Quantum illumination

Detecting a specific target in a cluttered environment is never easy. In 2008 a quantum detection scheme was proposed in which entanglement provided an advantage over the best possible classical illumination source of the same average power. Start with two entangled light beams created, for example, via parametric down-conversion. One beam, called the signal, is sent into a noisy environment to see if a particular object is located there; the other beam, dubbed the idler, is held close to home. As light returns from the targeted environment, the receiver combines it with the idler. If the signal beam reflects off the object before returning, then an unambiguous signal pops out of the noise—even if the original entanglement is lost. Two groups have now demonstrated the so-called quantum illumination (QI) experimentally. A group from Italy’s National Institute of Metrological Research in Turin and the University of Milan demonstrated QI detection that outperformed the best similarly powered classical protocol by orders of magnitude, independent of noise. Meanwhile, in a new twist, a group at MIT used the QI protocol for encrypted communication and showed that messages transmitted through a noisy environment not only survive but remain immune from passive eavesdropping. Both results show that entanglement-related enhancements can survive the loss of that very entanglement, with potential for practical uses in real-world environments. (E. D. Lopaeva et al., Phys. Rev. Lett. 110, 153603, 2013, doi:10.1103/PhysRevLett.110.153603 ; Z. Zhang et al., Phys. Rev. Lett., in press, doi:10.1103/PhysRevLett.110.226103 .)