Classical Control Systems for Photonic Quantum Computing

John R. Scott, Krishna C. Balram

Most of the architectural research on photonic implementations of measurement-based quantum computing (MBQC) has focused on the quantum resources involved in the problem with the implicit assumption that these will provide the main constraints on system scaling. However, the ‘flying-qubit’ architecture of photonic MBQC requires specific timing constraints that need to be met by the classical control system. This classical control includes, for example: the amplification of the signals from single-photon detectors to voltage levels compatible with digital systems; the implementation of a control system which converts measurement outcomes into basis settings for measuring subsequent cluster qubits, in accordance with the quantum algorithm being implemented; and the digital-to-analog converter (DAC) and amplifier systems required to set these measurement bases using a fast phase modulator. In this paper, we analyze the digital system needed to implement arbitrary one-qubit rotations and controlled-NOT (CNOT) gates in discrete-variable photonic MBQC, in the presence of an ideal cluster state generator, with the main aim of understanding the timing constraints imposed by the digital logic on the analog system and quantum hardware. We have verified that the design works using functional simulations, and have used static timing analysis of a Xilinx FPGA (7 series) to provide a practical upper bound on the speed at which the adaptive measurement processing can be performed, in turn constraining the photonic clock rate of the system. The design and testing system is freely available, for use as the basis of analysis of more complex designs, incorporating more recent proposals for photonic quantum computing.} Our work points to the importance of co-designing the classical control system in tandem with the quantum system in order to meet the challenging specifications of a photonic quantum computer.

Zoltán Kolarovszki

John R. Scott, Krishna C. Balram

Understanding the computational overheads imposed by classical control systems on quantum computing platforms becomes critically important as these quantum machines grow in scale and complexity. In this work, we calculate the overheads imposed by the implementation of real-time graph traversal algorithms needed to find computational paths through incomplete cluster states for the implementation of one-qubit gates; a necessary requirement for a realistic implementation of photonic measurement-based quantum computing. By implementing two different algorithms, a global breadth-first search that searches the entire cluster state and an incremental version that traverses a narrow subsection of the cluster state, we analyze the trade-off between the accuracy of finding viable paths and the speed at which this operation can be performed, which constrains the overall photonic clock cycle of the system. We also outline the broader implications of our results for implementing classical control systems for measurement-based photonic quantum computing.

Abhilash Amsanpally, Maeva Franco, A. Perrin +3 more

Yaxuan Wang, Yizhi Wang, Dongyang Wang +1 more

Causal theory provides a consistent framework for describing relations among events or variables, and has recently been applied to quantum systems. Here, we apply causal analysis to the remote-controlled quantum computing model and uncover a bidirectional causal structure rooted in quantum entanglement, which inherently admits a counterintuitive causal component. Unlike the unidirectional causal structure in classical computing, this framework allows the quantum operation to precede-and causally influence-the parameters that nominally control it, thereby exhibiting counterintuitive causal behavior. We experimentally verify this phenomenon on two photonic chips connected by two 15-meter fibers, achieving an average fidelity of 90.43±2.42% over 100 single-qubit unitaries. Timing analysis further shows that, in our setup, the control parameter can theoretically lag behind the operation by 129.14 ns. To our knowledge, this is the first systematic use of causal modeling to analyze and experimentally reveal the non-classical causal features induced by quantum entanglement in a quantum computing setting, with implications for both quantum computing and causal inference theory.

R. B. Dalton, Geoff Gillett, Geoff J. Pryde +4 more