In a small bit of irony, the word “quantum” means a indivisible bundle of some quantity that can only take on discrete values, but quantum mechanics sometimes predicts that some variables are definitely not quantized. The most famous example is Schrodinger’s Cat, for which the “aliveness” of the cat is not restricted to yes or no, but allowed to take on any amplitude in between. Recently, progress has been made in our effort to peek inside the box: Make sure you don’t look to closely. That is, the quantum weirdness that would be disrupted by a normal measurement – via the collapse of the wavefunction – remains if only a weak measurement is made. So there can even be degrees of “making a measurement,” as opposed to a sharp distinction between quantum and “classical” situations.
The blurring of quantum and “normal” is further evident in quantum biology. The remarkable efficiency of photosynthetic plants requires that the proteins that collect the energy from photons in sunlight employ quantum effects. However, it is not always immediately obvious when an effect is totally quantum in nature and not explainable using classical models. For example, quantum beating is observed in photosynthesis, but may have a non-QM origin. The smoking gun signature of quantumness is a negative probability – when the wavefunctions of two particles undergo destructive interference, so the presence of one actually suppresses the apperance of the other (as in the two-slit experiment, in which opening the second slit leads to fewer counts in certain spots). In a new paper, the degree of quantum weirdness is actually measured by observing how often particle occurrences fall below their expected probabilities. Destructive interference is useful in funneling energetic electrons, since the total probability of finding them somewhere is equal to 1, so cancelling the possibility of turning up in the wrong places insures they stay on track. This is similar to the light is controlled in antiglare coating using wave interference.