When we observe an object, we make a number of intuitive assumptions, among them that the unique properties of the object have been determined prior to the observation and that these properties are independent of the state of other, distant objects.
In everyday life, these assumptions are fully justified, but things are different at the quantum level. In the past 30 years, a number of experiments have shown that the behaviour of quantum particles - such as atoms, electrons or photons - can be in conflict with our basic intuition. However, these experiments have never delivered definite answers. Each previous experiment has left open the possibility, at least in principle, that the observed particles 'exploited' a weakness of the experimental set-up.
In a Bell test, pairs of particles, e.g. photons, are produced. From every pair, one photon is sent to a party usually called Alice, and the other photon is sent to Bob. They each make a choice which physical property they want to measure, e.g. which direction of their photon's polarization. For pairs that are quantum entangled, the correlations of Alice's and Bob's measurement outcomes can violate Bell's inequality. Quantum entanglement - a term coined by the Austrian physicist Erwin Schrödinger - means that neither photon taken by itself has a definite polarisation but that, if one party measures the polarisation of its photon and obtains a random result, the other photon will always show a perfectly correlated polarisation. Albert Einstein called this strange effect "spooky action at a distance".
In addition to its pre-eminent importance in foundational physics, quantum entanglement and Bell's inequality also play a quintessential role in the modern field of quantum information. There, individual quantum particles are the carriers of information and the entanglement between them promises absolutely secure communication as well as enhanced computation power compared to any conceivable classical technology.
In the last decades, Bell's inequality has been violated in numerous experiments and for several different physical systems such as photons or atoms. However, in experimental tests "loopholes" arise which allow the observed correlations - although they violate Bell's inequality - to still be explained by local realistic theories. The advocates of local realism can defend their world view falling back on three such experimental loopholes. In the "locality loophole" the measurement result of one party is assumed to be influenced by a fast and hidden physical signal from the other party to produce the observed correlations.
Similarly, in the "freedom-of-choice" loophole the measurement choices of Alice and Bob are considered to be influenced by the local realistic properties of the particle pairs. With a lot of effort, these two loopholes have already been closed in photonic experiments by separating Alice and Bob over large distances in space and enforcing precise timing of the photon pair creation, Alice's and Bob's choice events and their measurements. Then superluminal signals would be needed to explain the measured correlations. But influences which are faster than light are not allowed in the local realistic world view.
The third escape hatch for the local realist is called "fair-sampling loophole". It works in the following way: If only a small fraction of all the produced photons is measured, a clever advocate of local realism can conceive a model in which the ensemble of all produced photons as a whole follows the rules of local realism, although the "unfair" sample of the actually measured ones was able to violate Bell's inequality. Think of randomly flipping many fair coins, where those coins with heads up tend to hide and thus have a smaller probability of being observed than the ones with tails up. By having only access to the actually measured coins, it wrongly appears as if the coins had a special, i.e. unfair, distribution with more tails than heads up.
The way to close the fair-sampling loophole is to achieve a high detection efficiency of the produced particle pairs by avoiding losses and using very good measurement devices. As yet, this has not been accomplished for photons but only for other physical systems, e.g. atoms, for which, however, the other two loopholes are very hard to close and indeed have not been closed yet.
Quantum physics is an exquisitely precise tool for understanding the world around us at a very fundamental level. At the same time, it is a basis for modern technology: semiconductors (and therefore computers), lasers, MRI scanners, and numerous other devices are based on quantum-physical effects. However, even after more than a century of intensive research, fundamental aspects of quantum theory are not yet fully understood. On a regular basis, laboratories worldwide report results that seem at odds with our everyday intuition but that can be explained within the framework of quantum theory.
The physicists in Vienna report not a new effect, but a deep investigation into one of the most fundamental phenomena of quantum physics, known as 'entanglement'. The effect of quantum entanglement is amazing: when measuring a quantum object that has an entangled partner, the state of the one particle depends on measurements performed on the partner. Quantum theory describes entanglement as independent of any physical separation between the particles. That is, entanglement should also be observed when the two particles are sufficiently far apart from each other that, even in principle, no information can be exchanged between them (the speed of communication is fundamentally limited by the speed of light). Testing such predictions regarding the correlations between entangled quantum particles is, however, a major experimental challenge.
The young academics in Anton Zeilinger's group including Marissa Giustina, Alexandra Mech, Rupert Ursin, Sven Ramelow and Bernhard Wittmann, in an international collaboration with the National Institute of Standards and Technology/NIST (USA), the Physikalisch-Technische Bundesanstalt (Germany), and the Max-Planck-Institute of Quantum Optics (Germany), have now achieved an important step towards delivering definitive experimental evidence that quantum particles can indeed do things that classical physics does not allow them to do.
For their experiment, the team built one of the best sources for entangled photon pairs worldwide and employed highly efficient photon detectors designed by experts at NIST. These technological advances together with a suitable measurement protocol enabled the researchers to detect entangled photons with unprecedented efficiency. In a nutshell: "Our photons can no longer duck out of being measured", stated Anton Zeilinger.
This kind of tight monitoring is important as it closes an important loophole. In previous experiments on photons, there has always been the possibility that although the measured photons do violate the laws of classical physics, such non-classical behaviour would not have been observed if all photons involved in the experiment could have been measured. In the new experiment, this loophole is now closed. "Perhaps the greatest weakness of photons as a platform for quantum experiments is their vulnerability to loss - but we have just demonstrated that this weakness need not be prohibitive", explained Marissa Giustina, lead author of the paper.
Although the new experiment makes photons the first quantum particles for which, in several separate experiments, every possible loophole has been closed, the grand finale is yet to come, namely, a single experiment in which the photons are deprived of all possibilities of displaying their counterintuitive behaviour through means of classical physics. Such an experiment would also be of fundamental significance for an important practical application: 'quantum cryptography', which relies on quantum mechanical principles and is considered to be absolutely secure against eavesdropping. Eavesdropping is still theoretically possible, however, as long as there are loopholes. Only when all of these are closed is a completely secure exchange of messages possible.
An experiment without any loopholes, stated Anton Zeilinger, "is a big challenge, which attracts groups worldwide." These experiments are not limited to photons, but also involve atoms, electrons, and other systems that display quantum mechanical behaviour.
The experiment of the Austrian physicists highlights the photons' potential. Thanks to these latest advances, the photon is running out of places to hide, and quantum physicists are closer than ever to conclusive experimental proof that quantum physics defies our intuition and everyday experience to the degree suggested by research of the past decades.
This work was completed in a collaboration including the following institutions: Institute for Quantum Optics and Quantum Information - Vienna / IQOQI Vienna (Austrian Academy of Sciences), Quantum Optics, Quantum Nanophysics and Quantum Information, Department of Physics (University of Vienna), Max-Planck-Institute of Quantum Optics, National Institute of Standards and Technology / NIST, Physikalisch-Technische Bundesanstalt, Berlin.
This work was supported by: ERC (Advanced Grant), Austrian Science Fund (FWF), grant Q-ESSENCE, Marie Curie Research Training Network EMALI, and John Templeton Foundation. This work was also supported by NIST Quantum Information Science Initiative (QISI).
The paper titled "Bell violation with entangled photons, free of the fair-sampling assumption" is written by Marissa Giustina, Alexandra Mech, Sven Ramelow, Bernhard Wittmann, Johannes Kofler, Jörn Beyer, Adriana Lita, Brice Calkins, Thomas Gerrits, Sae Woo Nam, Rupert Ursin, Anton Zeilinger. It appears inNature(Advance Online Publication/AOP), April 14, 2013 - DOI: 10.1038/nature12012.