What is Quantum Entanglement?

IN THIS ISSUE SPRING 2011

Quantum Connections Through Space and Time
Author: Suzanne Nichols

Quantum entanglement is one of the most out-of-the-box areas of study in physics today, including research into cool sounding things such as quantum cryptography, quantum computing, and quantum teleportation. But whereas lots of big things can become entangled, like your ball of yarn or your hair, quantum entanglement refers to how tiny quantum entities like photons (particles of light) can come together, bond, and share quantum bits of information (or qubits, as they are called in the bizz). Once these particles have become entangled, they continue to share information no matter how far apart they become in space and, perhaps, in time as well.

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What is Quantum Entanglement? - Quantum Connections Through Space and Time

The term “entanglement,” used in physics to describe this inseparable relationship between quantum systems, was first introduced by the physicist Erwin Schrodinger in 1935. At the time, there were many profound discussions and interpretations of the revolutionary new science of quantum mechanics and its implications. Schrodinger believed that entanglement was one of the most important aspects of the quantum world, describing it as “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” One of the interpretations of quantum mechanics that Schrodinger disagreed with was the Copenhagen Interpretation, which suggested that in the quantum world all potentials exist simultaneously until one potential is observed. In an effort to illustrate the absurdity of such a notion, he decided to create a thought experiment, an imaginary experiment where he could model in mind what he could not do in a laboratory. His thought experiment came to be known as “Schrodinger’s Cat.”

He put an imaginary cat in a box along with some radioactive material. If the material decayed, it would set off a Geiger counter which would trigger a little hammer which would hit a vial of poison and kill the cat. The material had a 50-50 chance of decaying, so Schrodinger posed that his fictional cat would be both dead and alive until someone looked into the box! Absurd! Or not . . .

Albert Einstein created a thought experiment in 1935 as well. His thought experiment was designed to discredit Schrodinger’s idea of quantum entanglement. Einstein posed the question: what if two photons were entangled and then shot off in different directions, each traveling at the speed of light, which only photons can do, and you were to observe and measure one — would the other “know” instantaneously and change as a result? Of course not, reasoned Einstein, who was dedicated to the notion that nothing can travel faster than the speed of light, including information.

What is Quantum Entanglement? - Quantum Connections Through Space and Time

By the early 1980s science had evolved to the point that Einstein’s thought experiment could actually be done in a laboratory setting. French physicist Alain Aspect conducted a series of experiments which tested the nature of entangled photon pairs and how they might share information. What he found was amazing! In fact, the measurement of one photon did affect the state of its entangled partner, instantaneously! What Einstein had formerly called rather derisively “spooky action at a distance” was actually a quantum mechanical fact.

Since Aspect’s initial experiment, many scientists all over the world have continued to explore the implications and possible applications of quantum entanglement. In May of 2010, Chinese scientists successfully achieved the quantum teleportation of information over a distance of 10 miles. In January of 2011, physicists S. Jay Olson and Timothy Ralph of Australia’s University of Queensland produced the mathematics to support the quantum teleportation of information through time, from the past to the future. And an international team of physicists led by Stephanie Simmons and John Morton of Oxford University published a paper (January 19, 2011 in Nature online) outlining their ability to produce 10 billion entangled pairs of phosphorous within a medium of crystalized silicon. Given that silicon is widely used in conventional computers, this achievement is considered by many to be a significant step in the development of a solid state, siliconbased quantum information processor.

Who knows where quantum entanglement is taking us next? Maybe we should create a thought experiment . . .

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