 In 2019, a team of physicists at the University of Glasgow devised an actual experiment that used ghost images to prove quantum entanglement. First we'll cover what a ghost image is and how one is created. Then we'll cover how they proved quantum entanglement via a bell inequality. Here we have an argon laser sending its output into a beta barium borate crystal. These are unique crystals in that they can turn a photon into two entangled photons. The process is called spontaneous parametric down conversion. A beam splitter separates the photons. One collie idler proceeds through a liquid crystal spatial light modulator. There are many types of such modulators. This one has a thin gold image of the Greek letter lambda embedded in silicon. Given the idler photons' wavelength, they will pass through gold and be blocked by silicon. The photons that pass through enter a single photon detector. This detector then sends a signal to the camera. For each photon that travels to the spatial modulator, its entangled counterpart called the signal photon is guided to an intensified charge coupled device camera. This is the kind of camera technology we see in modern telescopes. We cover how they work and how far away is it video book chapter on planetary nebula exploding star. There is a delay loop in the photon's path to ensure that it enters the camera at exactly the same time that its entangled counterpart's signal reaches the camera, if indeed it did pass through the modulator. The match of one photon with one signal is called a coincidence count. When the camera senses a photon and a signal simultaneously, it lights the corresponding image pixel. If the camera gets a photon without a signal, it ignores it. As you can see over time, the lambda image is constructed. This is called a ghost image. The light that creates it never encountered the object itself. To ghost image a photon's polarization, the Glasgow team made some adjustments to this configuration to take advantage of the entangled polarization and the entangled orbital angular momentum created by the beta-barium boray crystal. First, the image in the spatial modulator is replaced with what they call a phase object that covers the outer edge of a photon's phase plane. This highlights the region of interest. If we ran with just this change, we'd see this ghost image. The next step is to introduce a second spatial modulator on the signal path of the photon heading to the camera. If the first angle is zero, we get this space image. If we change the angle with a new spatial modulator, say one with a 45-degree angle, the orientation of the image changes accordingly. This was done for 90 degrees and 135 degrees. Now the key to the experiment is that there is a relationship between the angular momentum of the photon and its orientation that shows itself in the light intensity profile measured as the number of coincidence counts. In other words, the intensity features of the ghost image reveal entanglement. The counts show a bell violation, proof that there are no hidden variables involved. Therefore, we see that the entanglement is real, but it is not spooky action at a distance as Einstein proposed. It is just the wave nature of reality as Bohr had proposed.