, another strikingly counter-intuitive quantum effect.

Dr Peruzzo, Research Fellow at the Centre for Quantum Photonics, said: "The measurement apparatus detected strong nonlocality, which certified that the photon behaved simultaneously as a wave and a particle in our experiment. This represents a strong refutation of models in which the photon is either a wave or a particle."

Professor O'Brien, Director of the Centre for Quantum Photonics, said: "To conduct this research, we used a quantum photonic chip, a novel technology pioneered in Bristol. The chip is reconfigurable so it can be programmed and controlled to implement different circuits. Today this technology is a leading approach in the quest to build a quantum computer and in the future will allow for new and more sophisticated studies of fundamental aspects of quantum phenomena."


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Wave–particle duality
Double-slit experiment
Wave theory