While software development and error correction rightfully receive most of the attention in quantum development today, hardware advances have hardly stopped. IBM showcased hardware milestones and refreshed QPU plans at its Quantum Developer Conference (QDC24) held two weeks ago. (Most of QDC24 dwelled on software tools and practices)
As part of IBM’s annual State of IBM Quantum address, Jerry Chow, IBM Fellow and director of quantum infrastructure, reviewed progress around Condor, Heron, Flamingo, Crossbill and Kookaburra platforms as well as introduction of Starling, a cryo-controller. Much of the hardware discussion centered on coupler technology. Broadly speaking, the entire quantum computer development community has embraced the idea that ganging together multiple chips (QPUs) will be necessary to adequately scale up quantum systems.
But before diving into couplers and future plans, Chow reviewed work done with its Heron QPU, introduced with 133 qubits last year and since upgraded to 156 qubits. It was introduced as the first step in IBM roadmap for building systems from multiple QPUs.
Chow said, “We made a really big deal about Heron last year, because it is a really big deal, but also I think it needs to be repeated again, because we’ve actually brought in so much more. Now the latest revision of Heron is the second one. We’ve actually managed to increase the number of qubits from 133 qubits to 156. We still preserve the heavy hex architecture, and we’re using the tunable couplers, which allows us to suppress our crosstalk errors down to the point 1% level.”
He singled out improvements made in Heron including an integrated two-level error mitigation controls and improvements to IBM calibrates the tunable couplers underlying the gate architecture. One result, said Chow, was better performance of our two-qubit gates given the coherence times available.
“Back in August, we updated Reno, our first Heron device, improving its median two-qubit gate performance from 5e minus three to 3e minus three. Now these new calibrations are actually typical across all of our new Heron deployments. Here is our fleet of Herons: three online right now, one just released right today, IBM Marrakesh, and a fourth one that’s coming into our EU Data Center shortly.”
Recently, IBM also added fractional one- and two-qubit gates, implementing gates with arbitrary angles. Previously doing this required compiling down into multiple gates. “With fractional gates, now we can actually run this natively with just a single gate.” There are many benefits. Chow cited improved circuit compiling and better user experience with dynamic circuits. “As we make modifications in the future, we’re going to be allowing for better, efficient parallel execution of conditional blocks that’s coming early next year,” he said.
As you have probably guessed, IBM names its QPU device generations after birds. Heron and Condor (a giant 1100-qubit device), both introduced last year, followed Eagle, which is still a mainstream device in IBM’s network. Condor, says IBM, was intended more as a demonstration of manufacturing expertise required to reach such a large size and the coincident introduction of Heron represented a fork in its approach and the need to develop smaller partition-able systems and technologies need for them.
Part of Heron’s role was to demonstrate IBM’s advancing coupler technology. IBM is working on three types of couplers — C-couplers that enable long-range on-chip connections for high-rate LDPC codes; L-couplers that enable joining logical memories to create large-scale systems; and M-couplers that enable multiple short-range chip-to-chip connections.
The centerpiece hardware innovation presented at QDC24 was work on Flamingo QPUs (156 qubits) and its advancing coupler and packaging technology.
“Last year, we told you that we’d start developing new couplers that actually run gates across multiple chips. Now I’m excited to report on those innovations,” said Chow. “Here you can see Flamingo connects two devices with four connectors measuring up to a meter long. We’ve demonstrated the technology for a high quality cable. But maybe what’s more important is we’ve shown ability to actually have a pluggable and re-pluggable connector to bridge across these devices and to really take this outside of just a research demonstration [and] towards a more confident deployable technology later next year. The L-couplers allow us to have modular system design to avoid wiring limitations, and it lets us better use the space within cryogenic infrastructure,” said Chow.
IBM is in the midst of testing the new technology. “We’ve benchmarked a distance C NOT gate between data qubits connected by an L-coupler with 96.5% fidelity on a test device in a loopback configuration. Now we’ve integrated that technology with Flamingo, and have already started to demonstrate transfer of excitation between qubits back and forth on separate Flamingo payloads connected by about a meter long cable. So all this gives us the confidence and learning to really promote this up towards deployment late next year,” said Chow.
Turning to M-coupler technology, Chow reviewed the Condor (1121-qubit) single chip.
“Condor was an experiment for us in scaling and yield, but it happened in a monolithic sense. The chip was about the size of a business card, and the package was actually about the size of a coffee table book. It certainly pushed the limits of what we could scale. But we also know that this type of monolithic scaling has an ultimate limit, and so that’s why we’re also introducing our latest innovations with M-couplers in Crossbill. Now M-couplers are interconnected between chips, in this case, actually across three by 160-qubit chips connected into a single package for. Total of 480 qubits. You add in all the tunable couplers of this architecture means we actually have over 1000 total quantum elements, which is the same complexity as Condor, but built in this modular fashion,” said Chow.
He showed (slide below) a side by side between Crossbill and Condor with the integrated connectors, noting much smaller the package for Crossbill is compared to Condor. “So I’m excited to say, with all the prototype innovations demonstrated, we’re entering assembly of Crossbill for the next phase of testing,” he said.
Chow noted that IBM had ‘made the cover of Nature” by demonstrating a new kind of error correction code, which reduced the number of qubits required for error correction, but which requires a lot of new, detailed hardware innovation. “First, it requires our qubits to have higher connectivity than any previous IBM Quantum chip,” he said. “So this year, we actually demonstrated six-way couplers. Now we can actually begin to connect the qubit to twice as many qubits without loss of gate fidelity and without errors from neighboring spectator qubits. We’re going to combine this with continued improvements to t1 and t2 to really achieve the error rate thresholds that are going to be needed by our quantum LDPC error correcting codes.”
The other advancement needed, said Chow, are C-couplers because of the type of connectivity required by the LDPC codes. “We need connectors that go outside of the plane in order to create more complex qubit graphs. We’re getting there with these low-loss buried interconnect technologies. So we’re expanding the full packaging technology that we’ve shown previously to have the ability to connect these C-couplers.” These technologies will be used with Kookaburra for demonstrating error correction.
Last, at least on the pure-play hardware front, IBM discussed Starling, a new cryo-controller chip due in 2028 on IBM’s roadmap. “We’re excited to show off a new CMOS controller that actually lives inside the dilution refrigerator. These are actual chips in the fridge, controlling two-qubit tunable coupled gates down to the 1e minus three level. And in fact, we are able to scale out a single chip out to a parallel set of 16 control lines.”
There was a good deal more on IBM hardware and a great deal more about software, tools, and practices in the IBM State of Quantum address and throughout QCD24. IBM has recordings from several sessions online.
Link to IBM State of Quantum: https://www.ibm.com/quantum
QDC24 Quick Hit: IBM Plows Ahead with Quantum Hardware Development
