[@0305-4470-34-35-315; @PhysRevLett.88.017901; @RevModPhys.84.1655], which quantifies correlations beyond entanglement, is considered a likely candidate. Unlike entanglement, discord is far more robust and can also survive in highly noisy conditions [@PhysRevLett.100.050502]. In fact, Weedbrook [*et al.*]{} have shown such a relation for discrete variables [@weedbrook2016discord]. Specifically, they showed that the performance advantage of quantum illumination—in terms of extra accessible information about whether an object is present—can be directly related to the amount of discord in the illumination protocol that survives after being subjected to entanglement-breaking noise. Does a similar relationship hold for continuous variables?

The aim of this work is to answer that question. We extend the framework relating discord and illumination to the continuous variable regime. This involves understanding how these relations generalize when a number of conditions specific to the discrete scenario no longer hold. The paper is organized as follows. In Sec. \[sec\_framework\] we describe the illumination protocol and the quantifiers of performance. In Sec. \[sec\_discord\] we describe discord and how it relates to quantum illumination. In Sec. \[sec\_results\] we present and discuss our results, demonstrating that there is a general relationship between discord and the quantum advantage of illumination in the continuous-variable regime.

The illumination framework {#sec_framework}
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