3.4.4.7  Quantum Entanglement

As we have explained in section 4.4.6, the theories and experiments, that followed the EPR, have all confirmed the existence of counter-intuitive non-local interaction between the strongly correlated particles. The EPR original thought experiment itself, in 1935, was meant to show that Quantum Mechanics was incomplete and required further hidden variables. The three authors wrote in their paper:

“We are thus forced to conclude that the quantum-mechanical description of physical reality given by wave-functions is not complete.” Kalckar (2013)

In response to this paper, Schroedinger wrote a letter to Einstein in German, in which he used the word “Verschrankung”, meaning: entanglement, to describe the correlations between two particles that interact and then separate, as in the EPR experiment. He then published a seminal paper defining and discussing the notion of entanglement, where he recognized the importance of the concept, and stated:

“I would not call [entanglement] one but rather the characteristic trait of Quantum Mechanics, the one that enforces its entire departure from classical lines of thought.” Bitbol (2012)

Schroedinger also expressed his dissatisfaction with the concept of entanglement, because it seemed to violate the principle of relativity, in the transmission of information faster than the speed of light. Einstein later famously ridiculed entanglement as “spooky action at a distance”.

Entanglement happens when a pair, or any group, of particles are described only by one quantum state for the whole system, and not by individual quantum states. This happens when the system is closed and its particles may only act on each other, without any interaction from outside. Although according to the uncertainty principle, discussed further in section 4.4.2, impose restrictions on how accurate we measure the momentum, but it is to know if the total momentum of the system changes when its particles act on each other. When the total momentum is always the same before and after the particles interact, then they are entangled, in which case we need to describe the whole system as a single quantum state which cannot be factored as a product of states of its local constituents.

In other words, the total state of a system is always the superposition, or the sum, of the states of its particles, but these particles are said to be entangled only if this sum has no more than one term. Therefore, entangled particles always act as an inseparable whole. This means that one particle cannot be fully described without considering the others. Entanglement is broken when the entangled particles decohere through interaction with the environment; for example, when a measurement is made.

For example, when a subatomic particle decays into two or more particles, they are always entangled as far as they do interact with the environment. A spin-zero particle could decay into a pair of spin-half particles, so the total spin before and after this decay must is still zero. In such a case, we will not be able to know if the spin of each particle is up or down, until we measure it, and when we do, we will automatically know the spin of the other particle no matter where it is in the Universe. In the June 16, 2017, issue of Science, Yin et al. reported the survival of photon pairs in entanglement for distance of 1203 km.

The electrons in any shell in the atom are always entangled. It has also been shown by femtosecond transition spectroscopy, that entangled photons exist in the photosynthesis of plants, which is critical for the efficient conversion of the photon energy into chemical energy.