*For the first time, physicists observed a quantum phenomenon that Einstein and two of his colleagues hotly debated over eighty years ago: the Einstein Podolsky Rosen paradox.*

In 1935, **Einstein**, and two other physicists, **Boris Podolsky** and **Nathan Rosen**, made a **thought experiment** that became famous and shed doubt over the quantum theory of physics.

## The EPR Paradox, a Reflection on Quantum Mechanics

First published in the *Physical Review* journal (PDF), the problem is known as the **Einstein-Podolsky-Rosen paradox**, or the **EPR Paradox**.

The **Einstein Podolsky Rosen argument** involves **quantum entanglement** and **superposition**.

According to the **superposition principle **of quantum mechanics, unless a particle is measured, it remains in an uncertain state.

Measurement, or observation, destroys superposition, and the more precisely a quantum system is measured, the more indeterminate is the measurement of the other.

When two quantum systems, A and B, are in a state of **entanglement**, the state of one affects the other, whatever the distance between them.

In a nutshell, the Einstein Podolsky Rosen Paradox suggests that an observer, only based on the measurements of system A, could deduce the state of system B more precisely than an observer with direct access to it.

Einstein’s goal, and the trio’s thought experiment as a whole, is a **criticism of the** **quantum theory** that points to its contradictions, overall incompleteness, and inconsistencies with other laws in physics.

## The Einstein Podolsky Rosen Paradox Observed

Now, after over eighty years since it was first brought up, the **EPR paradox** was observed in a quantum system.

Physicists at the **University of Basel** report the first observation of the EPR paradox as a quantum phenomenon in a system of several hundreds of atoms.

Using a laser, the team cooled hundreds of atoms to let quantum mechanics play its tricks. It then formed a **multi-particle system** in the form of **Bose-Einstein condensate**, the 5^{th} state of matter, also predicted by Einstein back in the 1920s.

In a **BEC**, particles become multi-entangled and behave as a **single system**.

Researchers were able to make precise predictions about the state of regions simultaneously based on measurements of other regions in the multi-particle system.

* “The results of the measurements in the two regions were so strongly correlated that they allowed us to demonstrate the EPR paradox,” *said the lead author of the study*. “It’s fascinating to observe such a fundamental phenomenon of quantum physics in ever larger systems. At the same time, our experiments establish a link between two of Einstein’s most important works.”*

Beyond confirming yet another prediction by Einstein, this demonstration of the EPR Paradox could provide the basis for practical applications such as **quantum sensors** and new **i****maging techniques**.