#### Extending Einstein

*Researchers at the University of Waterloo and the University of Calgary have carried out an experiment, using the quantum properties of three particles light, that could provide new insights into the philosophical arguments by Einstein about the foundations of quantum mechanics.*

In 1935 Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR) published a thought experiment designed to show that quantum mechanics, by itself, is not sufficient to describe reality. Using two entangled particles – particles that share correlations stronger than those allowed by classical physics – EPR tried to demonstrate that there must be some hidden parameters that quantum mechanics does not account for. The ensuing debate led to the pioneering work of John Bell who, in 1964, showed that by following the arguments of EPR to their logical conclusion one arrives at a contradiction with experiments; hidden parameters, like the ones EPR argued for, are incompatible with our observations of nature and the mystery at the heart of quantum mechanics remains intact. This has profoundly shaped our understanding of quantum theory, and today the entanglement between two particles that EPR first proposed is a valuable resource in emerging quantum technologies like quantum computing, quantum cryptography, and quantum precision measurements.

77 years after EPR's landmark work a new paper in Nature Physics, authored by physicists at the Institute for Quantum Computing in Waterloo and at the University of Calgary, has finally experimentally extended the original ideas of Einstein and his colleagues from two to three entangled particles. This new form of three-particle entanglement, based on the position and momentum properties of photons, may prove to be a valuable part of future communications networks that operate on the rules of quantum mechanics, and could lead to new fundamental tests of quantum theory that deepen our understanding of the world around us. According to group leader Thomas Jennewein, "It is exciting, after all this time, to be able to create, control, and entangle quantum particles in this new way. Using these states of light it may be possible to interact with and entangle distant quantum computer memories based on exotic atomic gases."

In the experiment the researchers took a highly energetic blue photon and passed it through a special crystal that caused it to split into a pair of red coloured daughter photons. They then repeated this process with one of the daughter photons to create three entangled photons. The energy of these three photons, through the conservation of energy, must be equal to the energy of the original blue photon. Because the splitting process is instantaneous, the three photons must arrive at the detectors at the same time. It is therefore possible to learn the precise energy (corresponding to their total momentum) of the three photons as well as their arrival times (corresponding to their position). At first this seems to be an apparent contraction with the Heisenberg uncertainty principle which states that it is impossible to simultaneously learn arbitrarily precise information about a particle's position and momentum. However, with entangled particles, it is possible to gain precise information about the sum and differences of their position and momentum in a manner not possible with classical particles. This is in part because each of the particles in an entangled state gives up its own individual identity–the properties of the particles are instead shared collectively. It still remains impossible to gain position and momentum information of any individual particle. Says lead author Krister Shalm, "It is as if you could only discover how many points two teams combined to score in a basketball game, but had no way of knowing how many points each individual team had scored." Co-author Deny Hamel adds, "Because the entangled photons cooperate with one another they can do things that classical particles are unable to."

The next step for the researchers is to try to combine the position and momentum entanglement between their three photons with more traditional types of entanglement based on angular momentum. This will allow the creation of hybrid quantum systems that combine multiple unique properties of light at the same time. According to Christoph Simon from the University of Calgary, "This work opens up a rich area of exploration that combines philosophy, quantum mechanics, and quantum technologies. The powerful insights by Einstein and his co-workers in 1935 are still informing the way we understand the world around us."