1. How do scientists explain quantum entanglement?.

In the video below, Caltech faculty members take a stab at explaining entanglement. Featured: Rana Adhikari, professor of physics; Xie Chen, professor of theoretical physics; Manuel Endres, professor of physics and Rosenberg Scholar; and John Preskill, Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter.

2. Unbreakable Correlation

When researchers study entanglement, they often use a special kind of crystal to generate two entangled particles from one.

How do you know that those particles are correlated?
If you don't know, than you have to perform the mentioned experiment 1000 time and actualy observe that the two particles are correlated.

The entangled particles are then sent off to different locations. For this example, let's say the researchers want to measure the direction the particles are spinning, which can be either up or down along a given axis. Before the particles are measured, each will be in a state of superposition, or both "spin up" and "spin down" at the same time.

How do you know that those particles are in superposition? What does superposition means?
The simplest solution to this problem is: not to use the concept superposition, but to assume that the state of each particle is already decided at the moment when both particles are created.

If the researcher measures the direction of one particle's spin and then repeats the measurement on its distant, entangled partner, that researcher will always find that the pair are correlated: if one particle's spin is up, the other's will be down (the spins may instead both be up or both be down, depending on how the experiment is designed, but there will always be a correlation). Returning to our dancer metaphor, this would be like observing one dancer and finding them in a pirouette, and then automatically knowing the other dancer must also be performing a pirouette. The beauty of entanglement is that just knowing the state of one particle automatically tells you something about its companion, even when they are far apart.

3. Are particles really connected across space?

But are the particles really somehow tethered to each other across space, or is something else going on? Some scientists, including Albert Einstein in the 1930s, pointed out that the entangled particles might have always been spin up or spin down, but that this information was hidden from us until the measurements were made. Such "local hidden variable theories" argued against the mind-boggling aspect of entanglement, instead proposing that something more mundane, yet unseen, is going on. Thanks to theoretical work by John Stewart Bell in the 1960s, and experimental work done by Caltech alumnus John Clauser (BS '64) and others beginning in the 1970s, scientists have ruled out these local hidden-variable theories. A key to the researchers' success involved observing entangled particles from different angles. In the experiment mentioned above, this means that a researcher would measure their first particle as spin up, but then use a different viewing angle (or a different spin axis direction) to measure the second particle. Rather than the two particles matching up as before, the second particle would have gone back into a state of superposition and, once observed, could be either spin up or down. The choice of the viewing angle changed the outcome of the experiment, which means that there cannot be any hidden information buried inside a particle that determines its spin before it is observed. The dance of entanglement materializes not from any one particle but from the connections between them.

4. Relativity Remains Intact

A common misconception about entanglement is that the particles are communicating with each other faster than the speed of light, which would go against Einstein's special theory of relativity. Experiments have shown that this is not true, nor can quantum physics be used to send faster-than-light communications. Though scientists still debate how the seemingly bizarre phenomenon of entanglement arises, they know it is a real principle that passes test after test. In fact, while Einstein famously described entanglement as "spooky action at a distance," today's quantum scientists say there is nothing spooky about it. "It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case," says Thomas Vidick, a professor of computing and mathematical sciences at Caltech. "There can be correlation without communication," and the particles "can be thought of as one object." Let's say you have two entangled balls, each in its own box. Each ball is in a state of superposition, or both yellow and red at the same time...
...until you observe the balls.
If the first one is yellow, the other will be yellow. If the first one is red, the other will be red.
The objects remain connected even over vast distances. Scientists think of entangled objects as really being a single object.
But what if one observer decides to look at their ball from a different angle or side of the box? The balls would revert back to a state of superposition and have a 50% chance of being yellow and 50% chance of being red.
The viewer might find a yellow ball now, even though the pair of balls had previously both been red!
Now, if the second observer also looks at their ball from the side view, it will match what the first observer saw.
The ball sare still entangled, but what the viewer sees depends on how they look at the ball. This is because the entangled information about color does not lie within any one ball but exists in the connection between the balls.
Credit: Lance Hayashida for Caltech Science Exchange

4. Networks of Entanglement

Entanglement can also occur among hundreds, millions, and even more particles. The phenomenon is thought to take place throughout nature, among the atoms and molecules in living species and within metals and other materials. When hundreds of particles become entangled, they still act as one unified object. Like a flock of birds, the particles become a whole entity unto itself without being in direct contact with one another.

You can't compare entanglement with a flock of birds.

The general defintion of Correlation is that there exist a (mathematical) relation between different parameters measured of certain objects or processes. For example: weight versus volume of an object.
However when you consider the trajectories of the planets around the Sun they are also correlated. For example: They more or less all move in the same plane and the shape of the trajectory of each resembles a circle.
However there are even more complex situations: When you observe a binary star system of both mass m, and one of these stars is hit by an object of 0.01m then the trajectory of that star will change but also the trajectory of the other star. The cause is the force of gravity or as a result the gravitational field, which changes as a result the collision, and which in inturn changes the trajectory of the other planet. In some sense the two stars are linked. The same is happening between all stars in the universe. To call that entangled?

A different situation arises in cases when in an experiment, as part of a reaction the process emits two photons in opposite directions. The parameter to consider is called polarization. To measure this paramater or polarization angle you need a beam splitter. That means you need a source which generates a sequence of photons, a beam splitter X and two detectors D1 and D2. The following picture shows an image.

D1 D3 | | | | X---------<-------Source ---->--------X | | | | D2 D4 Figure 1

What the left side of Figure 1 shows is, that when the source emits a photon towards the left, this photon will either detected by Detector D1 or D2. The cause is the beam splitter in combination of the angle of polarization of the photon, which can be UP or DOWN as a mention of convention. Or Left or Right. When this experiment is repeated you get a random sequence of results like: D1, D2, D1, D1, D2, D2, D1, D2 When the source also shows the results in the opposite direction, it shows: D4, D3, D4, D4, D3, D3, D4, D3 That means the results shows clearly correlated. The reality can be slightly different.

The question is what is the cause of this correlation. As far as I know there are two answers.

• The first answer comes from the Quantum theory. Maybe this is also part the Copenhagen interpretation.
  See: https://en.wikipedia.org/wiki/Copenhagen_interpretation#Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox
  What this means is, that when state of the first particle is measured the state of the second particle could be predicted. In the above document this is called: "Einstein–Podolsky–Rosen (EPR) criterion of reality"
• The second answer postulates (?) that the cause has its origin as part of the original reaction when the two (in this case) stable, photons are created. That means the direction of polarization is immediate opposite from each other. To establish, that the same reaction has to be repeated 1000 times, and both photons have to be measeared, to calculate the degree of polarization.
  The advantage of this solution is that this requires no faster than light communication and no action at a distance.
  The same method is also used when particle collisions are studied, with the created particles, which are stable. This is currently tested in the HSL. All these stable particles are more or less created at the same instant, very close to the original reaction.

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