Modular quantum computing takes shape with l-couplers

Key takeaways

• IBM researchers connected two IBM Quantum Nighthawk processors using four meter-scale l-couplers.
• The work achieved quantum state transfer fidelities of 99%, demonstrating reliable transmission of quantum information between processors.
• The new inter-chip connection design is compatible with IBM Quantum Nighthawk and can scale to IBM Quantum Starling and beyond.
• High-fidelity l-couplers provide a key building block for modular quantum computers.

For the first time, superconducting quantum processors have been connected using multiple meter-scale quantum channels with an average quantum state transfer fidelity of 99%. This research demonstrates a critical milestone for scaling quantum computers into the era of fault tolerance.

Previous work from IBM and other research outfits featured high-quality proof-of-concept demonstrations of quantum operations between connected modules. However, those approaches were not readily compatible with implementing multiple l-coupler connections between large superconducting qubit processors. Our recently published work introduces a new design for inter-chip connections that are compatible with IBM Quantum Nighthawk and can scale to the fault-tolerant IBM Quantum Starling computer and beyond.

How will quantum computers scale beyond a single chip?

Realizing the full potential of quantum computing requires us to scale to much larger quantum systems than exist today, a path that won't rely solely on building larger chips. Instead, we have determined that the future of quantum computing depends on many processors working together with information that can move between chips.

Modular quantum computers therefore consist of individual quantum processors joined by meter-scale interconnects inside of modular cryogenics—dilution fridges that scale but share a cryogenic environment. For IBM superconducting quantum processors, we call these long-range interconnects l-couplers, versus on-chip c-couplers. Meanwhile, earlier this year, we debuted our new modular cryogenic platform for scaling quantum computers inside of which these connected processors will sit.

Four aluminum l-couplers connect IBM quantum processor payloads inside a dilution refrigerator. The cables run between the front and rear payload assemblies to enable quantum information transfer between processors. (Credit: IBM)

What does the l-coupler work demonstrate?

A variety of previous papers from quantum hardware outfits in the industry have demonstrated a variety of connections between different quantum processors. However these methods are not compatible with how large-scale quantum processors are made and assembled. But now, we’re demonstrating reliable high-quality l-coupler connections capable of transmitting quantum information between quantum processors with low error rates. This is a critical component of the quantum computing architecture that truly unlocks scalability. With connected processors, we can run more powerful error correction codes and larger quantum algorithms across higher numbers of entangled qubits.

IBM research scientists work on hardware used to test l-couplers between IBM Quantum Nighthawk processors. The system includes four l-couplers and custom hardware designed to enable l-coupler installation after processor payloads are mounted inside a cryostat. (Credit: IBM)

Our l-couplers encompass all components required for inter-module quantum operation. This includes the aluminum coaxial cable connected to the associated components on the chip, the simplified “l-qubits” it connects to, the tunable-couplers that connect the l-qubits with the cables, and associated controllers. L-couplers are connected after the processors are mounted in the dilution refrigerator.

This work specifically demonstrates the connection between two IBM Quantum Nighthawk R1 processors with four l-couplers, each one with an 80-centimeter cable, linking the two chips inside of a dilution fridge. While the experiment wired out only a subset of the qubits for development purposes, the processor chips themselves are full Nighthawk processors.

We report l-coupler modes with internal quality factors of 940,000, where quality factor is a dimensionless number representing a systems ability to store energy—higher is better, and quality factors on the order of 10^6 are appropriate for transfer of superconducting quantum information. We benchmarked quantum state transfer fidelities of up to 99.3%, with an average fidelity of 99%. In other words, the quantum state is correctly transferred over the cable 99% of the time on average.

Finally, we used distant CZ gates constructed by transferring states across the different modules, achieving a two-qubit gate fidelity of 94.75% averaged over the four links. This distant CZ gate allowed us to generate entangled states shared between the two modules, including demonstrating of using two L-couplers simultaneously to generate independent entangled states.

Why is this paper important? or What does this work advance?

Building a scalable, modular quantum computer requires high-fidelity links between individual processing units located within a cryogenic cell or between adjacent cryogenic cells. However, these links must be compatible with scaling toward multiple connections in order to extend this technology to larger enterprise-ready use cases.

As far as we know, this is the first time that superconducting quantum processors have been connected using multiple quantum channels with an average quantum state transfer fidelity of 99%. This work establishes our technique for implementing l-couplers as a viable path forward for scaling quantum computers to the fault-tolerant regime.

What's next?

Our roadmap projects that quantum will scale at an accelerating pace. A major focus for our hardware will be implementing error correction codes which will require meter-scale couplers between neighboring chips. Therefore, we anticipate improving the yield and reproducibility of our l-couplers, reducing their error rates, and scaling to more l-couplers that can easily be plugged and unplugged between larger and more complex processors—as we build connected Nighthawks and work toward IBM Quantum Starling.

We have room to improve all of the metrics reported in our recent paper, and are aware of the steps required to make those improvements. We will continue performing research and development as we work to integrate the couplers into a larger system running more advanced workloads.

Our agile process for hardware development and quick cycles of learning continue to yield critical advances in the lab and translate them to production processes. All told, this work propels IBM further along our roadmap toward scalable, modular fault-tolerant quantum computing.