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arXiv:2608.19602

High fidelity control of superconducting qubits with optical transmitted signal

Yu-Huai Li, Daojin Fan, Na Li, Fusheng Chen, Shaowei Li, Dong-Dong Li, Yu Xu, Jin Lin, Ming Gong, He-Liang Huang, Hui Deng, Yulin Wu, Haoran Qian, Shaojun Guo, Futian Liang, Xiaobo Zhu, Cheng-Zhi Peng, Jian-Wei Pan

quant-phSubmitted Aug 20, 2026• Announced Aug 21, 2026• 0 citations
arXivHTMLPDF

Abstract

Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat load constraints. To overcome this issue, we experimentally implemented an optically-assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrated full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency tunable transmon qubits, achieving fidelities of 99.915% $\pm$ 0.005% and 99.676% $\pm$ 0.041%, respectively, which have reached the requirement of the surface code.

Journal reference
Phys. Rev. A 114, 032427 (2026)
DOI
10.1103/m1wg-synr
Comments
11 pages, 7 figures

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Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat load constraints. To overcome this issue, we experimentally implemented an optically-assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrated full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency tunable transmon qubits, achieving fidelities of 99.915% ±\pm± 0.005% and 99.676% ±\pm± 0.041%, respectively, which have reached the requirement of the surface code.
With the rapid development of superconducting quantum computing and the implementation of surface code, large-scale quantum computing is emerging as an urgent demand. In a superconducting computing system, the qubit is maintained in a cryogenic environment to avoid thermal excitation. Thus, the transmission of control signals, which are generated at room temperature, is needed. Typically, the transmission of these signals to the qubit relies on a coaxial cable wiring approach. However, in a large-scale computing system with hundreds or even thousands of qubits, the coaxial cables will pose great space and heat load to the dilution refrigerator. Here, to tackle this problem, we propose and demonstrate a direct-modulation-based optical transmission line. In our experiment, the average single-qubit XEB error and control error are measured as 0.139% and 0.014% separately, demonstrating the feasibility of the optical wiring approach and paving the way for large-scale superconducting quantum computing.
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Quantum computing is a rapidly evolving field with potential for solving complex problems beyond the reach of classical computers. Scalability in qubit design and signal processing architecture within cryostats is necessary for taking advantage of quantum computing's potential. Current state of the art qubit technology requires both a feed and readout line for each qubit within a quantum processor, with these lines generally being metallic coaxial cables. For stainless steel cables this represents a significant heat load into the cryostat. An alternative approach involves using optical fibers for signal transmission, as optical fibers have a thermal conductivity two orders of magnitude lower than stainless steel coaxial cables. The proposed method for optical signal transmission relies on electro-optic modulation, where a voltage applied to an electro-optic material changes its refractive index, allowing for a controllable phase change to be applied to any light passing through the material. This concept can be combined with on-chip photonic waveguide technology to build fiber-coupled phase modulators as well as intensity modulators using on chip interferometers. The proposed architecture uses a room temperature intensity modulator to encode microwave signals onto a laser signal, which is recovered at cryogenic temperatures using a photodiode and used as input for a device under test. The output of the test device is encoded onto light using a phase modulator and recovered at room temperature through interferometry. Proof-of-concept experiments have been carried out at room temperature, including noise characterization and preliminary tests of components at cryogenic temperatures.
This poster proposes an on-chip optical pulse modulation system for scalable superconducting quantum computers. Towards the scalable design, qubit control systems need to be located in the cryogenic environment to reduce the number of interconnections across the room temperature and the cryogenic regions. The cryogenic CMOS-based qubit control system is a promising approach, yet it is a bottleneck that exhausts the cooling capacity of the dilution refrigerator. In order to eliminate this bottleneck while providing the ability to control multiple quibits with a minimum necessary number of interconnections, this poster extends the RoF (Radio-over-Fiber) system where optical signals are directly fed from the Room Temperature (RT) region to a PD (PhotoDetector) mounted near the transmon qubit through low-loss and high-speed fibers. Unlike the conventional RoF system, the proposed system employs cryogenic silicon photonics-based modulators in the intermediate part for executing gate operations individually at each transmon qubit with a minimum necessary number of fibers across the RT and the cryogenic regions. Evaluation results show that the proposed system eliminates the power consumption by 79.5% compared with the cryogenic CMOS-based design. The proposed system achieves comparable gate fidelity to the control system generating fully customized RF pulses in the RT environment.
Direct entangling gates between spatially separated qubits are a key capability for modular quantum computing. They enable nonlocal quantum circuits to be executed across different processor modules, thereby alleviating single-chip scaling constraints and expanding the effective connectivity of the system. Here, we propose and numerically investigate a remote adiabatic controlled-Z gate between two frequency-tunable superconducting qubits connected by a multi-mode coaxial cable. The cable-mediated interaction enables the gate to be implemented without additional tunable couplers, preserving a simple circuit architecture. For a 30-cm cable, simulations incorporating ten standing-wave modes yield an optimized gate infidelity of 4.71×10−74.71\times10^{-7}4.71×10−7 with a duration of 376.7 ns in the absence of decoherence. The gate maintains high fidelity under qubit-frequency fluctuations and millimeter-scale cable-length variations. Calculations for 50- and 100-cm cable situations demonstrate its potential for even larger cryogenic systems. These results provide a promising approach to extending high-fidelity quantum operations beyond individual chips and toward scalable, interconnected superconducting quantum processors.