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Can a Dove prism change the past of a single photon? Report Title We reexamine the thought experiment and real experiment of Danan et al. (Phys Rev Lett 111:240402, 2013), Vaidman (Phys Rev A 89:024102, 2014), Vaidman (Phys Rev A 87:052104, 2013), by placing Dove prisms in the nested Mach–Zehnder interferometer arms. In those previous works, the criterion of whether a single photon was present, or not, was the presence of a “weak trace,” indicating the presence of a nonzero weak value. This “weak trace” was tested by slightly varying the mirror angle at a given frequency, and then verifying whether this variation led to a signal at the corresponding frequency on a position-sensitive detector. We show that the presence of the Dove prisms gives identical weak values everywhere to the previous configuration because the prisms change neither the path difference, nor the mode profile in the aligned case. Nevertheless, the same slight variations of the interferometer mirrors now give a signal at the first mirror of the nested interferometer, indicating the photons entered the inner interferometer by the weak trace criterion. We give several possible interpretations of this effect. Can a Dove prism change the past of a single photon? Approved for public release; distribution is unlimited. 63986.7-PH REPORT DOCUMENTATION PAGE (SF298) (Continuation Sheet) Continuation for Block 13