We report a technique for executing quantum computing protocols that are inspired by photonic matrix multiplication and exploits the inherent tensor product structure of transverse spatial modes.
Optical computing harnesses the speed of light to perform vector-matrix operations efficiently. It leverages interference, a cornerstone of quantum computing algorithms, to enable parallel computations. In this work, we interweave quantum computing with classical structured light by formulating the process of photonic matrix multiplication using quantum mechanical principles such as state superposition and subsequently demonstrate a well-known algorithm, namely, Deutsch–Jozsa’s algorithm. This is accomplished by elucidating the inherent tensor product structure within the Cartesian transverse degrees of freedom of light, which is the main resource for optical vector-matrix multiplication. To this end, we establish a discrete basis using localized Gaussian modes arranged in a lattice formation and demonstrate the operation of a Hadamard gate. Leveraging the reprogrammable and digital capabilities of spatial light modulators, coupled with Fourier transforms by lenses, our approach proves adaptable to various algorithms. Therefore, our work advances the use of structured light for quantum information processing.
Linear optical computing has emerged as a promising paradigm to bypass the thermodynamic constraints of von Neumann architectures and the Landauer dissipation limit (Emin = kBT ln2). However, existing integrated photonic accelerators remain fundamentally Markovian: they are restricted to commutative matrix-vector multiplications that systematically erase computational history during state aggregation. Here, we present the hardware architecture and operational principles of the Photonic Causal Tensor Processor (PCTP). Grounded in Causal Quaternionic Field Theory (CQFT), the PCTP natively executes the non-commutative Causal Tensor Product (⊠) governed by a rigorous chronological index (τ). We demonstrate how the four algebraic components of the quaternion (qa, qi, qj, qk) are directly mapped onto physical, orthogonal degrees of freedom of coherent optical wavepackets: real amplitude and longitudinal carrier phase, polarization states on the Poincaré sphere, and transverse orbital angular momentum (OAM) modes. We provide the physical layout of non-reciprocal causal logic gates utilizing low-loss Silicon Nitride (Si3N4) waveguides, magneto-optic Bismuth Iron Garnet (BIG) Faraday isolators, and synthetic diamond nitrogen-vacancy (NV) phase latches. The proposed processor executes deterministic causal tensor algebra at the speed of light with near-zero thermodynamic entropy generation, establishing an empirical hardware framework for post-quantum computing.
It is possible to prepare classical optical beams which cannot be characterized by a tensor product of vectors describing each of their degrees of freedom. Here we report the experimental creation of such a nonseparable, tripartite GHZ-like state of path, polarization, and transverse modes of a classical laser beam. We use a Mach-Zehnder interferometer with an additional mirror and other optical elements to perform measurements that violate Mermin's inequality. This demonstration of a classical optical analogue of tripartite entanglement paves the path to novel optical applications inspired by multipartite quantum information protocols.