Linear-optical realizations of Bell state measurement (BSM) on two single-photon qubits succeed with probability ps no higher than 0.5. However, predetection quadrature squeezing, i.e., quantum noise limited phase sensitive amplification, in the usual linear-optical BSM circuit, can yield ps≈0.643. The ability to achieve ps>0.5 has been found to be critical in resource-efficient realizations of linear-optical quantum computing and all-photonic quantum repeaters. Yet, the aforesaid value of ps>0.5 is not known to be the maximum achievable using squeezing, thereby leaving it open whether close-to-100% efficient BSM might be achievable using squeezing as a resource. In this paper, we report insights on why squeezing-enhanced BSM achieves ps>0.5. Using this, we show that the previously reported ps≈0.643 at single-mode squeezing strength r=0.6585 - for unambiguous state discrimination (USD) of all four Bell states - is an experimentally unachievable point result, which drops to ps≈0.59 with the slightest change in r. We, however, show that squeezing-induced boosting of ps with USD operation is still possible over a continuous range of r, with an experimentally achievable maximum occurring at r=0.5774, achieving ps≈0.596. Finally, deviating from USD operation, we explore a trade space between ps, the probability with which the BSM circuit declares a "success," versus the probability of error pe, the probability of an input Bell state being erroneously identified given the circuit declares a success. Since quantum error correction could correct for some pe>0, this tradeoff may enable better quantum repeater designs by potentially increasing the entanglement generation rates with ps exceeding what is possible with traditionally studied USD operation of BSMs.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics