Coherence-gated quantum devices via real-time weak measurement

Quantum Physics [Submitted on 20 Apr 2026] Coherence-gated quantum devices via real-time weak measurement Priyank Singh Single-photon routers in cavity and circuit QED direct photons based on which energy eigenstate a qubit occupies -- a projective decision that destroys coherence. We propose \emph{coherence-gated routing}, where the routing decision depends on the magnitude of quantum coherence, estimated in real time from simultaneous weak measurements of \sigma_x and \sigma_z. A photon is accepted depending on whether S(T) = \sqrt{\langle\sigma_x\rangle_c^2 + \langle\sigma_y\rangle_c^2} exceeds a tunable threshold~S_{\mathrm{th}}. Because coherence is certified at emission, the protocol enables two applications beyond conventional heralded sources: (i)~a quantum random number generator with min-entropy bounded by Bloch-sphere geometry, H_\infty \geq -\log_2\bigl(\frac{1+\sqrt{1-S_{\mathrm{th}}^2}}{2}\bigr), and (ii)~a phase-tracked photon source where independent certification at two nodes bounds the matter-matter entanglement fidelity after Bell-state measurement. The real-time estimator is a security primitive, not merely a numerical tool. We benchmark seven configurations across 3000 trajectories and show that deliberately underestimating detector efficiency (\eta_{\mathrm{a}} < \eta_{\mathrm{true}}) simultaneously stabilizes the numerics and suppresses overcertification. We trace this analytically through a purity monotonicity result, identify a geometric loophole amplifying purity undercertification into coherence overcertification by~45\times, and develop two pointwise bounds: an Ornstein-Uhlenbeck comparison yielding 4.5\% operational overcertification (validated at 3.6\% from 10^6 trajectories), and an exponential supermartingale establishing an exponential tail. The gap -- a single polynomial optimization -- defines a path to fully composable security. Comments: 13 pages, 15 figures Subjects: Quantum Physics (quant-ph) Cite as: arXiv:2604.18662 [quant-ph]   (or arXiv:2604.18662v1 [quant-ph] for this version)   https://doi.org/10.48550/arXiv.2604.18662 Focus to learn more Submission history From: Priyank Singh [view email] [v1] Mon, 20 Apr 2026 12:19:54 UTC (5,861 KB) Access Paper: HTML (experimental) view license Current browse context: quant-ph < prev   |   next > new | recent | 2026-04 References & Citations INSPIRE HEP NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)