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arXiv:2511.22920

CO-QLink: Cryogenic Optical Link for Scalable Quantum Computing Systems and High-Performance Cryogenic Computing Systems

Zheng Chang, Siqi Zhang, Wenqiang Huang, Tian Tian, Qichun Liu, Tiefu Li, Nan Qi, Yuanjin Zheng, Zhihua Wang, Yanshu Guo, Hanjun Jiang

quant-pheess.SPSubmitted Nov 28, 2025• Announced Dec 1, 2025• 0 citations
arXivHTMLPDF

Abstract

Cryogenic systems necessitate extensive data transmission between room-temperature and cryogenic environments, as well as within the cryogenic temperature domain. High-speed, low-power data transmission is pivotal to enabling the deployment of larger-scale cryogenic systems, including the scalable quantum computing systems and the high-performance cryogenic computing systems fully immersed in liquid nitrogen. In contrast to wireline and microwave links, optical communication links are emerging as a solution characterized by high data rates, high energy efficiency, low signal attenuation, absence of thermal conduction, and superior scalability. In this work, a 4K heat-insulated high-speed (56Gbps) low-power (1.6pJ/b) transceiver (TRX) that achieves a complete link between 4K systems and room temperature (RT) equipment is presented. Copackaged with a PIN photodiode (PD), the RX uses an inverter-based analog front-end and an analog half-rate clock data recovery loop. Connecting to a Mach-Zehnder modulator (MZM), the TX contains a voltage-mode driver with current-mode injection for low-power output-swing-boosting and 3-tap feed-forward equalization (FFE). This link has been demonstrated in the control and readout of a complete superconducting quantum computing system.

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Cryogenic systems necessitate extensive data transmission between room-temperature and cryogenic environments, as well as within the cryogenic temperature domain. High-speed, low-power data transmission is pivotal to enabling the deployment of larger-scale cryogenic systems, including the scalable quantum computing systems and the high-performance cryogenic computing systems fully immersed in liquid nitrogen. In contrast to wireline and microwave links, optical communication links are emerging as a solution characterized by high data rates, high energy efficiency, low signal attenuation, absence of thermal conduction, and superior scalability. In this work, a 4K heat-insulated high-speed (56Gbps) low-power (1.6pJ/b) transceiver (TRX) that achieves a complete link between 4K systems and room temperature (RT) equipment is presented. Copackaged with a PIN photodiode (PD), the RX uses an inverter-based analog front-end and an analog half-rate clock data recovery loop. Connecting to a Mach-Zehnder modulator (MZM), the TX contains a voltage-mode driver with current-mode injection for low-power output-swing-boosting and 3-tap feed-forward equalization (FFE). This link has been demonstrated in the control and readout of a complete superconducting quantum computing system.
In recent years, standard CMOS microprocessors have approached their maximum power dissipation per unit area, effectively placing a limit on computational power. This highlights the urgent need to explore alternative technologies. One promising avenue is the use of superconductors, which demonstrate zero resistivity below a critical temperature. However, circuits based on superconductors necessitate the use of cryostats to maintain low temperatures, presenting challenges in data transfer with the room temperature environment. While coaxial cables are often employed for this purpose, they suffer from limited data transfer rates and contribute significantly to heat load. On the contrary, photonics integrated circuits (PICs) coupled with optical fibers present a viable solution. They enable scalable, cost-effective, and power-efficient optical interconnections capable of supporting high data transfer rates while minimizing heat transfer. In this presentation, We will discuss the latest advancements in cryogenic PICs, focusing on their application in interfacing with cryogenic computing systems such as single-flux-quantum logic circuits and superconducting qubits.
High-speed digital data links from 4-K superconductor electronics to room temperature are challenging due to the fact that energy/bit for single flux quantum logic is many (six) orders of magnitude lower than that of standard room-temperature logic. Our approach of building an energy-efficient high-speed data link involves a joint electrical-thermal design of a temperature-distributed architecture using different electronic technologies. This differential digital data link design involves superconductor, multiple cryogenic semiconductor and additional room-temperature semiconductor circuitry. The current design involves three cryogenic semiconductor ICs for integration with a multistage cryocooled digital system. The first cryogenic semiconductor IC is designed to operate at 4 K with a power consumption of 0.3 mW and interface directly with the superconductor differential single flux quantum/dc drivers. For testing it, a superconductor carrier chip containing an analog-to-digital converter has been designed for the HYPRES dual-JcJ_{c}Jc​(4.5 and 20kA/cm2\hbox{kA/cm}^{2}kA/cm2) fabrication process.
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.
Superconducting quantum computing has the potential to revolutionize computational capabilities. However, scaling up large quantum processors is limited by the cumbersome and heat-conductive electronic cables that connect room-temperature control electronics to quantum processors, leading to significant signal attenuation. Optical fibers provide a promising solution, but their use has been restricted to controlling simple two-level quantum systems over short distances. Here, we demonstrate optical control of a bosonic quantum processor, achieving universal operations on the joint Hilbert space of a transmon qubit and a storage cavity. Using an array of cryogenic fiber-integrated uni-traveling-carrier photodiodes, we prepare Fock states containing up to ten photons. Additionally, remote control of bosonic modes over a transmission distance of 15 km has been achieved, with fidelities exceeding 95%. The combination of high-dimensional quantum control, multi-channel operation, and long-distance transmission addresses the key requirements for scaling superconducting quantum computers and enables architectures for distributed quantum data centers.
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.
We develop an energy efficient digital data link connecting cryogenic superconducting single flux quantum (SFQ) circuits to room-temperature electronics. The design is based on low-temperature (4 K) superconductor ERSFQ SFQ/dc drivers, high-temperature superconductor data cables spanning 4 K to 70 K temperature stages, mid-temperature (70 K) polarization modulation vertical cavity surface emitting lasers (PM VCSELs), and fiber optic links to room temperature electronics. The Energy-efficient Cryogenic Optical (ECO) data link design is based on balancing power dissipation and signal gain at each temperature stage to maximize overall energy efficiency following the recently introduced Thermo-Gain Rule. To achieve VCSEL light emission with two switchable distinct polarization modes, a cruciform-shaped anisotropic optical cavity is formed by fabrication of a photonic crystal with etched periodic air holes surrounding the unetched cruciform region. In this report, we present the results of design, fabrication, and preliminary testing of the ECO data link components.