Superconducting processor based on single flux quantum (SFQ) technology is a promising solution for cryogenic computing with high-speed operation at extremely low power. However, energy-efficient data link from$\sim 4\text{K}$operation to room temperature (RT) users has been remained as a key challenger to be resolved. In this work, we report the 5K performance of 51.56 Gb/s PAM-4 optical data link from a VCSEL, coupled by free-space optics inside cryostat to RT receiver through cryostat optical window. Finally, a superconducting bit pattern generator (BPG) is used to modulate a VCSEL at 4 K: 12.5 Gb/s NRZ error-free optical data transmission and 20 Gb/s NRZ open eye-diagrams are demonstrated via an OM4 fiber coupled directly to the VCSEL at 4 K inside cryostat and linked to the outside receiver at RT.
Superconducting processor based on single flux quantum (SFQ) technology is a promising solution for cryogenic computing with high-speed operation at extremely low power. However, energy-efficient data link from∼4Koperation to room temperature (RT) users has been remained as a key challenger to be resolved. In this work, we report the 5K performance of 51.56 Gb/s PAM-4 optical data link from a VCSEL, coupled by free-space optics inside cryostat to RT receiver through cryostat optical window. Finally, a superconducting bit pattern generator (BPG) is used to modulate a VCSEL at 4 K: 12.5 Gb/s NRZ error-free optical data transmission and 20 Gb/s NRZ open eye-diagrams are demonstrated via an OM4 fiber coupled directly to the VCSEL at 4 K inside cryostat and linked to the outside receiver at RT.
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.
The development of quantum computing is progressing rapidly, but there are still important technological and scaling challenges to overcome to reach the expected potential. Generally speaking, one of the main challenges of scaling quantum computers arises from the delicacy of the quantum states. For example, the fact that superconducting-based quantum processors need to be connected to the ambient environment with high-speed (microwave regime) signals, brings in scaling limitations related to the ability of preserving very low operation temperatures [1]. An important step in overcoming this challenge would be replacing traditional microwave connectors between the outside environment and the mK-range processors with optical fibers. This would simultaneously increase the bandwidth available and drastically reduce the thermal load due to heating by minimizing the number of thermal bridges to outside environment [2]. Another intermediate solution is to interface quantum processes with a superconducting processor based on single flux quantum (SFQ) technology, operating at more relaxed cryogenic conditions (4 K instead of mK range) [3]. Such hybrid solution would also need to have bidirectional communication links to external environments, and an optical link exploiting efficient light sources operating at cryogenic temperatures appears to be instrumental to it.
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.
Superconductor single-flux-quantum (SFQ) logic family has been recognized as a promising technology for cryogenic applications (e.g., quantum computing, astronomy, metrology) thanks to its ultra-fast and low-energy characteristics. Therefore, recent efforts in SFQ-based computing have focused on developing fast and low-power SFQ processors for cryogenic applications. However, there still has been little progress toward a convincing SFQ processor design due to the critical performance challenges originating from its extremely deep pipeline. In this paper, we propose a super-fast and low-power in-order SFQ processor by tackling the challenges from the deep pipeline. First, we develop a minimal-depth SFQ processor pipeline with novel architecture-level ideas. Next, we conduct in-depth performance analyses and identify three real performance bottlenecks in the deeply pipelined SFQ processors (i.e., stall/flush logic, RAW stall, fetch unit). Finally, we propose SuperCore, our super-fast SFQ-based processor architecture, with three SFQ-friendly solutions that effectively resolve the identified bottlenecks. With our solutions applied, SuperCore achieves 11 times speed-up over the SFQ processor baseline. In addition, SuperCore achieves six times speed-up and consumes up to 193 times less power compared to in-order CMOS processors running at 4K.
Scalable, high speed data transfer between cryogenic (0.1-4 K) and room temperature environments is instrumental in a broad range of fields including quantum computing, superconducting electronics, single photon imaging and space-based communications. A promising approach to overcome the limitations of conventional wire-based readout is the use of optical fiber communication. Optical fiber presents a 100-1,000x lower heat load than conventional electrical wiring, relaxing the requirements for thermal anchoring, and is also immune to electromagnetic interference, which allows routing of sensitive signals with improved robustness to noise and crosstalk. Most importantly, optical fibers allow for very high bandwidth densities (in the Tbps/mm2 range) by carrying multiple signals through the same physical fiber (Wavelength Division Multiplexing, WDM). Here, we demonstrate for the first time optical readout of a superconducting nanowire single-photon detector (SNSPD) directly coupled to a CMOS photonic modulator, without the need for an interfacing device. By operating the modulator in the forward bias regime at a temperature of 3.6 K, we achieve very high modulation efficiency (1000-10,000 pm/V) and a low input impedance of 500 Ω with a low power dissipation of 40 μW. This allows us to obtain optical modulation with the low, millivolt-level signal generated by the SNSPD. Optical communication is becoming the preferred I/O solution in modern, room temperature high performance systems, and this work demonstrates its suitability for scalable cryogenic readout, which could help achieve the full potential of superconducting technologies.