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Linear optics and the problem of Bell-like state discrimination

Jatin GhaiSibasish Ghosh
Feb 2024
A linear optics-based scheme to implement various quantum information processing tasks is of paramount importance due to ease of implementation and low noise. Many information-theoretic tasks depend on the successful discrimination of Bell states. A no-go theorem has been proved in literature which tells that it is not possible to perfectly discriminate among the four Bell states by restricting measurement apparatus to linear optical elements. The success probability is only $50\%$. Through using extra resources such as hyper entanglement, ancillary entanglement, and even a minimum amount of non-linearity complete Bell-state discrimination can be achieved. The success probability for Bell-like state discrimination is only $25\%$. We find that this can be boosted up to $50\%$ using hyperentanglement in polarization, momentum, or OAM degrees of freedom of the photons which is in contrast to the Bell-state discrimination scenario where $100\%$ can be achieved. Furthermore, we find that by using correlation in time of the photons all four Bell states can be distinguished with $100\%$ success probability while for the Bell-like state discrimination, it strictly lies between $25\%$ and $50\%$ depending on the state parameter with only three Bell-like states being distinguishable. We also observe a similar contrast when we use ancillary entangled photons. While the success probability for all four Bell-state discrimination increases as $1-\frac{1}{2^N}$ where N is the number of ancillary photons for Bell-like states it depends again on the state parameters and can be less than $25\%$ in some cases. Also adding further ancillary photons decreases the success probability. We then show that using non-linear gadgets namely SFG $100\%$ success probability can be achieved even for Bell-like state discrimination.
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