According to Wikipedia (sorry, citing Wikipedia is not ideal; but indeed works here), which provides a “simplified definition”**,**Quantum entanglement is one of the truly amazing, mysterious and counter-intuitive actions in quantum physics. Quantum entanglement occurs when particles such as photons, electrons, molecules and even small diamonds interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description (state), which is indefinite in terms of important factors such as position, momentum, spin, polarization, etc.

In 1935, a group led by Albert Einstein, including Boris Podolsky and Nathan Rosen, commonly referred to as EPR, published a thought experiment designed to show that quantum mechanics, by itself, cannot describe reality to any degree of confidence. Using two entangled particles EPR tried to demonstrate that there must be some other hidden parameters or particles that quantum mechanics overlooks.

In subsequent years, a number of theoretical physicists showed that the kind of hidden parameters EPR had in mind were indeed incompatible with observations. The mystery at the heart of quantum mechanics therefore remained. But the entanglement first proposed by EPR is now a valuable resource in emerging quantum technologies like quantum computing, quantum cryptography, and quantum precision measurements.
Quantum entanglement is one of the central principles of quantum physics, which is the science of sub-atomic particles. *Multiple particles, such as photons, are connected with each other even when they are very far apart and what happens to one particle can have an effect on the other one at the same moment, even though these effects cannot be used to send information faster than light.*

Now, physicists at the University of Calgary and at the Institute for Quantum Computing in Waterloo have published new research in Nature Physics which builds on the original ideas of Einstein and adds a new ingredient: a third entangled particle.

The new form of three-particle entanglement demonstrated in this experiment, which is based on the position and momentum properties of photons, may prove to be a valuable part of future communications networks that operate on the rules of quantum mechanics, and could lead to new fundamental tests of quantum theory that deepen our understanding of the world around us.

“This work opens up a rich area of exploration that combines fundamental questions in quantum mechanics and quantum technologies,” says Christoph Simon, paper co-author and researcher at the University of Calgary. This research extends the theories of Einstein, seventy-seven years later.

“It is exciting, after all this time, to be able to finally create, control, and entangle, quantum particles in this new way. Using these new states of light it may be possible to interact with and entangle distant quantum computer memories based on exotic atomic gases, ” says Thomas Jennewein, whose group at the University of Waterloo carried out the experiment.

The next step for the researchers is to try to combine the position and momentum entanglement between their three photons with more traditional types of entanglement based on angular momentum. This will allow the creation of hybrid quantum systems that combine multiple unique properties of light at the same time.

**Quantum entanglement** is a form of quantum superposition. When a measurement is made and it causes one member of such a pair to take on a definite value (e.g., clockwise spin), the other member of this entangled pair will at any subsequent time be found to have taken the appropriately correlated value (e.g., counterclockwise spin). Thus, there is a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may have been separated by the Planck length ^{1} (the smallest unit of measurement in physics) and is too small to see even with the most powerful electron microscope or be arbitrarily large distances such as 10 light years ^{2}. In quantum entanglement, part of the transfer happens instantaneously. Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there’s no slower-than-light influence that can pass between the entangled particles. Recent experiments have shown that this transfer occurs at least 10,000 times faster than the speed of light.

**Note 1**: The Planck length is denoted as a unit of length, equal to 1.616199(97)×10−35 metres. It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, Planck’s constant, and the gravitational constant.

It is much to small cannot be seen by any microscope and is the smallest length defined in physics.

**Note 2: **10 light years is is defined as

≈ 5,878,625 million miles times 10

≈ 58,786,250 million miles

≈ 58,786,250,000,000,000 jillion miles

≈ 58.8 jillion miles

≈ 58.8 ^{14} miles

≈ the distance light will travel in 10 light years

≈ really, really, really far away