[论文解读] Quantum Resource Theory of Lasers

Yannik Brune, Marius Cizauskas+1 more|arXiv (Cornell University)|Feb 23, 2026

Quantum Information and Cryptography被引用 0

一句话总结

Paper develops a quantum resource-theoretical framework for laser light, showing how coherence is limited by mixedness and the purity of the closest incoherent state, and validates this with displaced thermal states experimentally, linking coherence to qubit initialization performance.

ABSTRACT

Lasers serve as the fundamental workhorses of photonic quantum technologies, with perfectly coherent light fields being essential for many protocols that generate nonclassical light, implement coherent control schemes, and initialize qubits. However, no laser is absolutely ideal and the implications of deviations from perfect coherence in quantum technological tasks remain unclear. In this study, we theoretically and experimentally explore the quantum coherence properties of lasers from a resource theory perspective, establishing a significant connection between photonics, quantum optics, and quantum information science. We demonstrate that the maximum achievable quantum coherence for laser light is constrained by spontaneous emission and the purity of the dephased laser field state. As a critical example application in quantum information protocols, we show that the quantum coherence of a laser field with a given mean photon number directly governs the maximum purity attainable when initializing a qubit in a superposition state through resonant driving. Our findings are highly relevant for bridging applied physics and engineering with integrated photonic quantum technologies and resource theories, paving the way for reliable benchmarking of various coherent light sources for applications in photonics and quantum protocols.

研究动机与目标

• Motivate a resource-theoretic view of laser coherence for quantum technologies.
• Characterize quantum coherence, mixedness, and incoherent purity in displaced thermal states.
• Establish how spontaneous emission and dephasing constrain maximum coherence.
• Demonstrate experimentally how coherence relates to qubit initialization and coherent control tasks.

提出的方法

• Define quantum coherence as C(ρ) = sum_{m≠n} |ρ_{m,n}|^2, equivalently C(ρ) = P(ρ) − P(ρ_inc).
• Relate coherence to mixedness M(ρ) and incoherent purity P(ρ_inc) via C(ρ) + M(ρ) + P(ρ_inc) = 1.
• Construct a four-step resource-theoretical toy model of lasing starting from vacuum to displaced thermal states.
• Experimentally generate displaced thermal states by mixing coherent and thermal fields and perform quantum state tomography to obtain ρ, C(ρ), M(ρ), P(ρ_inc).
• Analyze how dephasing to ρ_deph increases M(ρ) and reduces C(ρ).
• Investigate how coherence constrains and informs coherent control tasks, particularly qubit initialization and interferometric vs synchronization scenarios.

实验结果

研究问题

• RQ1 How is quantum coherence of laser light bounded by mixedness and the purity of the closest incoherent state?
• RQ2 How do spontaneous emission and dephasing impact maximum achievable coherence in displaced thermal states?
• RQ3 How does the coherence of a laser field govern the maximum purity obtainable when initializing a qubit via resonant driving?
• RQ4 Can displaced thermal states serve as a realistic model for evaluating lasers used in coherent control tasks?
• RQ5 What experimental signatures validate the resource-theoretical relationships among C, M, and P_inc?

主要发现

• Maximum quantum coherence is constrained by the level of mixedness and the purity of the closest incoherent state.
• Even a small thermal contamination (e.g., ⟨N⟩_th ≈ 0.5) can significantly reduce achievable coherence.
• Coherence saturates to a value below unity when the coherent contribution dominates due to the limiting role of P_inc.
• Increasing the mean photon number can reduce P_inc and hence increase the possible coherence, but absolute thermal photon number still bounds C.
• Experimental results show excellent agreement with theory for displaced thermal states across varied coherent/thermal ratios.
• Coherence of the laser field directly governs the maximum purity achievable when initializing a qubit in a superposition via resonant driving.