Compensation-free broadband entangled photon pair sources

Abstract

Quantum sources that provide broadband biphotons entangled in both polarization and time-energy degrees of freedom are a rich quantum resource that finds many applications in quantum communication, sensing, and metrology. Creating such a source while maintaining high entanglement quality over a broad spectral range is a challenge, which conventionally requires various compensation steps to erase temporal, spectral, or spatial distinguishabilities. Here, we point out that in fact compensation is not always necessary. The key to generate broadband polarization-entangled biphotons via type-II spontaneous parametric downcoversion (SPDC) without compensation is to use nonlinear materials with sufficiently low group birefringence that the biphoton bandwidth becomes dispersion-limited. Most nonlinear crystals or waveguides cannot meet this condition, but it is easily met in fiber-based systems. We reveal the interplay of group birefringence and dispersion on SPDC bandwidth and polarization entanglement quality. We show that periodically poled silica fiber (PPSF) is an ideal medium to generate high-concurrence (>0.977) polarization-entangled photons over a broad spectral range (>77nm), directly and without compensation. This is the highest polarization-entanglement concurrence reported that is maintained over a broad spectral range from a compensation-free source.

Turn-key diode-pumped all-fiber broadband polarization-entangled photon source

“…Due to the low birefringence property of the PPSF, the absolute amplitudes |f − | and |f + | are spectrally similar [31]. Previously we have also demonstrated the direct polarization entanglement in PPSF with narrowband filtering and theoretically show that their frequency-dependent phase difference, which indicate the amount of walk-off between two states, is negligible over the whole bandwidth [21]. Hence no compensation or erasure of distinguishability is needed at the PPSF output.…”

Biphoton shaping with cascaded entangled-photon sources

“…(3)], biphotons generated from the cascade structure ( Fig. 2c) are also entangled in the polarisation DOF as well 26,51 .…”

“…Note that both of the scaling factors mentioned here generally applies for degenerate SPDC processes in which the signal and idler have the same polarisation. However, as the group birefringence of PPSF is negligible 51 over the bandwidth of the downconverted photons (see supplement, section 8.2), the scaling factors mentioned above also apply for the type-II SPDC phase-matching in the case of PPSF. Figure 3 shows the brightness (3a) and the emission bandwidth (3b) of N identical PPSFs that are coherently cascaded.…”

Optimizing the spectro-temporal properties of photon pairs from Bragg-reflection waveguides

“…Some applications such as the generation of polarization entanglement, which we investigate in more detail in Section 5, are very sensitive to this average differential group delay (DGD). Only if it is negligible, such a scheme can be implemented without external optical delay compensation [19,33]. The average DGD between the signal and idler wavepackets is given by…”