Quantum entanglement is a fundamental phenomenon in quantum mechanics where two or more particles become interconnected in such a way that the state of one particle is dependent on the state of the other, regardless of the distance between them. This entanglement can occur between particles at the quantum level, such as electrons or photons, and it has been experimentally observed and confirmed.
When we talk about something physical being entangled in a quantum state, we are referring to macroscopic objects or systems that are composed of a large number of particles, such as molecules, atoms, or even larger structures. Although quantum entanglement is typically associated with microscopic particles, it is possible for the collective behavior of a large number of particles to exhibit entanglement, leading to the entanglement of macroscopic objects.
One example of macroscopic entanglement is known as "entanglement of collective variables." In such cases, the quantum state of a macroscopic object or system is described in terms of collective variables, which are properties that emerge from the interactions of many individual particles. These collective variables can be entangled, meaning that the state of the entire system depends on the states of the constituent particles.
For instance, in a superconducting system, such as a superconducting qubit, the macroscopic state of the system, such as the presence or absence of a current, can be entangled with the quantum state of individual electrons within the material. Similarly, in Bose-Einstein condensates, which are collections of a large number of ultra-cold atoms, the collective behavior of the condensate can exhibit entanglement.
It's important to note that while macroscopic entanglement has been observed and studied in various systems, the fragility of these entangled states increases with the size and complexity of the system. Macroscopic objects are more prone to environmental disturbances and decoherence, which can rapidly destroy the delicate quantum states required for entanglement.
In summary, macroscopic objects or systems can exhibit entanglement through the entanglement of collective variables, where the quantum state of the entire system depends on the states of its constituent particles. Although macroscopic entanglement is possible, it is more challenging to achieve and maintain due to increased sensitivity to environmental influences.