PERSPECTIVES New Tool for Genome Surgery MOLECULAR BIOLOGY

A bacterial system that uses RNA to edit DNA is harnessed for engineering mammalian genomes. The output probability distribution from a quantum random walk process does not simply correspond to a sum of the indepen-dent probabilities for the individual par-ticles even if there is no physical interac-tion between the particles (panel B of the fi gure). The extra interactions arise because the state of the system must obey the rule that Ψ(x1, x2) = ±Ψ(x2, x1) when two indis-tinguishable particles are swapped or inter-changed. The plus sign applies to particles that are known for historical reasons as bosons, whereas the minus sign applies to particles known as fermions. Applying this rule to the wave function produces an effec-tive attraction between identical bosons and an effective repulsion between identical fer-mions. For example, the probability of fi nd-ing two identical fermions at the same loca-tion x1 = x2 is zero because that corresponds to Ψ(x1, x1) = –Ψ(x1, x1). These effects are commonly referred to as exchange forces, even though there is no physical interac-tion between the particles. Roughly speak-ing, the exchange forces provide an effec-tive interaction that can be used to perform certain calculations. Spring et al., Broome et al., and Tillmann et al. used indistinguishable particles of light (photons) to implement a quantum random walk of this kind called boson sampling. The photons propagated through a series of conducting channels known as waveguides that were fabricated on the surface of a chip. Neighboring channels were coupled to each other by bringing them sufficiently clos