Quantum Entanglement
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Quantum entanglement is a physical phenomenon that occurs when a pair or group of particles generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.
LATEST BREAKTHROUGH IN THE FIELD OF QUANTUM ENTANGLEMENT
- In three independent experiments in 2013 it was shown that classically communicated separable quantum states can be used to carry entangled states.The first loophole-free Bell test was held in TU Delft in 2015 confirming the violation of Bell inequality.
- In 2015, Markus Greiner's group at Harvard performed a direct measurement of Renyi entanglement in a system of ultra cold bosonic atoms.
- From 2016 various companies like IBM, Microsoft etc. have successfully created quantum computers and allowed developers and tech enthusiasts to openly experiment with concepts of quantum mechanics including quantum entanglement.
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WHAT IS QUANTUM ENTANGLEMENT?
Quantum entanglement is a physical phenomenon that occurs ridiculous when a pair or group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the pair or group cannot be described independently of the state of the others, including when the particles are separated by a large distance.
HISTORY
The counter intuitive predictions of quantum mechanics about strongly correlated systems were first discussed by Albert Einstein in 1935, in a joint paper with Boris Podolsky and Nathan Rosen. In this study,the three formulated the Einstein-Podolsky-Rosen paradox(EPRparadox),a thought experiment that attempted to show that"the quantum-mechanical description of physical reality given by wave functions is not complete."However, the three scientists did not coin the word entanglement,nor did they generalize the special properties of the state they considered. Following the EPRpaper, Erwin Schrödinger wrote a letter to Einstein in German in which he used the word Verschränkung to describe the correlations between two particles that interact and then separate, as in the EPR experiment."John Stewart Bell proved that one of their key assumptions, the principle of locality, as applied to the kind of hidden variables interpretation hoped for by EPR, was mathematically inconsistent with the predictions of quantum theory.
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SCHRÖDINGER'S CAT THEORY
One can even set up quite rediculous cases. A cat is penned up in a steel chamber, along with the following device(which must be secured against direct interference by the cat):in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays,but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if mean while no atom has decayed.The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat(pardon the expression)mixed or smeared out inequal parts.
HIDDEN VARIABLE THEORY
A possible resolution to the paradox is to assume that quantum theory is in complete, and the result of measurements depends on predetermined "hidden variables". The state of the particles being measured contains some hidden variables, whose values effectively determine, right from the moment of separation, what the out comes of the spin measurements are going to be.This would mean that each particle carries all the required information with it, and nothing needs to be transmitted from one particle to the other at the time of measurement. Einstein and others(see the previous section) originally believed this was the only way out of the paradox, and the accepted quantum mechanical description(with a random measurement outcome)must be incomplete.
VIOLATIONS OF BELL'S INEQUALITY
Local hidden variable theories fail, however, when measurements of the spin of entangled particles along different axes are considered. If a large number of pairs of such measurements are made(on a large number of pairs of entangled particles),then statistically, if the local realist or hidden variables view were correct, the results would always satisfy Bell's inequality.
NATURALLY ENTANGLED SYSTEMS
The electron shells of multi-electron atoms always consist of entangled electrons. The correct ionization energy can be calculated only by consideration of electron entanglement.
CONCLUSION
To conclude with, I would like to propose a more wide-ranging theoretical task to arrive at a set of principles like energy and momentum conservation, but which apply to information, and from which much of quantum mechanics could be derived. Two tests of such ideas would be whether the EPR-Bell correlations thus became transparent, and whether they rendered obvious the proper use of terms such as 'measurement' and 'knowledge'.