Quantum entanglement
‘ Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels…’ (certain discrete values of energy a particle, i.e. an electron, can have in the structure) ‘…in order to transport energy from where it is collected by photon absorption, to reaction centres where the energy is used to drive chemical reactions ’ .[10] The species of bacteria used in the experiment, Chlorobaculum tepidum , is amember of the family Chlorobiaceae (see [11]) and absorbs light with complexes called chlorosomes. An exciton is created for the purpose of carrying energy, and is transferred to the protein baseplate and then to the reaction centre. The experiment shows that one can access the exciton-photon coupling (i.e. interaction) regime and form so-called polariton states through coherent energy exchange in an optical microcavity, created by suspending a bacterial solution between two semi-transparent metallic mirrors. These ‘ polaritons have an energy distinct from that of the exciton and photon, and can be tuned in situ via the microcavity length. This results in real-time, non-invasive control over the relative energy levels within the bacteria. ’ [10] To check whether the bacteria are alive during strong exciton-photon coupling, a cell viability stain known as trypan blue is used. Trypan blue permeates through compromised membranes of dead cells, binding to intracellular proteins, thus staining the bacteria. It was shown afterwards that the cells of the bacteria remained unstained during the experiment. While the cavity acts to restrict the intensity of light reaching the bacteria, the species used is known to be capable of surviving in extremely low light environments, and even displays low mortality rates in the presence of no light. The bacteria under investigation remained unstained for the duration of the experiment, which lasted for several hours.[10] The results of the paper were further analysed by a research team based at the University of Oxford. The results of the analysis, published in 2018 in a paper titled Entanglement between living bacteria and quantized light witnessed by Rabi splitting , have revealed some evidence for quantum entanglement, indicated by strong coupling between the excitons in the chlorosomes and the photons in the microcavity. According to the researchers, the coupling is strong if the leakage of light trapped in the microcavity is slow compared to the energy exchange rate between light and the bacteria. This results in a modification of the energy spectrum of the exciton and cavity modes by the introduction of two new energy levels corresponding to the aforementioned polaritons.[12] The two resulting peaks representing the polariton branches have higher and lower energies in comparison to the uncoupled exciton energies. The energy difference between the two peaks when the uncoupled cavity and exciton modes have the same energy is called Rabi splitting and is used as an ‘entanglement witness’, a physical observable that ‘reacts’ differently to entangled and disentangled states. The coupling is strong if the Rabi splitting is greater than the sum of uncoupled photon and exciton energies. According to the analysis, this condition has been met in the experiment, which demonstrates that the bacteria and the light are entangled, or more specifically, the excitons in the bacteria and the photons in the cavity.[12] The researches point out that the same results (including the observed Rabi splitting) can be modelled completely classically, which is not a contradiction because the entanglement witness they use works under the assumption that both systems (i.e. the bacteria and the light) are quantum and checks whether the subsystems are quantum as well. The classical result that is obtained is identical to the result of the quantum analysis shown in the paper.[12]
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