Entanglement is “the characteristic trait of quantum mechanics,” as Schrödinger phrased it in the early days of quantum theory in 1935. It is, however, not only a fundamental physical phenomenon that exhibits counterintuitive correlations between two or more distant objects. Especially when shared by many different entities, entanglement is a resource for applications in quantum information, such as quantum communication networks or even quantum computation.
Proper entangled states for such applications require significant engineering and are typically not available at the push of a button. Only specific forms of entanglement are directly obtainable as the output of a laser light source, when the laser operates in the frequency-comb regime (with many equally spaced spectral components) and when its frequency modes are subject to the effect of parametric down-conversion (with a single photon converted into a photon pair whose total energy equals that of the input photon). However, in Physical Review A, R. Medeiros de Araújo, from the Kastler Brossel Laboratory, France, and colleagues report an optical experiment [1], in which they show that a much larger variety of multimode entangled states is already embedded in a down-converted frequency comb—including states of immediate use in quantum information processing. To reveal the different entangled states, their trick is to choose the right spectral-mode basis both for the source and the detector, with no need for changing the optical elements at different target states. In a way, this is like a painter who first mixes together all the different colors on the palette and then applies the new colors with a paintbrush in any desired form.
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