An international collaboration and a newly published paper may have just settled a century old physics debate.

Quantum mechanics is spooky. Entanglement – a component of quantum mechanics – tells us that two particles can be directly connected even across vast distances. If you measure the spin of one particle, you immediately know the spin of its counterpart. Physicists have labeled this behavior as “spooky” as it doesn’t follow everyday logic. Common sense tells us that objects across the universe cannot possibly be connected, yet in the quantum realm, they are. Quantum mechanics also says that properties of particles are only fixed when the particle is observed.

Some physicists, including Albert Einstein, opposed this notion as it went against the very nature of the real world. In the 1930s when quantum mechanics was an emerging field, Einstein was a proponent of “local realism,” arguing that only close objects could affect each other. Einstein and other physicists developed the ‘hidden variables theory’ to explain the spooky behavior. They argued that our knowledge of quantum mechanics was incomplete and there could be hidden variables that we didn’t yet understand.

In the 1960s a physicist named John Bell devised a mathematical expression – called an inequality – to test for these so-called hidden variables. He realized that if these hidden variables did indeed exist, there would be a limit to how connected the particles were. If they exceeded the set limit then the hidden variables did not exist. However, the experiment – known as Bell’s Inequality – did not definitively close the door on local realism. The tests involved entangled photons, which can get lost along the way, and experimenters might not detect all photons produced.

In the new experiment, led by Professor Ronald Hanson of Delft University of Technology in the Netherlands, we have two researchers – we will call them Alice and Bob – in two laboratories 1.3 kilometers apart. Each laboratory is set up with a diamond chip containing an electron whose spin was entangled with a photon. The photons were then sent to a third lab in between Alice and Bob, where a detector records the arrival time. If two photons arrived at the same time they would be entangled, resulting in the electrons being entangled as well.

The experiment took place over a span of nine days. In that time, researchers recorded 245 successful entanglements. While other tests over the last few decades have also supported Bell’s limit, this new experiment learns from their shortcomings to overcome experimental pitfalls. Previous test used inefficient detectors, only measuring a small number of the particles passing through them. Recent experiments used near-perfect detectors, but the entangled particles were close enough to potentially communicate. In the new experiment, the team used high-quality detectors and measurements collected before the electrons could possibly exchange signals with each other, making it the first to close both loopholes.

The results of this experiment have big implications for the world of quantum cryptography – meaning entangled photons could potentially create secure encryption keys. Closing the loopholes would ensure that computer systems could detect if anyone tried to intercept the keys, as it would break the entanglement and trigger an alarm.