# Quantum Mechatronics
## 1 Introduction
### Quantum Technologies  
The field of quantum technologies (in the sense of possessing genuine quantum effects, such as entanglement and superposition) emerged in the past few decades as a disruptive area that may provide significant advantages in the processing and transmission of information, as well as sensing and measurement in physical systems [5]. Quantum computers promise to carry out exponentially faster computations in areas such as quantum simulation as well as factoring in cryptography. Quantum technologies are applicable to the microscopic domain, which behaves quantum-mechanically. Among others, atomic, molecular and quantum optical systems, as well as solid state systems in the genuine quantum realm, such as superconductors and quantum dots, are some of the quantum platforms experimentally studied. For digital quantum processors, two quantum technologies have special prominence: superconducting circuits and trapped ions. They allow for efficient single- and two-qubit gates, which constitute a universal set for arbitrary quantum operations. They can also be initialized and measured with high fidelities. However, for our practical purposes, when aiming at connecting the fields of mechatronics, a mechanical engineering area, and quantum technologies, a more direct quantum platform to consider is that of quantum photonics. This technology employs the quantum particle of light, the photon, as a carrier of quantum information. The latter can be encoded in horizontal and vertical polarization states of each photon, which constitutes a qubit, or in other degrees of freedom, such as dual rail or time-bin encodings. Photons can interact via nonlinear media, such as parametric down-conversion crystals, or Kerr devices. They can, in this way, become entangled, and be used, for example, in Bell tests for nonlocality. For a review on this field, see Ref. [6]. Photons can propagate at long distances with almost no decoherence, preserving their genuine quantum properties of superposition and quantum entanglement. They can also be efficiently connected to macroscopic devices via single-photon sources and detectors. Therefore, they constitute a natural candidate to establish quantum links between macroscopic systems such as robots, drones, or both. One should clarify here that the quantum links we refer to are photons that can be entangled before they are detected by the robot and/or drone detectors. Once measured, the entanglement and quantum properties disappear, as the robotic systems are macroscopic and cannot sustain genuine quantum features, such as coherence and entanglement. However, shared optical entanglement can lead to a host of quantum communications applications. The most accessible is in the field of quantum cryptography, such as establishing secret key bits via quantum key distribution (QKD). QKD provides a provable information-theoretic security model [7], which distinguishes itself from the computational security associated with traditional key distribution systems. A longer-term application of distributing entanglement could be to enhance the sensitivity of distributed sensor systems [8; 9]. Classical sensing systems can use mobile platforms to optimize sensor positions [10], but extending the sensor network capabilities to allow for shared entanglement can, in principle, allow much more sensitive measurements. In such a scenario, the optimal sensor location depends, in part, on the entanglement resources available and other system-level parameters, such as loss between the sensors. The union of quantum technologies and mechatronics can possess the features of being integrated, unified, unique, and systematic, exposed above in the area of mechatronics. The combined field is integrated because it considers a unified system involving classical, macroscopic devices (e.g., robots and/or drones) and quantum systems (e.g., photonic quantum channels, entangled photons).