Direct and indirect couplings in the interconnection of open two level quantum systems

Recently, the influences and applications of indirect and direct couplings in coherent feedback control of linear quantum stochastic systems have been investigated. However, bilinear quantum stochastic systems escape the realm of the current known results. Furthermore, as fast tunable coupling of a set of two level quantum systems (qubits) with high coherence plays a vital role in quantum computing operations, in this paper we study direct and indirect couplings in the interconnection of qubits. A physical model described by bilinear quantum stochastic differential equations is presented, and physical realizability conditions (necessary and sufficient algebraic constraints) are provided to guarantee these bilinear quantum stochastic differential equations to correspond to the dynamics of directly or indirectly coupled open two level systems. These results shed light on network synthesis of bilinear quantum stochastic systems and coherent control of multiple qubits.