# Towards Deep Integration of Electronics and Photonics
## 4 From Macroscopic to Microscopic Integration
It is remarkable that no decoherence occurs between photons because of the interaction with macroscopically large amount of electrons. Surface plasmons can exist as both macroscopic modes and purely quantum objects with just one photon involved. They can also be excited not only on macroscopic surfaces of metals, but at microscopically small objects where the features of an internal structure come into play. Quantum size effect can manifest itself as broadening, shifting and additional fine structure of the plasmon resonance [9]. Following this line of thought, it is possible to observe how at some point (some characteristic size), classical electrodynamics and the Drude model, usually used in applied plasmonics, become insufficient. This effect can be observed, for instance, in quantum materials like graphene [87]. At the typical thickness of a graphene sheet below 10 nm, the size effect becomes dominant. Quantum effects are discovered for plasmons that are formed in subnanometre gaps between nanoparticles [88]. The described principles can be applied in the future to construct quantum plasmonic circuits [89]. The implementation of this idea meets multiple challenges. It requires small quantum emitters with efficient coupling between single photons and SPP. One elegant way using nitrogen vacancies in nano diamonds attached to silver nanowire is mentioned above [49]. Another question is how to manipulate and control SPPs at this level. This includes the development of quantum plasmonic waveguides, which should be not only small, but also homogeneous enough to guide the single SPP. Lithography has a limited precision and does not exclude impurities. One idea available in this field is the bottom-up chemical synthesis of metallic waveguides. Crystalline, smooth, defect-free structures can be obtained in this way [90]. Single plasmon detectors is another technical challenge. In [91], near field electrical interfaces are suggested to improve the detection efficiency. In their design, SPPs are converted to excitons interacting with a semiconducting nanowire. They are collected and measured in an electrical circuit as a current. The scheme is efficient enough to detect light emitted by a single quantum dot. The concept of 'dark' optoelectronic devices, where only optical near fields are used, is proposed in the paper. Obviously, to implement such ideas, one should invent a way how to transfer loss compensation and gain techniques into the quantum regime [78]. Active elements, like transistors and modulators, are expected to play a role in quantum circuits as well. Molecular transistors, operated by a small amount of electrons, are already demonstrated. Photonic transistors can be used to control the propagation of the optical signal, using another optical 'gate field' in a non-linear fashion ('gate field' may contain just a single photon). Single-photon nonlinearities are usually weak (photon interactions are rare) and such devices are hard to produce. In [92] it is proposed to use SPPs to provide strong coupling between single photons. It opens a direct way to a single-photon transistor. In the suggested theoretical model (see Figure 4) waveguided photons are converted to SPPs supported by a silver nanowire. These modes are strongly coupled to a three-level quantum gate, which can be practically realized as a quantum-dot nanocrystal or a color center in diamond. The transition from the ground state of the gate \(\ket{g}\) to the excited state \(\ket{e}\) is strongly coupled to the SPP. An additional metastable state \(\ket{s}\) is coupled to \(\ket{e}\) via an external optical control field, and decoupled from \(\ket{g}\). Such a system works as an atomic memory capable of storing one photon. An incoming photon is stored if the optical control field is switched on. According to the computations, subsequent incoming signals (individual or multiple photons) can be transmitted or reflected depending on the population of the gate (similar to the photon blockade described in [93]).