Hardware-Efficient Continuous-Variable Quantum Computation: Finite-$N$ Non-Gaussian Gates in a Floquet-Engineered Cavity-BEC

Abstract

Continuous-variable (CV) quantum computation in bosonic modes offers a hardware-efficient route toward fault tolerance, yet synthesizing deterministic, macroscopic non-Gaussian gates with Wigner negativity remains challenging. Here, we establish two physical breakthroughs and two tensor-network innovations using a single-cavity 87Rb Bose-Einstein condensate (BEC) as a macroscopic bosonic logical mode. Physically, building on our orthogonal optical dual-driving architecture, we propose deterministic, unitary Floquet-engineered Two-Axis Twisting (TAT) non-Gaussian gates without post-selection, and introduce a paradigm shift from quantum metrology to CV quantum computation: via a normal-ordered Holstein-Primakoff expansion and non-perturbative spin simulations, we show that the over-squeezed regime ($t>t_{opt}$, where Wineland squeezing ${\xi }_R^2$ degrades) harnesses finite-$N$ Bloch-sphere curvature - manifesting at $O(1/S)$ as cubic shearing and quartic Kerr $(a^{†}a{)}^2$ interactions - to deterministically generate Wigner negativity. Methodologically, we formulate an exact automaton Matrix Product Operator in the compact SU(2) spin basis with an $N$-independent Liouvillian bond dimension ($D_L=6$), bypassing Fock cutoff errors ($N_{max}$) and $O(N^3)$ permutational-invariant scaling. Finally, we decouple $4^N$-dimensional Lindblad evolution from collective observation via an Exact Bilateral Dynamic Programming Dicke Projector with log-gamma normalization, extracting the $(N+1)\times (N+1)$ symmetric projection ${\tilde{\rho }}_{n_L,n_R}$ without moment truncation while tracking symmetry retention $p_{sym}(t)$. Our proof-of-principle $N=50$ benchmark resolves deep Wigner negativity ($W_{min}\approx -0.214$ at $t=2.00\mu$s, after ${\xi }_R^2\approx 0.124$ at $t_{opt}\approx 0.64\mu$s), establishing a scalable architecture for $N\sim 10^3-10^5$ macroscopic BEC processors.