The first "loophole-free" measurement of the violation of Bell's inequality by a quantum system has been claimed by physicists in the Netherlands, Spain and the UK. Their experiment involves entangling spins in diamonds separated by 1.28 km and then measuring correlations between the spins. The large separation between the diamonds and the relative ease with which the spins can be measured ensures that the experiment is performed properly and its result confirms the existence of the seemingly bizarre concept of quantum-mechanical entanglement.

The idea of entanglement first arose back in 1935, when Albert Einstein, Boris Podolsky and Nathan Rosen pointed out that two quantum particles such as electrons can be in a state in which a measurement on one particle instantaneously affects the other – no matter how far apart they may be. This apparent paradox upset the trio because, in the world of classical physics, it would require information to travel faster than the speed of light. This relationship between particles was later dubbed entanglement and subsequent work showed that entanglement can be determined by looking at correlations between measurements made on the two particles, such as the direction in which the two electrons are spinning. Entangled particles have much stronger correlations than are allowed in classical physics – a property that can be exploited in quantum computers and other quantum technologies.

Upper limit

In 1964 the Northern Irish physicist John Bell famously calculated an upper limit on how strong these correlations could be if they were caused by classical physics alone – what has become known as Bell's inequality. Correlations stronger than this limit, Bell reasoned, could occur only if the particles were entangled. Experiments using photons, ions and other entangled particles have confirmed that Bell's inequality is indeed violated. However, these experiments are plagued by one or more loopholes that allow unforeseen effects of classical physics to cause the violation.

In this latest work, Ronald Hanson and colleagues at the Delft University of Technology, along with researchers at the Institute of Photonic Sciences in Barcelona and the diamond-maker Element Six in Oxford, have eliminated what they consider to be the two most significant loopholes that can arise in Bell-violation experiments. Crucially, they have done so simultaneously in one experiment, which had not been done before.

Channels unknown

One is the "locality" loophole, whereby information about the measurements is exchanged between detectors via unknown classical communication channels – thereby increasing the apparent correlation between the particles. Because this communication is classical, it cannot be transmitted faster than the speed of light and therefore this loophole can be closed by increasing the separation distance between the particle detectors and/or reducing the time it takes to make the measurement so that communication is impossible.

The second is the "detection" loophole, whereby an experimentalist is fooled into thinking a large correlation exists because an unknown aspect of the experiment causes it to favour the detection of particles with large correlations over those with small correlations.

Best of both particles

The locality loophole is easily eliminated by using photons as quantum particles, because photons are able to travel many kilometres without being scattered or absorbed. However, it is very difficult to detect each and every photon in such an experiment, which leaves it open to the detection loophole. Conversely, experiments involving electrons suffer from locality problems because they cannot be done over large distances. However, electron experiments can beat the detection loophole because electrons can be more reliably detected. What Hanson and colleagues have done is to use both photons and electrons in their experiment.

Their set-up consists of two diamonds separated by 1.28 km. Each diamond has a single nitrogen vacancy (NV) centre, which is essentially an electron spin. The measurement process begins with each NV centre emitting a photon that is entangled with its parent NV electron. Both photons travel to a third location that is hundreds of metres away from both diamonds. There, the photons are detected and when this measurement occurs, the NV electrons become entangled in a process called "entanglement swapping". The next step is to quickly measure the spin states of the two electrons, which is done using a very efficient fluorescence technique.

The team ran 245 trials of the Bell test over a total measurement time of 220 h and found a very strong violation of Bell's inequality. Furthermore, the team calculates that the large separation between the two diamonds and the rapid readout time of the spins closes the locality loophole, while the high efficiency of the spin readout technique closes the detection loophole.

No more freedom of choice

While the team points out that no Bell experiment can be free of every conceivable loophole, the researchers say that their experiment places the strongest restrictions to date on classical theories of quantum entanglement. They also say that their experiment could be modified to close more exotic loopholes such as "freedom of choice", whereby unbeknown to the experimentalist the design of the experiment is somehow limited in a way that boosts the measured correlations.

The experiment is described in a preprint on arXiv. Update: The research was published in the journal Nature on 21 October 2015.