Apparatus for coupling two signals

1. An apparatus for providing coupling between at least a first input signal (S in1 ) with a first signal frequency, and a second input signal (S pi_in2 ) with a second, different signal frequency, the first input signal being a microwave signal, and the second input signal being an optical signal, the apparatus comprising:

a first input port;

a second input port;

a first output port;

a second output port;

a first waveguide disposed between the first input port and the first output port;

a second waveguide disposed between the second input port and the second output port, the second waveguide comprising non-linear material such that a first electromagnetic field generated by a first-waveguide signal in the first waveguide and a second electromagnetic field generated by a second-waveguide signal in the second waveguide are arranged to overlap in the non-linear material;

a periodic structure at least within the second waveguide and/or on a surface of the second waveguide and/or outside of the second waveguide for creating at least one photonic band gap; and

a phase-matching arrangement to cause the first-waveguide signal in the first waveguide and the second-waveguide signal in the second waveguide to be phase-matched or quasi-phase-matched,

wherein the first and second waveguides are non-resonator waveguides configured to allow travelling waves to propagate through the first and second waveguides.

2. The apparatus according to claim 1 , wherein the first input signal (S in1 ) is a microwave signal, and the second input signal is an optical signal (S pi_in2 ), and wherein the non-linear material has at least a second order nonlinearity capable of generating a sum frequency signal and/or a difference frequency signal from the first and second electromagnetic fields.

3. The apparatus according to claim 1 , wherein the first waveguide comprises at least a first electrode and a second electrode separated from the first electrode by a dielectric.

4. The apparatus according to claim 3 , wherein the second waveguide is disposed between the first electrode and the second electrode, or wherein the first and second electrodes are disposed on one side of the second waveguide.

5. The apparatus according to claim 1 , wherein the apparatus further comprises a pump source for generating one of the input signals (S in1 or S pi_in2 ).

6. The apparatus according to claim 1 , wherein the non-linear material has at least a second-order nonlinearity capable of generating a sum-frequency signal and/or a difference-frequency signal from the first and second electromagnetic fields, and wherein the periodic structure is configured so that the difference and/or sum signal(s) fall(s) into at least one photonic band gap.

7. The apparatus according to claim 1 , wherein one or more photons of the first input signal received at the first input port is/are arranged to be fed to the second output port, and/or one or more photons of the second input signal received at the second input port is/are arranged to be fed to the first output port.

8. The apparatus according to claim 1 , wherein the second waveguide comprises a non-linear component comprising the non-linear material, and a linear component comprising linear material.

9. The apparatus according to claim 1 , wherein the periodic structure comprises a periodic set of holes within the second waveguide and/or outside of the second waveguide, and/or the periodic structure comprises a periodic set of blocks on the second waveguide, and/or the periodic structure is formed by a periodically varied cross-section of the second waveguide along a longitudinal axis of the second waveguide.

10. The apparatus according to claim 1 , wherein the phase-matching arrangement is placed within at least one of the first and second waveguides or capacitively connected to the first waveguide.

11. The apparatus according to claim 1 , wherein the phase-matching arrangement comprises at least any one of the following means: one or more delay lines, one or more coupled resonators, a periodically varying distance between the first and second waveguides, a periodically varying size of at least one of the first and second waveguides, local modifications of a refractive index, and a periodically poled structure.

12. The apparatus according to claim 1 , wherein the periodic structure comprises elements arranged at predefined intervals, and wherein a dimension of the elements substantially orthogonal to a longitudinal axis of the second waveguide is in the range of 0.1 μm to 10 μm, or in the range of 0.5 μm to 5 μm.

13. The apparatus according to claim 1 , wherein the non-linear material is made of any noncentrosymmetric optical crystalline material, or is made of any centrosymmetric optical crystalline material, or any combination thereof.

14. The apparatus according to claim 13 , wherein said noncentrosymmetric optical crystalline material comprises any one of LiNbO3, GaP, BaTiO3, LiTaO3, ß-BaB2O4, AlN, LiB3O5, KTiOPO4, KNbO3, KH2PO4, LiIO3, (NH4)(H2PO4), CdSe, GaAs, InN, InSb, ZnTe, Pb[ZrxTi1−x]O3, and said centrosymmetric optical crystalline material comprises any one of SiNx, SiO2, Al2O3, Si, Gd3Ga5O12, Diamond, SiC, SiGe, CaF2, and MgF2.

15. The apparatus according to claim 1 , wherein the apparatus further comprises a direct current electric field generator to apply a direct current electric field across the non-linear material.

16. The apparatus according to claim 1 , wherein the apparatus is a memory-free apparatus.

17. The apparatus according to claim 1 , wherein the apparatus is an acoustic signal-free apparatus.

18. A quantum logic gate comprising the apparatus according to claim 1 , wherein the quantum logic gate is a quantum entangling gate for a predefined active interaction length L defined by the length of the non-linear material along the second waveguides, and a quantum SWAP gate for an interaction length 2L.