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How to Make a Quantum Entanglement

Einstein was so disturbed by the implications of quantum mechanics that he said, "no reasonable definition of reality could be expected to permit this." The "this" he was talking about was quantum entanglement. Quantum entanglement is the creation of identical particles with linked characteristics. For example, a pair of electrons can be created with linked magnetic fields, such that if one has its field pointing up (called "spin up"), the other must have its spin down. Imagine one of the electrons is sent to the moon. You then measure the electron on earth and find it is spin down. You automatically know the electron on the moon is spin up. This is one of those rare cases when it was not the theory that was wrong, but Einstein himself. Quantum entanglement can be created in the lab.

Things You'll Need

Beta-barium borate crystal
405-nm laser
Beamsplitters
Single-photon detectors
Linear polarizers

Instructions

- 1
  Send the beam from the laser through a linear polarizer whose axis is oriented at 45 degrees from the vertical.
- 2
  Transmit the resultant beam through the BBO crystal. A small percentage of the incident light will be down-converted --- changed into two photons, each with half the energy of the incident photon. The down-converted photons will be entangled, with the same polarization, perpendicular to the polarization of the incident beam.
- 3
  Place a vertically oriented polarizer in front of each of two single-photon detectors. Place a polarizer/detector combination in each of the two output beams from the BBO crystal.
- 4
  Measure the output from each of the two detectors. Roughly half the photons will be blocked by the polarizer, so the output from any single detector will be a random string of "hits" and "misses."
- 5
  Correlate the outputs of the two detectors. Even though either detector, looked at by itself, has a random string of hits and misses, the two photons are correlated, such that they both have the same random string.
- 6
  
  When you can't tell which particle is which, yet they are inextricably linked, you have achieved entanglement.
  
  Move one detector farther along the propagation path of one of the down-converted beams. The correlation will be preserved. This means that the "random" output of the second detector can be predicted using the measurement made at the first detector --- a sign of entanglement.

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