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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2604.18662 |
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| _version_ | 1866916027223244800 |
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| author | Singh, Priyank |
| author_facet | Singh, Priyank |
| contents | Single-photon routers in cavity and circuit QED direct photons by the qubit's energy eigenstate -- a projective decision that destroys coherence. We propose a different primitive: coherence-gated routing, where the decision depends on the magnitude of the qubit's quantum coherence, estimated in real time from simultaneous weak measurements of $σ_x$ and $σ_z$. A photon is accepted if the coherence score $S(T) = \sqrt{\langleσ_x\rangle_c^2 + \langleσ_y\rangle_c^2}$, extracted from the conditional density matrix via the stochastic master equation, exceeds a tunable threshold $S_{\mathrm{th}}$.
Certifying coherence at emission enables two applications conventional heralded sources cannot: (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 whose two-node coherence certification bounds the matter--matter entanglement fidelity after Bell-state measurement.
The estimator is itself a security primitive. Benchmarking seven configurations, we find that underestimating detector efficiency ($η_{\mathrm{a}} < η_{\mathrm{true}}$) both stabilizes the numerics and suppresses overcertification. We trace this via a purity-monotonicity result, identify a geometric loophole amplifying purity undercertification into coherence overcertification by an order of magnitude ($\sim$12$\times$), and prove two complementary tail bounds: an Ornstein--Uhlenbeck comparison giving $9.0\%$ overcertification (empirical $6.3\%$ from $10^6$ trajectories) and an exponential supermartingale establishing structural exponential decay. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_18662 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Coherence-gated quantum devices via real-time weak measurement Singh, Priyank Quantum Physics Single-photon routers in cavity and circuit QED direct photons by the qubit's energy eigenstate -- a projective decision that destroys coherence. We propose a different primitive: coherence-gated routing, where the decision depends on the magnitude of the qubit's quantum coherence, estimated in real time from simultaneous weak measurements of $σ_x$ and $σ_z$. A photon is accepted if the coherence score $S(T) = \sqrt{\langleσ_x\rangle_c^2 + \langleσ_y\rangle_c^2}$, extracted from the conditional density matrix via the stochastic master equation, exceeds a tunable threshold $S_{\mathrm{th}}$. Certifying coherence at emission enables two applications conventional heralded sources cannot: (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 whose two-node coherence certification bounds the matter--matter entanglement fidelity after Bell-state measurement. The estimator is itself a security primitive. Benchmarking seven configurations, we find that underestimating detector efficiency ($η_{\mathrm{a}} < η_{\mathrm{true}}$) both stabilizes the numerics and suppresses overcertification. We trace this via a purity-monotonicity result, identify a geometric loophole amplifying purity undercertification into coherence overcertification by an order of magnitude ($\sim$12$\times$), and prove two complementary tail bounds: an Ornstein--Uhlenbeck comparison giving $9.0\%$ overcertification (empirical $6.3\%$ from $10^6$ trajectories) and an exponential supermartingale establishing structural exponential decay. |
| title | Coherence-gated quantum devices via real-time weak measurement |
| topic | Quantum Physics |
| url | https://arxiv.org/abs/2604.18662 |