Full figure

Fig. 1. Review of some prominent quantum phenomena. (a) Quantum physics emphasizes that our world is built on discrete particles that are bound in finite systems of discontinuous energies. This becomes evident in the finite number of wavelengths , respectively, colors that an atom emits. (b) The quantum delocalization and wave-particle duality of light and matter can be demonstrated using a double-slit experiment (see text). (c) Quantum wave effects allow tunneling through an energy barrier which would classically be insurmountable. (d) A Mach–Zehnder interferometer allows to split a wave into two widely separated paths. (e) A quantum measurement generates objective randomness. A photon beam that is divided by a 50/50 beam splitter will hit either detector randomly and yield an absolutely random sequence of zeros and ones. (f) Entanglement is the inseparable quantum correlation of two or more particles or degrees of freedom. Here, we sketch the creation of a polarization-entangled pair of red photons when a single UV pump photon interacts with a nonlinear crystal (Kwiat et al., 1995). A measurement of both photons shows perfectly anticorrelated polarizations although the result on each side individually appears to be absolutely random. First citation in article

Full figure

Fig. 2. The wave-particle duality of the biodye tetraphenylporphyrin can be revealed by diffracting the molecules in a near-field interferometer of the Talbot–Lau type (Hackermüller et al., 2003). Molecules passing the first grating are diffracted and delocalized over several micrometers at the second grating. Diffraction there leads to interference fringes, i.e., a molecular density pattern, at the position of the third mask. This is imaged by scanning grating G₃ and by recording all transmitted molecules in a mass spectrometer. First citation in article

Full figure

Fig. 3. Some recent explorations of quantum aspects in the life sciences. (a) The nuclear spins of amino acids have been used as qubits in quantum computing demonstrations (Jones et al., 2000). (b) Electron tunneling on nanometer scales has been established as a common phenomenon in life, for instance, in reactions with cytochrome (De Vault and Chance, 1966). (c) Electron spin entanglement and coherent spin transport are part of a possible explanation for the magnetic orientation of migratory birds (Ritz et al., 2000). (d) Speculations about the influence of quantum physics on human consciousness are often regarded as inspiring but as of today they are not substantiated by any experiment [robin picture: David Jordan, CC-BY-SA (http://commons.wikimedia.org/wiki/Commons:Reusing_content_outside_Wikimedia)]. First citation in article

Full figure

Fig. 4. The FMO complex is composed of three protein-pigment structures. Each of them contains seven bacteriochlorophyll-a molecules (Blankenship, 2002). Electronic excitation transfer from the FMO complex to the reaction center is a key process in the light-harvesting of green photosynthetic bacteria. Two-dimensional Fourier transform spectroscopy (Engel et al., 2007) was able to document long-lived excitonic coherences across neighboring molecules in this structure (picture credits: Tronrud et al., 2009). First citation in article