C3-VQA: Cryogenic Counter-based Co-processor for Variational Quantum Algorithms

TL;DR

This work tackles the thermal and bandwidth bottlenecks in cryogenic quantum computers by introducing C3-VQA, a cryogenic counter-based co-processor built with ultra-low-power SFQ logic that operates at 4 K. By precomputing part of the VQA expectation-value calculations and buffering intermediate results inside the cryostat, C3-VQA dramatically reduces inter-temperature bandwidth, cutting total heat dissipation by up to 30–81% in SS and PS regimes and achieving up to 87% heat reduction in a quantum-chemistry case study with 10,000 qubits. The architecture relies on simple bit operations, MSB-only data transfer, and overlapped non-MSB collection to maintain convergence while minimizing power; PS workloads particularly benefit from these reductions, bridging the gap toward fault-tolerant quantum computing. The paper provides a concrete SFQ-based implementation (1046 JJs, 0.319 mm^2) and power estimates showing feasibility under ERSFQ, with scalable behavior demonstrated for VQE, QAOA, and QML across large qubit counts. Overall, C3-VQA demonstrates a viable path to scalable cryogenic quantum computing by co-locating near-data processing with qubits to alleviate interconnect heat and bandwidth challenges, and suggests applicability to future FTQC-related syndrome data compression and QEC workloads.

Abstract

Cryogenic quantum computers play a leading role in demonstrating quantum advantage. Given the severe constraints on the cooling capacity in cryogenic environments, thermal design is crucial for the scalability of these computers. The sources of heat dissipation include passive inflow via inter-temperature wires and the power consumption of components located in the cryostat, such as wire amplifiers and quantum-classical interfaces. Thus, a critical challenge is to reduce the number of wires by reducing the required inter-temperature bandwidth while maintaining minimal additional power consumption in the cryostat. One solution to address this challenge is near-data processing using ultra-low-power computational logic within the cryostat. Based on the workload analysis and domain-specific system design focused on Variational Quantum Algorithms (VQAs), we propose the Cryogenic Counter-based Co-processor for VQAs (C3-VQA) to enhance the design scalability of cryogenic quantum computers under the thermal constraint. The C3-VQA utilizes single-flux-quantum logic, which is an ultra-low-power superconducting digital circuit that operates at the 4 K environment. The C3-VQA precomputes a part of the expectation value calculations for VQAs and buffers intermediate values using simple bit operation units and counters in the cryostat, thereby reducing the required inter-temperature bandwidth with small additional power consumption. Consequently, the C3-VQA reduces the number of wires, leading to a reduction in the total heat dissipation in the cryostat. Our evaluation shows that the C3-VQA reduces the total heat dissipation at the 4 K stage by 30% and 81% under sequential-shot and parallel-shot execution scenarios, respectively. Furthermore, a case study in quantum chemistry shows that the C3-VQA reduces total heat dissipation by 87% with a 10,000-qubit system.

Figures (2)

• Figure 1: (a) Overview of SFQ implementation of our architecture for VQE, QAOA, and QML. (b) Dot product and modulo arithmetic unit and counter. (c) to (f) Circuit diagram and time chart of each SFQ subcircuit.
• Figure 2: Required bandwidth (top) and power dissipation (middle, solid line) of the baseline and C3-VQA architecture are shown for (a) SS and (b) PS scenarios. The bottom plots show the execution time of C3-VQA normalized to that of the baseline.