Loophole-free Bell experiment –
If Einstein knew this
“It seems hard to sneak a look at God’s cards. But that he plays dice and uses ‘telepathic’ methods (as the present quantum theory requires of him) is something that I cannot believe for a single moment.” This statement was written by Albert Einstein in a letter to Cornelius Lanczos in the year 1942.
Quantum mechanics, a branch of quantum physics, allows the entanglement of the states of two particles that are far away from each other. Analyzing the state of one particle allows direct conclusions on the state of the other. This strange behavior is one of the reasons why Einstein always rejected the theory of quantum mechanics. Einstein assumed that this type of interaction would be explicable by hitherto unknown or hidden variables.
In 1964, physicist John Stewart Bell proposed an experiment to prove this entanglement, the so-called Bell experiment. This experiment was meant to refute the theory of the hidden variables. Even though the validity of quantum mechanics has been confirmed for decades, all Bell experiments have left a few loopholes up to now. The most important loopholes are the so-called detection and locality loopholes.
There is a possibility that when an entanglement is detected, only a part of the particles is measured, because the detector does not work properly. The detector could then only measure particles which show the correlation required by quantum mechanics without showing particles that follow other principles. Hence quantum theory only provides a partial description of reality and could be replaced by a more comprising theory. This is called the detection loophole. Instead of the particles being entangled, it could also have been that a signal was sent from one particle to the other. Both particles would then no longer be a holistic system as required by quantum mechanics, but two separate systems which exchange information. According to the theory of relativity, this information exchange could maximally take place at the speed of light. Even though this explanation is rather unrealistic, it is not completely impossible. This is called the locality loophole.
So far, it has been possible to close one of the loopholes in experiments but never both at the same time. For the first time, Bas Hensen and his colleagues of the Technical University of Delft were now able to close both loopholes at the same time in a Bell experiment . For the experiment, two diamonds with nitrogen vacancy centers were cooled down to a few Kelvin. Due to the electron arrangement, this defect is a common single photon source. The two diamonds were in different laboratories and were coupled with a 1.3 km long fiber. The large distance allowed a short time period of a few millionths of a second, during which measurements were performed on the electrons. The short time ensured that no information was exchanged between the systems or the detectors. This closed the locality loophole. The measurement of all of the entangled electron pairs of the nitrogen vacancy center also closed the detection loophole.
The measured correlation of the states of the two electrons has fully confirmed the quantum mechanics prediction and refuted Einstein’s hidden variable theory. The experiment was performed with a Cryostation from Montana Instruments. The Cryostation offers a low-interference environment at 4 K and flexible optical access.
 Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers. Nature, Hensen et al., 2015