Tikhonov-regularised projected gradient flow for equality-constrained bilinear quantum control
带Tikhonov正则化的投影梯度流用于等式约束的双线性量子控制
Tanveer Ahmad
AI总结 本文提出一种带Tikhonov正则化的投影梯度流方法,用于解决等式约束下的双线性量子控制优化问题,通过引入移动Gram矩阵Γ( s )并证明其正则化后在连续时间和离散化时的稳定性与收敛性。
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我们研究了一种投影型梯度流,用于在H=L²(0,T;R)上最大化一个光滑的双线性控制目标函数,通过一个(M+1)×(M+1)的移动Gram矩阵Γ(s)_{ℓℓ'}=∫₀ᵀS(t)c_ℓ(s,t)c_{ℓ'}(s,t)dt消除Lagrange乘子。该流在连续时间中产生单调上升,但在离散化时变得不稳定;现有的实现依赖于启发式的步长安全措施,缺乏严格的证明。我们通过将Γ替换为Γ_ε:=Γ+ε²I并证明:(i)一个精确的谱恒等式给出κ(Γ_ε)=(σ_max²+ε²)/(σ_min²+ε²);(ii)目标单调性dJ/ds≥0对于所有ε≥0;(iii)约束漂移|h_m-C_m|=O(ε²)具有可计算的预因子;(iv)正则化轨迹在L²(0,T)中以O(ε²)的速度收敛到无正则化的轨迹,当Γ均匀可逆时;(v)一个离散CFL准则ΔsG‖Γ_ε⁻¹‖≤α<2保证了前向欧拉方案的目标单调性,直到O(Δs²)局部截断误差。该理论在三能级双线性基准上得到验证,用于全光贝尔态制备,其中κ(Γ)∈[10⁹,10¹¹],预测的ε²速率在八个数量级上得到确认,适度的正则化消除了步长拒绝并减少了约束漂移,同时保持最终保真度不变。
We study a projection-type gradient flow for equality-constrained maximisation of a smooth bilinear control objective on $\mathcal{H}=L^2(0,T;\mathbb{R})$, eliminating Lagrange multipliers through an $(M{+}1)\times(M{+}1)$ moving Gram matrix $Γ(s)_{\ell\ell'}=\int_0^T S(t)\,c_\ell(s,t)\,c_{\ell'}(s,t)\,\mathrm{d}t$. The flow generates monotonic ascent in continuous time but becomes unstable on discretisation; existing implementations rely on heuristic step-size safeguards lacking rigorous justification. We close this gap by replacing $Γ$ with $Γ_{\varepsilon}:=Γ+\varepsilon^{2}I$ and prove: (i) an exact spectral identity giving $κ(Γ_{\varepsilon})=(σ_{\max}^{2}+\varepsilon^{2})/(σ_{\min}^{2}+\varepsilon^{2})$; (ii) objective monotonicity $\mathrm{d}J/\mathrm{d}s\ge 0$ for all $\varepsilon\ge 0$; (iii) constraint drift $|h_{m}-C_{m}|=\mathcal{O}(\varepsilon^{2})$ with a computable prefactor; (iv) convergence of the regularised trajectory to the unregularised one in $L^{2}(0,T)$ at rate $\mathcal{O}(\varepsilon^{2})$ under uniform invertibility of $Γ$; and (v) a discrete CFL criterion $Δs\,G\,\|Γ_{\varepsilon}^{-1}\|\leα<2$ guaranteeing objective monotonicity of the forward-Euler scheme up to $\mathcal{O}(Δs^{2})$ local truncation error. The theory is validated on a three-level bilinear benchmark for all-optical Bell-state preparation, where $κ(Γ)\in[10^{9},10^{11}]$, the predicted $\varepsilon^{2}$ rate is confirmed over eight decades, and moderate regularisation eliminates step rejections and reduces constraint drift by more than an order of magnitude at unchanged final fidelity.