**1. Introduction**

Semiconductor quantum dots (QDs), for example, CdSe/ZnS is an attractive quantum object in classical space. Attractiveness is based on the unique characteristics of nanoscale structures with a large quantum limitation. The high quantum yield of exciton luminescence (up to 80%), the narrow band of this luminescence, the long photo stability and the rearrangement of the exciton luminescence band in a wide spectrum range, depending on the size of the nanoparticles, have specific unique characteristics of these crystalline nanostructures [1]. These characteristics provide potential applications in photovoltaic and laser devices, thin-film transistors, light-emitting diodes and luminescent labels in biology and medicine [2–6]. In this article, I want to justify no less, but rather more, meaningful applications, both in fundamental and in applied physics.

We are talking about the properties of crystalline structures to have on their surface quantum defects called surface trap states. These surface traps capture photoinduced charge carriers, usually an electron, and delay its recombination for a fairly long time [7–9]. In [10], the exciton luminescence of CdSe/ZnS QDs was recorded in the millisecond time range and its intensity was six orders of magnitude less than the intensity of exciton luminescence immediately after the photo excitation pulse. Such a quantum state with a long existence of an exciton is called a metastable exciton. A metastable exciton is an electron—hole pair, in which an electron is captured by a surface trap with a long lifetime [10].

The typical relaxation time of exciton luminescence is the nanosecond time range. Consequently, relaxation of all excited quantum states of QDs takes place in the nanosecond range, whereas relaxation of metastable excitons takes place in the millisecond range, which is six orders of magnitude greater than the lifetime of all other quantum states of QDs. In other words, QDs with a metastable exciton are a quasistable quantum state, and can play the role of a second stable state |1〉 of QDs, as a qubit. The first stable state of such a qubit |0〉 is QD in the ground quantum state. The irradiation of a colloid QDs is a simple and practically free way to continuously generate an array of qubits in two stable quantum states, naturally, when the energy of the optical beam quanta exceeds the QD bandgap.

Obviously, we must receive the result of any quantum process, for example, quantum computing or teleportation in the classical space, in the space where we all function. For example, a digital computer operates in its "digital space." We will need it only when it "produces" a result that is understandable to us, for example, a graph or a picture, but not as a set of numbers. QDs with a metastable exciton, as a qubits in the quantum state |1〉, have unique nonlinear optical characteristics. The fact is that the electron capture by the surface trap separates the charge carriers by a distance that coincides with the size of the QD, which are several nanometers. Such a large separation of charge carriers is the source of a very large light-induced dipole moment *p* of an individual QD with a metastable exciton. The dipole moment *p* is responsible for the value of light-induced change in the refractive index [11]. Therefore, a large value of *p* makes it possible to record the distribution of the concentration of individual QDs with a metastable exciton using conventional interferometry. It is this distribution of the concentration of QDs in the quantum state that is the result of the quantum process in the quantum space, which is the "quantum box" by definition of the founding fathers of quantum mechanics.

We see that quantum dots can be in two stable quantum states, which allows them to be used as a qubit in all modern quantum technologies. One of these quantum states has a significantly different classical refractive index. This property makes it possible to register individual QDs in this quantum state |1〉 by interferometry methods and, thus, to register the results of quantum processes in classical space. The experimental results of this chapter substantiate and realize this possibility, which opens up new areas for the use of quantum dots in fundamental and applied physics.
