# Towards Deep Integration of Electronics and Photonics
## 4 From Macroscopic to Microscopic Integration  
Devices and ideas discussed above are already available for implementation at the present time. Nevertheless, merging of electronics and photonics at the nanoscale is just a first step in the process which should continue in the future at a more fundamental level. The trend of miniaturization, in spite of many obstacles, unavoidably leads to molecular electronics, where single molecules are used as building blocks in devices, operated by a small amount of electrons. Integration of electronics with photonics at this level also requires single photon generation and manipulation. Description of light-matter interaction at this level leads to quantum plasmonics, among other things [78]. From a quantum mechanical point of view, volume plasmons are bosonic quasi-particles formed by collective oscillations of correlated electrons. The first quantum model is presented by [PERSON] in 1953 [79]. The quantum model of SPPs was suggested by [PERSON] and [PERSON] in 1971 [80]. A more detailed model, which took ohmic losses into account, was introduced later by [PERSON] [81]. LSPs in nanoparticles were also successfully quantized [82]. Since that time many experiments were performed revealing quantum properties of plasmons including coherence, entanglement and wave-particle duality. One of the first fundamental questions considered in quantum plasmonics is about the possibility to excite a SPP using one single photon. Experiments demonstrate that such modes exist [49]. SPPs are generated on a silver nanowire attached with nano-diamonds, where a single nitrogen-vacancy center works as a single photon emitter excited by a laser pulse. The emitted SPP then propagates along the wire and, reaching the end, is converted to a far field photon, which is subsequently detected. By accomplishing this, both wave and particle-like properties of a single SPP were discovered. In the first case, self-interference of the SPP is observed, qualitatively similar to self-interference of single photons in double slit experiments. The phase difference is provided by different distances between the emitter and one of the wire ends. Particle behavior is observed in the antibunching experiments measuring fluorescence at the wire ends. Remarkable to note is that many electrons are involved in a single SPP, but it still behaves as a single quantum particle. SPPs reproduce many aspects of quantum behavior typical for photons. Two-photon quantum interference experiments of Hong-Ou-Mandel [83] are repeated for plasmons [84]. In the waveguided version of the experiment, a pair of photons propagates inside dielectric waveguides fabricated on a silicon chip. A directional coupler is used instead of a beam splitter to initiate the interference. To provide the conversion of photons to a SPP and back, dielectric-loaded plasmonic waveguides are integrated into ordinary dielectric waveguides. And, consequently, a plasmonic directional coupler is used instead of a dielectric one to provide the interference. Statistics at the output of two waveguides is analyzed to monitor the interference, which is clearly detected. A close correspondence between interference in dielectric and plasmonic waveguides is found. It reveals a high degree of coherence between SPP. As one of the conclusions, the authors suggest plasmonic components to be used in quantum computing. Like photons, SPPs can be entangled (when their wave function cannot be factorized into single particle wave functions), which once again demonstrates their true quantum nature. In the experiments of [85] a pair of entangled photons (obtained using a beta barium borate nonlinear crystal, where photons are down-converted into twin photons with the doubled wavelength) passesthrough an optically thick metal film, perforated with a periodic array of subwavelength holes. Such a structure converts photons to SPP and then back to photons again at the output [86]. Coincidence counting measurements prove that the entanglement survives such a conversion.