In classical physics, the concept of correlation refers to the statistical relationship between two or more variables. Correlation implies that two variables are related or linked in some way, but it does not necessarily imply any causal connection between them. For example, the price of ice cream and the number of swimming pool accidents may be correlated, but it does not mean that the price of ice cream causes swimming pool accidents or vice versa.
In quantum mechanics, the concept of correlation is fundamentally different from classical physics due to the phenomenon of entanglement. Entanglement is a quantum mechanical phenomenon that occurs when two or more quantum systems become correlated in a way that is not possible in classical physics. When two or more quantum systems are entangled, the state of one system is correlated with the state of the other system in such a way that their joint state cannot be described by a simple combination of the states of the individual systems.
One way to understand the difference between classical correlations and entanglement is to consider a simple example of two quantum particles, such as electrons or photons. In classical physics, the state of a system is described by a set of variables, such as position and velocity, that can be measured independently of each other. In quantum mechanics, however, the state of a system is described by a wave function that encodes all possible outcomes of a measurement. When two particles are entangled, their wave functions are linked in such a way that the outcome of a measurement on one particle is correlated with the outcome of a measurement on the other particle, even if the particles are separated by large distances.
To illustrate this point, consider the following thought experiment known as the Einstein-Podolsky-Rosen (EPR) paradox. In this experiment, two particles, such as photons, are produced in a way that ensures they are entangled. The entangled photons are then sent to two distant locations, labeled A and B. At each location, a measurement is performed on one of the photons, and the outcome of the measurement is recorded.
According to quantum mechanics, the outcome of a measurement on an entangled particle is not determined until the measurement is performed. However, once a measurement is performed on one particle, the state of the other particle becomes correlated with the outcome of the first measurement. This means that the outcome of the second measurement is not random, but is determined by the outcome of the first measurement. In other words, the two particles are correlated in a way that cannot be explained by classical physics.
One of the key differences between classical correlations and entanglement is that classical correlations can be explained by local hidden variables, whereas entanglement cannot. Local hidden variables are hypothetical variables that describe the properties of a system in a way that is consistent with both classical physics and the statistical predictions of quantum mechanics. However, the existence of entanglement implies that local hidden variables cannot fully describe the state of a quantum system. This was proven by John Bell in 1964, who derived a mathematical inequality that can be violated by entangled particles but not by classical systems with local hidden variables.
Another important difference between classical correlations and entanglement is that classical correlations can be shared between many particles, whereas entanglement is a property that is specific to two or more particles. This is because entanglement arises from the non-separability of the joint state of the entangled particles, whereas classical correlations can be described by a joint probability distribution that is separable.
In conclusion, the difference between entanglement and classical correlations lies in the nature of the correlation itself. Classical correlations arise from statistical relationships between variables, whereas entanglement arises from the non-separability of the joint state of two or more quantum systems. This fundamental difference has important implications for our understanding of the nature of reality, and has important practical applications in areas such as quantum information processing and quantum communication.
In particular, entanglement has been proposed as a resource for quantum information processing tasks such as quantum teleportation, quantum cryptography, and quantum computing. For example, in quantum teleportation, the quantum state of one particle can be transmitted to another particle that is entangled with it, without the need for a physical transfer of the particle itself. Similarly, in quantum cryptography, entangled particles can be used to generate shared secret keys that are provably secure against eavesdropping.
In addition, entanglement has been studied as a potential means of improving the performance of sensors and measurement devices. For example, entangled particles have been used to improve the sensitivity of interferometers, which are used for measuring small displacements, by reducing the noise due to quantum fluctuations.
In summary, the difference between entanglement and classical correlations is a fundamental one that arises from the non-separability of the joint state of entangled quantum systems. While classical correlations can be explained by statistical relationships between variables, entanglement cannot be explained by local hidden variables and has no classical analog. This difference has important implications for our understanding of the nature of reality and has led to the development of new technologies such as quantum information processing and quantum sensors.