Vladimirov, IgorPetersen, Ian2026-06-132026-06-130363-0129ORCID:/0000-0003-4856-9450/work/217153606https://hdl.handle.net/1885/733811348This paper considers a risk-sensitive optimal control problem for a field-mediated interconnection of a quantum plant with a coherent (measurement-free) quantum controller. The plant and the controller are multimode open quantum harmonic oscillators governed by linear quantum stochastic differential equations, which are coupled to each other and driven by multichannel quantum Wiener processes modeling the external bosonic fields. The control objective is to internally stabilize the closed-loop system and minimize the infinite-horizon asymptotic growth rate of a quadratic-exponential functional which penalizes the plant variables and the controller output. We obtain first-order necessary conditions of optimality for this problem by computing the partial Frechet derivatives of the cost functional with respect to the energy and coupling matrices of the controller in the frequency domain and state space. An infinitesimal equivalence between the risk-sensitive and weighted coherent quantum LQG control problems is also established. In addition to variational methods, we employ spectral factorizations and infinite cascades of auxiliary classical systems. Their truncations are applicable to numerical optimization algorithms (such as the gradient descent) for coherent quantum risk-sensitive feedback synthesis.This work wass supported by the Australian Research Council, grants DP210101938 and DP200102945.31en©2025 The authorscoherent quantum feedbackfirst-order optimality conditionsopen quantum harmonic oscillatorpartial Frechet derivativesquadratic-exponential costquantum risk-sensitive control202510.1137/23M1593371105009263024