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Entangled Polarized Photons Crystals can produce pairs of photons, heading in different directions. These pairs always show the same polarization. ? These are said to be entangled photons. If one is measured to be vertically polarized, then its partner kilometers away will also be vertical. ? Entanglement 1) Does a polarizing filter act by a) selecting light with certain characteristics, like a sieve selects grains larger than the hole size or by b) changing the light and rotating its polarization, like crayons and a grid Measurement-Reality Niels Bohr and Einstein argued for 30 years about how to interpret quantum measurements like these. Niels Bohr codified what became the standard view of quantum mechanics. The filter is like a grid for crayons - the photon has no polarization until it is measured. It is in a superposition of states. Einstein felt that the filters were like a sieve. The photons must contain characteristics that determine what they will do. The information from the measurement of one can’t possibly fly instantaneously to its partner. He referred to this as ‘spukhafte Fernwirkungen’ which is usually translated as ‘spooky action-at-adistance’. Most physicists felt that there was no way to test these views. The topic was better left to philosophers. They kept busy using quantum mechanics to advance physics and technology. nuclear power microelectronics medical imaging Then in 1964 John Bell devised a test. He looked at what happens if the filters are in different orientations. 2)Four entangled pairs of photons head toward two vertical polarizers. If four make it through on the left, how many make it through on the right? ? If four make it through on the left, four will make it through on the right. In this situation, Both Einstein and Bohr predict 100% agreement between the photon pairs. Bell recognized that to distinguish between the two models, we need to use filters at different angles. Turn the filter on the right to 30o. 3) Which of the following would Einstein expect to see on the right if 4 photons made it through the vertical filter on the left? a) b) c) d) 4) What percentage agreement would he expect on average between the photon pairs? a) 0% b) 25% c) 75% d) 100% 5) If the right filter is vertical and the left is placed at –30o, what agreement would he expect? a) 0% b) 25% c) 75% d) 100% Next we combine the two experiments. The left polarizer is at –30 and the right at +30. 6)Which of the following is possible? a) left b) right c) both d) neither 7) How much agreement is expected? a) 25% b)50% c)75% d)100% 8) How much agreement does quantum mechanics predict? Hint: The two filters are at 60 degrees to each other. a) 0% b) 25% c) 50% d) 75% Hidden Variables: If the photons have a polarization before measuring - the agreement will be between 100% and 50%. Quantum: The photons have no polarization before measuring. The agreement should be 25%. In 1983 Alain Aspect did the experiment. He used crystals, not filters, so that each photons’ polarization was clearly detected and accounted for. The results were conclusively in support of quantum mechanics, not Einstein. The entanglement of photons has been demonstrated with the photons separated by over 20 km. Somehow, measuring one photon, instantly affects its partner 20 km away. Entanglement is an important part of explaining superconductors. Entanglement is an important part of superfluidity. Entanglement is an important part of quantum computing and cryptography. Erwin Schrodinger said that entanglement was “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”