How is quantum entanglement proven

Twin research

According to quantum theory, particles that have once been in interaction can no longer be regarded as separate objects, even if they are spatially far apart. This claim has far-reaching consequences.

Polarization of light

In 1935 Einstein made his last big blow against quantum theory. He wanted to prove that things have an objective reality regardless of an observer. We intuitively agree with Einstein right away. Because a car, for example, naturally has a certain speed even if the speedometer has failed and we therefore cannot measure it. However, quantum theory claims that the state of microscopic objects before a measurement is not only not known, it is completely indeterminate. In order to expose this claim as nonsensical, Einstein, together with his colleagues Boris Podolsky and Nathan Rosen, devised a thought experiment that went down in history as an EPR argument after the first letters of its inventors.

In a modern version, the thought experiment works in such a way that laser light of a certain wavelength traverses a crystal with special optical properties. This creates pairs of light particles (photons) in the crystal. Immediately after a pair has formed, the two photons fly apart again in different directions. According to the rules of quantum theory, the polarization directions of these “twin photons” are indefinite. That means: the light “decides” for a certain polarization only at the moment of the measurement. However, based on the laws of conservation of physics, we know that the polarization directions of both photons are always perpendicular to one another.

Secret agreement

This leads to a strange consequence: If the polarization direction of only one photon is measured, the polarization direction of the twin particle must also be determined at the same moment (namely perpendicular to the measured one). Einstein, Podolsky and Rosen saw in this a contradiction to the theory of relativity. Because theoretically one could wait so long until the two photons were at opposite ends of the universe. If you then measure the one photon, it will choose a polarization direction quite randomly. This would also automatically determine the polarization direction of the twin (just perpendicular to it). The first photon should therefore tell the twin with faster than light speed (namely instantaneously!) Which direction it has chosen. Since exceeding the speed of light is forbidden according to the theory of relativity, Einstein, Podolsky and Rosen concluded that the state of each photon had to be determined before the measurement.

In Einstein's time, the contradictions of the EPR argument could not be resolved. It was not until 1982 in Paris that the French physicist Alain Aspect succeeded in credibly confirming the “spooky action at a distance” that Einstein had doubted in a laboratory test. In particular, he was able to prove that the state of the photons is actually not determined by "hidden variables" before the measurement. Researchers led by Nicolas Gisin from the University of Geneva have recently even succeeded (with a somewhat complicated experiment) in demonstrating this “secret collusion” between photons over a distance of ten kilometers.

Generation of pairs from twin photons

The pair of twins was split up so that one photon traveled the route between Geneva and Bellevue, while the other photon made its way to Bernex. Shortly before the end of the racetrack, each photon went through a measuring device in which it could choose between several outputs. For example, if one chose the right exit, the twin photon did the same. In 2008, Gisin's team even expanded the experiment. This time, the researchers sent the entangled photons from Geneva to Satigny and Jussy, which are 18 kilometers apart. The scientists were not only able to confirm the mysterious connection between the photons once more. By taking into account the west-east axis, on which the two locations are approximately, and the rotation of the earth, they also showed that the exchange between the photons, if it should not be infinitely fast, must take place at least 10,000 times the speed of light.

Entanglement and teleportation

Such a finally quick exchange would also have another consequence. At the beginning of the 20th century it was experimentally ruled out that a universal reference system, which distinguishes itself from all other physical reference systems, exists for classical information. But if the “quantum information” between the photons should not be exchanged instantaneously, there would have to be a “quantum ether” as a consequence.

But how can this extraordinary behavior be explained if the transmission of information at faster than light speed is excluded according to Einstein's theory of relativity? The answer is: no information is transmitted at all! Rather, the twin photons behave like a pair of dice that show the same number of pips with every throw. Since the result of such an experiment is completely random, this phenomenon cannot be used to convey any meaningful data. For example, you cannot “Morse code” with such an apparatus.

Entangled photons made visible

The scientists explain the astonishing outcome of the experiments with the fact that two particles, once they have interacted with one another, apparently become parts of an indivisible system. Erwin Schrödinger coined the term entanglement for this. This initially only hypothetical concept has now been confirmed by numerous research projects. At the Ecole Nationale Supérieure in Paris, a research group led by Serge Haroche was able to prove that there are not only entangled photons, but also entangled atoms. Anton Zeilinger's group in Vienna even managed to entangle four light particles. One of the most exciting experiments based on the entanglement of particles is the “teleportation” of quantum states.

When the radio message “Beam me up, Scotty!” Reaches the transporter room of the Enterprise, Star Trek fans know that in the next instant, Captain Kirk and his companions will materialize in the spaceship. Although this idea still belongs in the field of science fiction, Charles Bennett from the IBM research laboratory in Yorktown Heights, USA, made a serious suggestion as early as 1993 as to how teleportation could be physically implemented. The entanglement between twin photons played a central role. Bennett's idea was successfully implemented in 1997 by Anton Zeilinger's group: They succeeded in teleporting a photon for the first time. In the meantime, teleportation has also been successful for other objects. Jeff Kimble at Caltech in Pasadena, USA, for example, teleported a light field with the help of two interlaced light beams. His colleague Raymond Laflamme in Los Alamos, on the other hand, teleported the state of one atom to another. Although both atoms were only a short distance apart - they were inside the same molecule - this type of teleportation could also be useful. The researchers believe that this is how data processing could take place in future quantum computers.

Quantum cryptography

One of the first practical applications of such quantum wizardry emerged in cryptography - the art of encrypting secret messages. The principle is based on the fact that the code that is needed to encrypt and later decipher the secret data is generated using pairs of photons. The transmitter, usually called Alice, has an apparatus for this that produces pairs of twin photons. One photon is measured by Alice, the second is transmitted to the receiver, usually called Bob, and measured there. Since a twin photon pair resembles a pair of dice that always show the same number, Alice and Bob can communicate using a key for later data transmission.

If Alice transmits the twin photon to Bob, an eavesdropper on the line can easily be exposed if a specific, somewhat complicated procedure is used. The basic principle, however, is simple: eavesdropping corresponds to a measurement that can change the state of the twin photon. The specialty of the procedure is not that it is bug-proof, but that Alice and Bob notice the eavesdropper (and can then start all over again). Both of them determine the presence of the uninvited listener during a phone call by comparing randomly selected results from their measurement protocols. Already when comparing only 10 to 15 randomly selected measured values, it is very likely to detect a photon that has been changed by an eavesdropper. If there was no error, Alice and Bob simply delete the test readings from the log and then use the remaining code to send the actual messages.

That this principle works has been experimentally confirmed several times. Nicolas Gisin's group at the University of Geneva demonstrated the practicality of the process three years ago by sending encrypted messages through a Swisscom fiber optic cable under Lake Geneva. However, the tap-proof code cannot be generated in two locations that are far apart. Because to do this one would have to amplify the “stream” of photons - and this amplification corresponds (just like listening) to a measurement! A research group led by Richard Hughes in Los Alamos holds the previous route record over forty kilometers.

Nevertheless, quantum cryptography could work for inner-city areas or within limited security areas, for example in ministries: "The industry is showing increasing interest in this process," says Harald Weinfurter from the University of Munich. A module is currently being developed in his working group that is suitable for distances between two and five kilometers. Once such a device is in operation, spies have to be careful. It is very likely that you could be caught during the bugging.