German scientists have overcome a theoretical hurdle by transmitting a pulse of squeezed light over a long distance through the air. “We have now succeeded in transmitting a flash of light, namely a pulse which contains many photons, through the atmosphere in a particularly sensitive quantum state,” said Christian Peuntinger, a researcher at the Max Planck Institute for the Science of Light. The researchers said they demonstrated “the distribution of bright polarization-squeezed quantum states of light through an urban free-space channel of 1.6 km.” Specifically, a photon packet was sent in a straight line, in daylight, from the Max Planck Institute to the University of Erlangen-Nuremberg (FAU). The squeezed form of the quantum state registered at the receiver. The short transverse and long longitudinal axis here represent the spread of two properties which are linked to each other via Heisenberg's uncertainty principle. The property, which is represented on the short axis, assumes values only within a narrow range. The property shown on the long longitudinal axis has a larger spread of measured values. The transitions between the two extremes result in the elliptical shape. The corresponding representation of conventional, non-squeezed states has a circular contour. Courtesy of the Max Plank Institute for the Science of Light. The result hints at applications in quantum cryptography, such as detection of eavesdropping. Quantum communications using flashes of light is possible only if the constituent photons exist in the low-noise, squeezed state. This state is easily destroyed if an intense flash of light passes through air turbulence, however. The researchers circumvented this problem by relying on polarization, which cannot be changed by turbulence. “Even if the turbulences make the wavefronts of a light flash appear terribly deformed, the polarization is maintained,” said Dr. Christoph Marquardt, leader of the Max Planck Quantum Information Processing Group. The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.113.060502). For more information, visit www.mpg.de.