Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become correlated in such a way that the state of one particle cannot be described independently of the state of the other particles. In its simplest form, quantum entanglement can be understood through a concept known as "spin entanglement."
Spin entanglement involves particles with an intrinsic property called "spin," which is a type of angular momentum. For example, let's consider a pair of entangled electrons. Each electron can have its spin pointing either "up" or "down" along a particular direction, denoted as spin-up (↑) and spin-down (↓).
In a simplified scenario, we can imagine that these two electrons are initially prepared in a special quantum state called a singlet state. In this state, the total spin of the entangled electron pair is zero, which means that their spins are perfectly correlated. However, the individual spins of the electrons are not fixed and can only be determined when measured.
Now, here's where entanglement becomes intriguing: If we measure the spin of one electron and find it to be, for instance, spin-up (↑), the state of the other electron instantly becomes spin-down (↓), regardless of the physical distance between them. Similarly, if we measure the spin of one electron and find it to be spin-down (↓), the state of the other electron will be spin-up (↑).
The remarkable aspect of entanglement is that the state of each electron is undefined until measured, and the measurement of one particle's spin instantaneously determines the spin state of the other particle, regardless of the spatial separation between them. This instantaneous correlation, occurring faster than the speed of light, is often referred to as "spooky action at a distance" by Albert Einstein.
Quantum entanglement extends beyond spin and applies to other properties of particles as well. It forms the basis for various quantum technologies, including quantum computing, quantum cryptography, and quantum teleportation. The concept of entanglement lies at the heart of quantum mechanics and remains a fascinating and puzzling aspect of the quantum world.