Directionality in Engineered Quantum Systems

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In general an interaction process between two quantum systems is reciprocal. This means that forward and backward processes are inherently present and both systems are influenced by the interaction. One may question whether it is possible to break this symmetry, i.e., can one realize a uni-directional interaction between two quantum systems? This is indeed possible, as any factorizable (coherent) Hamiltonian interaction can be rendered directional if balanced with the corresponding dissipative interaction. Here dissipation is the crucial element to obtain directionality; a dissipative interaction can be realized simply by coupling both systems to a third (highly damped) auxiliary system which mediates an indirect interaction between the two systems.The powerful concept of balancing a coherent interaction with the corresponding dissipative interaction can be exploited to engineer unidirectional devices for quantum information processing, computation and communication protocols - for example, to achieve control over the direction of propagation of photonic signals, enabling the construction of circulators, optical isolators or directional amplifiers. Moreover, having control over the direction of the interaction between two systems opens up an interesting route for quantum state transfer protocols, teleportation and feedback control algorithms.This recipe to realize nonreciprocity forms the basis of this research proposal. Based on this method, new designs for nonreciprocal devices shall be investigated. Among these is a cascaded quantum-limited amplifier enabling robust directional amplification of weak signals. Additionally, a new nonreciprocal device shall be introduced: the directional squeezer, a device generating broadband squeezed light and a promising candidate to outperform existing cavity-based squeezing protocols. Besides practical applications, this proposal aims to study the generalization of this basic recipe for achieving nonreciprocity to higher dimensional systems, systems with nonlinear interactions, and to answer important open and fundamental questions. For example, the nonreciprocal system's ability to generate useful entanglement and the relation to non-Hermitian Hamiltonian (PT-symmetric) systems has yet to be investigated. This proposal aims to explore whether concepts for nonreciprocity via dissipation engineering are also applicable to realize topologically non-trivial states in optomechanical or superconducting array structures. Additionally, the transfer of concepts of dissipation engineering and directionality to fermionic and thermodynamic transport shall be explored.