Quantum Photonics
Quantum networks will rely on photons to transport quantum entanglement between the quantum memories. One can create entangled photons using many methods, but the most common method is based on a non-linear optics processes; spontaneous parametric downconversion, or SPDC in short. By pumping a non-linear crystal, maybe a LiNbO3 crystal, a pump photon (blue) can be split into two entangled photons (red and green) through the SPDC process, while conserving energy of course:
To increase the probability to create a pair of entangled photons we use the strong confinement of optical waveguides fabricated on LiNbO3 crystals. The waveguides can be several centimeters long, which further increases the non-linear effect. Below is an example of second-harmonic generation, clearly showing the generated blue light inside a narrow LiNbO3 crystal waveguide:
In our lab we have developed several sources of entangled photons based on SPDC, working with time-bin entanglement, polarization entanglement and hyper entanglement. An example of a SPDC source pumped by a powerful green laser is seen below, the non-linear crystal waveguide is the element at bottom-right:
As we strive to actually demonstrate key capabilities fo quantum repeaters, we have also used these SPDC sources to demonstrate storage of entangled quantum states in our rare-earth quantum memories. Our pioneering work has demonstrated storage of a range of differnent quantum states in solid-state quantum memories, including hyper entanglement, displaced single photon states, and multidimensional entanglement.
Recently we have also started working on optical quantum frequency converters. These are also based on non-linear optical effects, with the goal of converting a photonic qubit from visible wavelengths to the telecommunications C-band. This will be a key device for interfacing rare-earth quantum memories with existing fiber communication networks.
Further reading
Some of our key publications in quantum photonics: (full publication list):
- STRASSMANN, Peter Clemens et al. Spectral noise in frequency conversion from the visible to the telecommunication C-band. In: Optics Express, 2019, vol. 27, n° 10, p. 14298. doi: 10.1364/OE.27.014298
- TIRANOV, Alexey et al. Quantification of multidimensional entanglement stored in a crystal. In: Physical Review. A, 2017, vol. 96, n° 040303. doi: 10.1103/PhysRevA.96.040303
- TIRANOV, Alexey et al. Temporal Multimode Storage of Entangled Photon Pairs. In: Physical Review Letters, 2016, vol. 117, n° 24, p. 240506. doi: 10.1103/PhysRevLett.117.240506
- TIRANOV, Alexey et al. Storage of hyperentanglement in a solid-state quantum memory. In: Optica, 2015, vol. 2, n° 4, p. 279. doi: 10.1364/OPTICA.2.000279
- CLAUSEN, C. et al. A source of polarization-entangled photon pairs interfacing quantum memories with telecom photons. In: New Journal of Physics, 2014, vol. 16, n° 093058. doi: 10.1088/1367-2630/16/9/093058
- CLAUSEN, Christoph et al. Quantum storage of photonic entanglement in a crystal. In: Nature, 2011, vol. 469, n° 7331, p. 508-511. doi: 10.1038/nature09662
