In their famous Physical Review article published in 1935, A. Einstein, B. Podolsky and N. Rosen have considered a decay of a particle into two products. In their thought-experiment, two products of decay were projected in exactly opposite directions, or more scientifically speaking their momenta were anti-correlated. It would not be a mystery within the framework of classical physics, however when applying the rules of the Quantum theory, the three researchers quickly arrived at a paradox. The Heisenberg uncertainty principle, dictating that position and momentum of a particle cannot be measured at the same time within arbitrary precision, lies at the center of this paradox.
Only today we know that this experiment is not, in fact, a paradox. The mistake of Einstein and co-workers was to use one-particle uncertainty principle to a system of two particles. If we treat these two particles as described by a single quantum state, we learn that the original uncertainty principle ceases to apply, especially if these particles are entangled.
From right: Michal Parniak uses the green laser to shining the glass cell with quantum memory, holding by Wojciech Wasilewski. Michal Dabrowski makes a simultaneous measurement of position and momentum of photons generated inside the memory.
In the Quantum Memories Laboratory at the University of Warsaw, the group of three physicists was first to create such an entangled state consisting of a macroscopic object – a group of about one trillion atoms, and a single photon – a particle of light. “Single photons, scattered during the interaction of a laser beam with atoms, are registered on a sensitive camera. A single registered photon carries information about the quantum state of the entire group of atoms. The atoms may be stored, and their state may be retrieved on demand.” – says Michal Dabrowski, PhD student and co-author of the article.
Visualization of a hybrid bipartite entanglement between a single photon (blue) and an atomic spin-wave excitation inside quantum memory glass cell, subsequently confirmed in the detection process of a second photon (red). Presented setup enables the demonstration of Einstein-Podolsky-Rosen paradox with true positions and momenta.
The experiment performed by the group from the University of Warsaw is unique in one other way as well. The quantum memory storing the entangled state, created thanks to “PRELUDIUM” grant from the National Science Centre and “Diamentowy Grant” from the Polish Ministry of Science and Higher Education, allows for storage of up to 12 photons at once. This enhanced capacity is promising in terms of applications in quantum information processing. “The multidimensional entanglement is stored in our device for several microseconds, which is roughly a thousand times longer than in any previous experiments, and at the same time long enough to perform subtle quantum operations on the atomic state during storage” – explains Dr. Wojciech Wasilewski, group leader of the Quantum Memories Laboratory team.
The entanglement in the real and momentum space, described in the Optica article, can be used jointly with other well-known degrees of freedom such as polarization, allowing generation of so-called hyper-entanglement. Such elaborate ideas constitute new and original test of the fundamentals of quantum mechanics – a theory that is unceasingly mysterious yet brings immense technological progress.
Contacts and sources:
Dr. Wojciech Wasilewski
Institute of Experimental Physics, Faculty of Physics, University of Warsaw
M. Dąbrowski, M. Parniak, W. Wasilewski
Optica 4, 272-275 (2017); DOI: 10.1364/OPTICA.4.000272