No, quantum entanglement does not have to be restricted to particles. While the most commonly discussed examples of entanglement involve particles like photons or electrons, entanglement can occur between any quantum systems.
In quantum mechanics, entanglement refers to a phenomenon where two or more quantum systems become correlated in such a way that the state of one system is intrinsically linked to the state of the other(s), regardless of the physical distance between them. The entangled systems exhibit a type of correlation that is stronger than any classical correlation.
These quantum systems can be particles, such as photons or electrons, but they can also be larger entities like atoms, molecules, or even macroscopic objects. As long as the systems are described by quantum mechanics and can exist in superposition states, they can potentially become entangled.
For example, in experiments with superconducting circuits, it has been possible to create entangled states between artificial atoms, known as qubits, which are macroscopic electrical circuits that exhibit quantum behavior. This demonstrates that entanglement can occur on a larger scale than individual particles.
Additionally, entanglement has been explored in various quantum systems such as ions, Bose-Einstein condensates, and even in the context of quantum information processing using quantum bits (qubits) encoded in different physical systems like photons, ions, or superconducting circuits.
The fundamental principles of quantum entanglement apply to any quantum system, irrespective of its size or nature. It is a fascinating and essential concept in quantum physics that has implications for fields such as quantum computing, quantum communication, and fundamental tests of quantum mechanics.