Whether a quantum object behaves like a wave or like a particle depends on the tool used to observe the system, and thus on the type of measurement performed. Now, a team of physicists at the University of Vienna, Austria, and the Austrian Academy of Sciences has taken this phenomenon to the extreme by performing measurements on two photons placed 144 km apart.

In all past experiments, the possibility that the choice of measurement has a causal influence on the actual observation (by transmission of information slower than light) still remained, at least in principle. In two new experiments, the physicists ruled out that possibility of a causal explanation.

In this quantum eraser experiment, no physical communications between choice and interference is possible, according to Einstein’s interpretation of the principle of locality. Nevertheless, the team observed that whether the first photon behaved like a wave or like a particle still depended on the measurement performed on the second photon.

In their search for an explanation for this complementary behavior of quantum systems, the scientists employed entanglement, a characteristic trait of quantum mechanics according to the Austrian physicist Erwin Schrödinger. “Specifically, we utilized hybrid path-polarization entangled photon pairs in our quantum eraser experiment,” says Dr Xiaosong Ma, post doc in the Zeitlinger Group at the University of Vienna.

The experiments, conducted in Vienna and on the Canary Islands, could bring upon some scientific paradigm shifts. “While the results of such experiments are fully consistent with quantum physics, a clear explanation in terms of causality is impossible, as, according to Einstein’s relativity theory, any transfer of information is limited to the speed of light,” Ma says. He and his collaborators suggest fellow scientists should completely abandon the view that a quantum system might, at a certain point in time, appear definitely as a wave or definitely as a particle.

The observations made during this research work could play a major role in the development of future quantum technologies. “The technology implemented in both experiments is crucial for quantum communication and photon-based quantum information processing, which hold the promises of unconditional security and exponential speed-up in computation,” Ma says. “We expect that many of the features implemented here will be key blocks for a new area of fascinating experiments.”

The discovery could have an impact on various quantum-enhanced technologies. Ma says, ”The applications are manifold and reach from quantum communication, quantum metrology to quantum information processing.”

The question then arises — are quantum events independent from space and time? “The fact that it is possible to decide a quantum event, in our case whether a wave or particle feature manifests itself even non-locally with respect to the measurement, teaches us that we should not have any naive realistic picture for interpreting quantum phenomena,” Ma encourages. “Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded.”

Going forward, the Austrian team strives for a long list of accomplishments. “We are planning to develop a brighter photon pair source, low-noise single-photon detectors, faster optical modulators with higher duty cycle and more precise clock synchronization,” reports Ma, who believes that research areas like quantum communication and quantum information processing will benefit tremendously from his team’s research.

Image: Long time exposure photography viewing from Tenerife to La Palma. A green laser beam indicates the free-space link between the two laboratories (picture: Thomas Herbst, IQOQI Vienna).

Written by Sandra Henderson, Research Editor, Novus Light Technologies Today