Quantum Resource Theory of Lasers

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.

Figures (11)

• Figure 1: Wigner functions for the intermediate states arising in the resource-theoretical laser model. (a) vacuum state. (b) Thermal state with $\langle N\rangle_\mathrm{th}$=10. (c) Displaced thermal state with $|\alpha_0|=\sqrt{60}$. (d) Dephased displaced thermal state.
• Figure 2: Experimental scheme for generating and analyzing displaced thermal states of tailored composition. A fs-laser acts as source of both local oscillator (LO) and coherent signal, while a super-luminescent diode (SLD) provides the thermal signal Doronin2019. The displaced thermal signal state is created by mixing both input signals on a beamsplitter. Its field quadratures are measured using a 4-port homodyne detector. The quadrature sampling axis is driven by a piezo mirror.
• Figure 3: Quantum coherence (black), mixedness (red) and purity of the closest incoherent state (blue) for displaced thermal states with $\left<N\right> = 4.5$ with varying ratio between coherent and thermal photon numbers. Dots denote experimental results, while solid lines represent the theoretical prediction.
• Figure 4: Quantum coherence (black), mixedness (red) and purity of the closest incoherent state (blue) for displaced thermal states with $\left<N\right> = 20$ with varying ratio between coherent and thermal photon numbers.
• Figure 5: Time traces of the probability to find the qubit in the excited state (black line) and the purity of the atomic state (red line). Upper panels shows the results for a light field initially in a coherent state for $\langle N \rangle=30$. Middle panels show the results for a thermal state with $\langle N \rangle=30$. Bottom panels shows the results for a coherent state with $\langle N\rangle=30$ with all off-diagonal matrix elements removed. Panels on the right side show close-ups of the initial collapse of Rabi oscillations.