Distinguishability in quantum interference with multimode squeezed states

Distinguishability theory is developed for quantum interference of the squeezed vacuum states on unitary linear interferometers. It is found that the entanglement of photon pairs over the Schmidt modes is one of the sources of distinguishability. The distinguishability is quantified by the symmetric part of the internal state of n pairs of photons over the spectral Schmidt modes, whose normalization q(2n) is the probability that 2n photons interfere as indistinguishable. For two pairs of photons q(4) = (1 + 2P)/3, where P is the purity of the squeezed states (K = 1/P is the Schmidt number). For a fixed purity P, the probability q(2n) decreases exponentially fast in n. For example, in the experimental Gaussian boson sampling of H.-S. Zhong et al., [Science 370, 1460 (2020)], the achieved purity P approximate to 0.938 for the average number of photons 2n >= 43 gives q(2n) less than or similar to 0.5, i.e., close to the middle line between n indistinguishable and n distinguishable pairs of photons. In derivation of all the results, the first-order quantization representation based on the particle decomposition of the Hilbert space of identical bosons serves as an indispensable tool. The approach can be applied also to the generalized (non-Gaussian) squeezed states, such as those recently generated in the three-photon parametric down-conversion.