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While there has been a comprehensive and concerted effort towards developing diagnostics for non-Markovian dynamics, all existing measures rely on time-domain measurements which are typically slow and expensive as they require averaging several runs to resolve small transient features on a broad background, and scale unfavorably with system size and complexity. In this work, we propose a spectroscopic measure of non-Markovianity which can detect persistent non-Markovianity in the system steady state. In addition to being experimentally viable, the proposed measure has a direct information theoretic interpretation: a large value indicates the information loss per unit bandwidth of making the Markov approximation. In the same vein, we derive a frequency-domain quantum master equation (FD-QME) that goes beyond the standard Born-Redfield description and retains the full memory of the state of the reduced system. Using the FD-QME and the proposed measure, we are able to reliably diagnose and quantify non-Markovianity in several system-environment settings including those with environmental correlations and retardation effects.<\/jats:p>","DOI":"10.22331\/q-2025-09-24-1863","type":"journal-article","created":{"date-parts":[[2025,9,24]],"date-time":"2025-09-24T11:34:11Z","timestamp":1758713651000},"page":"1863","update-policy":"https:\/\/doi.org\/10.22331\/q-crossmark-policy-page","source":"Crossref","is-referenced-by-count":3,"title":["Quantifying spectral signatures of non-Markovianity beyond the Born-Redfield master equation"],"prefix":"10.22331","volume":"9","author":[{"given":"A.","family":"Keefe","sequence":"first","affiliation":[{"name":"Department of Physics and Applied Physics, University of Massachusetts, Lowell, MA 01854, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"N.","family":"Agarwal","sequence":"additional","affiliation":[{"name":"Department of Physics and Applied Physics, University of Massachusetts, Lowell, MA 01854, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"A.","family":"Kamal","sequence":"additional","affiliation":[{"name":"Department of Physics and Applied Physics, University of Massachusetts, Lowell, MA 01854, USA"},{"name":"Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"9598","published-online":{"date-parts":[[2025,9,24]]},"reference":[{"key":"0","doi-asserted-by":"publisher","unstructured":"Daniel Manzano. ``A short introduction to the Lindblad master equation''. 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