The following diagram represents the experiment setup. The two photons that result from the annihilation of the positron have all what is need in order to form a single quantum system, from which it follows that the two gamma photons are entangled one to another.

From theoretical considerations also it is known that they have spin phased out by π/2. We have already described in the post on the annihilation of the positron that the two gamma photons of 511 keV, for the conservation of momentum, are emitted on the same line but in opposite directions. The experiment described in this post is the repetition of the famous experiment of Wu – Shaknov in which it will demonstrate the angular correlation of gamma photons emitted from the annihilation of the positron and subsequently scattered by a compton scatterer. (This is called the spin anti-correlated case and if the prior probabilities for measuring each spin are equal, the pair is said to be in the singlet state.) Experiment Description Since the total spin before and after this decay must be zero (conservation of angular momentum), whenever the first particle is measured to be spin up on some axis, the other (when measured on the same axis) is always found to be spin down. For instance, a spin-zero particle could decay into a pair of spin-½ particles. The decay events obey the various conservation laws, and as a result, the measurement outcomes of one daughter particle must be highly correlated with the measurement outcomes of the other daughter particle (so that the total momenta, angular momenta, energy, and so forth remains roughly the same before and after this process). Entanglement is broken when the entangled particles decohere through interaction with the environment for example, when a measurement is made.Īs an example of entanglement: a subatomic particle decays into an entangled pair of other particles. Quantum systems can become entangled through various types of interactions. Note that the state of a composite system is always expressible as a sum, or superposition, of products of states of local constituents it is entangled if this sum necessarily has more than one term. If entangled, one constituent cannot be fully described without considering the other(s). It thus appears that one particle of an entangled pair “knows” what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances.Īn entangled system is defined to be one whose quantum state cannot be factored as a product of states of its local constituents (e.g., individual particles). However, this behavior gives rise to paradoxical effects: any measurement of a property of a particle can be seen as acting on that particle (e.g., by collapsing a number of superposed states) and in the case of entangled particles, such action must be on the entangled system as a whole.

For example, if a pair of particles is generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, then the spin of the other particle, measured on the same axis, will be found to be counterclockwise because of the nature of quantum measurement. Measurements of physical properties such as position, momentum, spin, polarization, etc., performed on entangled particles are found to be appropriately correlated.

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently - instead, a quantum state may be given for the system as a whole.

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