Quantum entanglement

SPDC figure

NYT May 4, 1935

Von Neumann entropy for bipartite system plot

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles become interconnected in such a way that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance. The term was first coined by Erwin Schrödinger in 1935, as a translation of the German Verschränkung ("entanglement"). This phenomenon is a fundamental aspect of quantum mechanics, and it has been a subject of much research and debate in the physics community.

Overview

Quantum entanglement is a counterintuitive aspect of quantum mechanics that defies the classical understanding of the world. When two particles become entangled, the properties of one particle (such as its spin, polarization, or position) can instantaneously affect the properties of the other, no matter how far apart the two are. This phenomenon was famously referred to by Albert Einstein as "spooky action at a distance", and it challenges the classical notion of locality and causality.

Historical Context

The concept of quantum entanglement was first brought to light through the Einstein-Podolsky-Rosen paradox (EPR paradox) in 1935, which questioned the completeness of quantum mechanics. Einstein, along with physicists Boris Podolsky and Nathan Rosen, proposed a thought experiment that aimed to demonstrate that quantum mechanics could not be a complete theory if it required such "spooky" actions at a distance. However, subsequent experiments, most notably those based on the Bell's theorem proposed by physicist John Stewart Bell in 1964, have provided strong evidence in favor of quantum entanglement, showing that the predictions of quantum mechanics regarding entanglement are indeed observed in reality.

Experimental Evidence

The first significant experimental tests of quantum entanglement were conducted in the 1970s and 1980s, notably by Alain Aspect and his team in 1982. These experiments involved measuring the polarization states of entangled photons and provided empirical support for the existence of quantum entanglement, thus challenging the classical assumptions about the separability and independence of distant objects.

Applications

Quantum entanglement has potential applications in various cutting-edge technologies, including quantum computing, quantum cryptography, and quantum teleportation. In quantum computing, entanglement is used to link qubits in a way that enhances computational power beyond what is possible with classical systems. Quantum cryptography utilizes entanglement to create secure communication channels that are theoretically immune to eavesdropping. Quantum teleportation, on the other hand, exploits entanglement to transmit the state of a quantum system from one location to another without physical transfer of the system itself.

Challenges and Future Directions

Despite its proven existence and potential applications, quantum entanglement raises profound questions about the nature of reality, information, and causality. The non-local nature of entanglement challenges our classical intuitions about space and time, and it has implications for the interpretation of quantum mechanics. Researchers continue to explore the limits of entanglement, including how it can be harnessed for practical applications and what it reveals about the fundamental principles of the universe.