We demonstrate generation of high-purity photon pairs at 1560 nm in a single spatial mode from a periodically-poled KTiOPO4 (PPKTP) waveguide. With nearly lossless spectral filtering, the PPKTP waveguide source shows approximately 80 % single-mode fiber coupling efficiency and is well suited for high-dimensional time-energy entanglement-based quantum key distribution. Using high-count-rate self-differencing InGaAs single-photon avalanche photodiodes configured with either square or sinusoidal gating, we achieve > 1 Mbit/s raw key generation with 3 bits-per-photon encoding, and, to the best of our knowledge, the highest reported Franson quantum-interference visibility of 98.2 % without subtraction of accidental coincidences.
A high generation rate photon-pair source using a dual element periodically-poled potassium titanyl phosphate (PP KTP) waveguide is described. The fully integrated photon-pair source consists of a 1064-nm pump diode laser, fiber-coupled to a dual element waveguide within which a pair of 1064-nm photons are up-converted to a single 532-nm photon in the first stage. In the second stage, the 532-nm photon is down-converted to an entangled photon-pair at 800 nm and 1600 nm which are fiber-coupled at the waveguide output. The photon-pair source features a high pair generation rate, a compact power-efficient package, and continuous wave (CW) or pulsed operation. This is a significant step towards the long term goal of developing sources for high-rate Quantum Key Distribution (QKD) to enable Earth-space secure communications. Characterization and test results are presented. Details and preliminary results of a laboratory free space QKD experiment with the B92 protocol are also presented.
Entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled potassium titanyl phosphate (KTP) waveguide that up-converts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to energy entangled photon pairs at 800 nm and 1600 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1600 nm photons are detected by a superconducting nanowire with efficiency over 60% and dead time less than 50 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a singlesource, high efficiency, high data rate, quantum communication system to enable Earth-space quantum networks. Of particular interest is a source of entangled photons that is amenable to utilization in aircraft and spacecraft under rigorous flight and environmental conditions. Test results that characterize the entangled photon pair creation and detection capabilities of our system will be presented.
We demonstrate a compact and stable source of polarization-entangled pairs of photons, one at 810 nm wavelength for high detection efficiency and the other at 1550 nm for long-distance fiber communication networks. Due to a novel Sagnac-based design of the interferometer no active stabilization is needed. Using only one 30 mm ppKTP bulk crystal the source produces photons with a spectral brightness of 1.13 x 10(6) pairs/s/mW/THz with an entanglement fidelity of 98.2%. Both photons are single-mode fiber coupled and ready to be used in quantum key distribution (QKD) or transmission of photonic quantum states over large distances.
In a commercial entanglement-distribution system over telecom fibre, the single-photon detectors are the largest part of the hardware cost. In entanglement-based quantum key distribution (QKD), both photons of each pair must be detected. With two telecom photons, each arm needs either an InGaAs single-photon avalanche diode (SPAD) of low efficiency or a superconducting nanowire single-photon detector (SNSPD) with its own cryostat. We study a highly nondegenerate photon-pair source that needs a telecom detector on one arm only. Two periodically poled KTiOPO
4 (ppKTP) crystals pumped at 405~nm emit pairs at 548 and 1550~nm. A Si-SPAD with 82\% efficiency detects the 548~nm photon at the source node, and only the 1550~nm photon travels through fibre. A first, unoptimised module in a polarisation-correlated configuration generates about
1.5×106 pairs/s per mW of pump power. It collects 13\% of the pairs on the 548~nm arm and 23\% on the 1550~nm arm. In an asymptotic BBM92 model, we compare its links with generic
2×1550~nm links that use a bright published source at the same pump power. At 50~km, our link with one SNSPD costs about 25\% more per secret bit than the generic link with one SNSPD. With optimistic target values of 52\% collection and twice the brightness, it costs about 40\% as much as the generic link with two SNSPDs. Our link without a cryostat then costs about 70\% less than the generic link with one SNSPD. If every link runs at its own best pump power, the two-SNSPD link is again the cheapest. The source can also serve clock synchronisation and detector calibration, which need only time-correlated pairs. All key rates assume the entangled configuration of the source, which is under development.