Loss-robust crossband entanglement generation beyond the direct-transduction limit

Abstract

Entanglement across distant frequency bands is a crucial resource in quantum networking. However, directly entangling crossband photons, e.g., microwave and optical, is challenging. Furthermore, distributing crossband entanglement via direct quantum transduction is fundamentally limited, regardless of input engineering with unconstrained source brightness. We propose to utilize intraband entanglement to overcome such direct-transduction limits by a factor that increases with the input intraband entanglement brightness in the ideal case. In the presence of experimental loss 5% and assuming 10dB of squeezing in both optical and microwave bands, we show that our protocol can generate a violation of the separability criterion equivalent to 2.38 ebits, compared with the baseline protocol limited to 0.082 ebits. The proposed protocols rely only on off-the-shelf components and provide advantages robust to a substantial amount of loss.