Mathematical Foundations of Quantum Pricing Theory
Tian Xin, Liang Aoqin
TL;DR
The paper develops a rigorous operator-algebraic framework for pricing in markets where information flows are classical yet embedded in a noncommutative market algebra. Prices arise from a dynamic, state-preserving projection operator constructed from an abelian information filtration, with a Local Informational Efficiency Principle that localizes martingale pricing to available information. It shows that, under modular compatibility (Takesaki theory) and a pricing state $\varphi^*$, a robust dynamic pricing map $\Pi_t$ enjoys normality, complete positivity, unitality, and intertemporal consistency, and it recovers classical risk-neutral valuation in the commutative limit. The work further develops an $L^2$-prediction theory and introduces operator-valued Fisher information and a noncommutative Cramér–Rao bound, providing quantitative limits on conditional mean-square prediction under information constraints; in diffusion limits these bounds align with familiar Black–Scholes-type structures, while the noncommutative setting yields a rich information-theoretic perspective on arbitrage and pricing. The framework thereby unifies pricing, information flow, and prediction under a single operator-algebraic umbrella with clear pathways to quantum extensions and nontrivial information-geometry bounds.
Abstract
Let $M$ be a von Neumann algebra and let $(N_t)_{t\in[0,T]}$ be an increasing family of abelian von Neumann subalgebras encoding a (classical) information flow. Fix a faithful normal state $\varphi_ρ$ on $M$ and assume a filtration of normal $\varphi_ρ$-preserving conditional expectations $E_t:M\to N_t$ satisfying the tower property. For self-adjoint observables affiliated with $M$, we introduce a truncation-stable notion of $(N_t,E_t)$-martingales via bounded functional-calculus cutoffs $f_n$, and formulate a \emph{Local Informational Efficiency Principle} requiring symmetrically discounted traded prices to be martingales in this localized sense. Assuming the existence of a pricing state $\varphi^\star$ and a compatible family of normal $\varphi^\star$-preserving conditional expectations $(E_t^\star)$, we define for bounded terminal payoffs $X\in M_T$ the dynamic pricing operator \[ Π_t(X):=B_t^{1/2}\,E_t^\star\!\bigl(B_T^{-1/2}XB_T^{-1/2}\bigr)\,B_t^{1/2}, \] where $(B_t)$ is a strictly positive numéraire adapted to $(N_t)$. We prove that $(Π_t)_{t\in[0,T]}$ is normal, completely positive, unital, $N_t$-bimodular, and time-consistent; in the commutative reduction it coincides with classical risk-neutral valuation by conditional expectation. We further develop an $L^2(M,\varphi_ρ)$-prediction theory: $E_t$ acts as the $L^2$-optimal predictor and yields a canonical innovation decomposition. For differentiable parametric families of normal states, we introduce an operator-valued Fisher information relative to $(N_t)$ and derive a noncommutative Cramér--Rao inequality giving a quantitative lower bound on conditional mean-square prediction error under information constraints; the bound is computed explicitly for compound Poisson lattice-jump models under the risk-neutral constraint $\sum_αγ_α(e^{αΔx}-1)=r$.