The capability to distribute entanglement to remote locations is a precursor for future quantum systems. Of particular interest are entanglement-based quantum communication systems compatible with existing standard fiber telecom networks. The development of telecom compatible, compact and reliable sources of entangled photons is a prerequisite to pave the way for a successful implementation of future quantum information technologies into real world applications. Therefore, some of our current activities address the development of rugged and efficient entangled photon (pair) sources.

Most of entanglement-based quantum cryptography systems and advanced quantum networks rely on entangled photons favourably in the 1.5 μm telecom band. In our group, we develop an integrated source of non-degenerated polarization entangled photon pairs in the telecom regime, which is generated in a Ti-indiffused waveguide in periodically poled lithium niobate (Ti:PPLN). The basic idea is to use specifically tailored domain structures to simultaneously provide phase-matching for two type II phase-matched PDC processes. In each process a pair is generated with one photon being ordinary and the other one extraordinary polarized. Polarization entanglement is obtained if the wavelength of the ordinary and the extraordinary photons are interchanged in the two processes.

Practically, such devices are realized by using an interlaced domain pattern with alternating sections of poling period Λ₁ and Λ₂ as shown in Fig. 1.

This interlaced domain grating in the PPLN waveguide enables the direct generation of non-degenerate, polarization entangled photon pairs with high brightness of B = 7 × 10³ pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness. Visibilities exceeding 95 % (as shown in Fig. 2) and a violation of the Bell inequality with S = 2.57±0.06 has been demonstrated [1].

N00N states are a very useful class of quantum states, because it leads to a better resolution than possible for a conventional laser of the same wavelength in quantum metrology or lithography. The usual way of generating such states is the combination of two different waveguide sources on an integrated beam splitter.

Our integrated way even goes one step further. We can harness the path degree of freedom known from bulk sources and therefore produce the two-photon N00N state in a single non-linear optical device, instead of putting many different elements together. We achieve this by combining the linear functionalities of an integrated directional coupler and the non-linear properties of a photon pair source via parametric down-conversion [2], as shown in Fig.3.

Due to the intrinsic structure of our source, we can use the pump wavelength of our PDC process to spatially tune the properties of our generated quantum state. This tunability is directly integrated into the source and does not require any additional overhead during the generation process, nor the application of any post-processing in the experimental implementation.

In order to prove that really a pure N00N state is generated and that the photons coming out of the two waveguides are in a superposition, the double fringing expected from a two-photon N00N state is shown in Fig. 4. Thus, we have shown in our lab and find a very nice fringing amplitude, which proves, that we indeed can generate a post-processing free two-photon N00N state in a single non-linear device.

[1] Post-selection free, integrated optical source of non-degenerate, polarization entangled photon pairs
Harald Herrmann, Xu Yang, Abu Thomas, Andreas Poppe, Wolfgang Sohler, Christine Silberhorn
[PDF, Opt. Exp., 21(23), 27981 (2013)]

[2] 

On-chip dual-path entangled photon pair sources by coupled non-linear waveguides