Indistinguishable photons, quantum

Figure 6. a - Schematic of a simplified version of nonlinear sign-shift (NS) operation constructed by a non-polarizing beam splitter of reflectivity II. T i /v) and hour) are the quantum states of input and output photons. The operation is successful when the single-photon detector in ancilla mode 4 counts a single photon, b - The two paths that lead to the detection of exactly one photon in output mode 4. As long as all of the photons are indistinguishable, these two paths can interfere. 

Now photons may reasonably be taken to be indistinguishable one from another, so that the statistical considerations which apply in the light of quantum mechanics to molecules should apply at least equally well here. We may take all photons corresponding to frequencies between v and v- -dv to possess energy and, as we have seen, there will be states of this energy, where has the value which has just been calculated. 

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