Using the Uncertainty Principle to Detect Entanglement of One Photon Shared Among Four Locations
Members of the Caltech team, from left to right, undergraduate Garrett Drayna, postdoctoral scholar Hui Deng, graduate student Kyung Soo Choi, and postdoctoral scholar Scott Papp.
A team of physicists at the California Institute of Technology has demonstrated a new method to detect entanglement in the form of one photon shared among four optical paths. Their work is reported in the May 8 issue of the journal Science . In their experiments, led by H. Jeff Kimble, entanglement is detected using quantum uncertainty relations for the regime of discrete variables, in which photons are taken one by one. Their approach builds on the famous Heisenberg uncertainty principle that places a limit on the precision with which the momentum and position of a particle can be known simultaneously.
Link to Professor Jeff Kimble's Quantum Optics group at Caltech >>
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Entanglement, which lies at the heart of quantum physics, is a state in which the parts of a composite system are more strongly correlated than is possible for any classical counterparts, regardless of the distances separating them. Entanglement in a system with more than two parts, or multipartite entanglement, is a critical tool for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. In the future, a ‘quantum internet’ will rely on entanglement for the teleportation of quantum states from place to place .
For some time physicists have studied bipartite entanglement, and techniques for classifying and detecting the entanglement between two parts of a composite system are well known. But that isn’t the case for multipartite states. Their classification is much richer, and detecting their entanglement is extremely challenging.
In the Caltech experiment, a pulse of light was generated containing a single photon—a massless bundle, with both wave-like and particle-like properties, that is the basic unit of electromagnetic radiation. The team split the single photon to generate an entangled state of light in which the quantum amplitudes of the photon propagate among four distinct paths, all at once. This so-called W state plays an important role in quantum information science.
To enable future applications of multipartite W states, the entanglement contained in them must be detected and characterized. This task is complicated by the fact that entanglement in W states can be found not only among all the parts, but also among a subset of them. To distinguish between these two cases in real-world experiments, collaborators Steven van Enk and Pavel Lougovski from the University of Oregon developed a novel approach to entanglement detection based on the uncertainty principle. (See also the recent theoretical article by van Enk, Lougovski, and the Caltech group .)
The new approach to entanglement detection used in the Caltech experiments makes use of non-local measurements of a photon propagating through all four paths. The measurements indicate whether a photon is present, but not which path it takes. From this information the scientists can estimate the level of correlation in the photon’s paths. Correlations above a certain level signify entanglement among all the paths – even partially entangled W states do not attain a similar level of correlation. A key feature of this approach is that only a relatively small number of measurements must be performed.
Due to their fundamental structure, the entanglement of W states persists even in the presence of some sources of noise. This is an important feature of W states for real-world applications conducted in noisy environments. The Caltech experiments have directly tested this property by disturbing the underlying correlations of the entangled state. When the correlations are purposely weakened, the Caltech team detects a reduction in the number of paths of the optical system that are entangled. Yet, as predicted by the structure of W states, the entanglement amongst a subset of the paths still remains.
The work was funded by the Intelligence Advanced Research Projects Activity, the National Science Foundation, and Northrop Grumman Space Technology.
 “Characterization of Multipartite Entanglement for One Photon Shared Among Four Optical Modes”
S. B. Papp, K. S. Choi, H. Deng, P. Lougovski, S. J. van Enk, and H. J. Kimble, Science 324, 764 (2009). Abstract.
 “The Quantum Internet” H. J. Kimble, Nature 453, 1023 (2008). Abstract.
 “Verifying multi-partite mode entanglement of W states”
P. Lougovski, S. J. van Enk, K. S. Choi, S. B. Papp, H. Deng, and H.J. Kimble at http://xxx.lanl.gov/abs/0903.0851.