IBM's Heron processor, released in late 2023 and now the backbone of IBM Quantum's cloud fleet, introduces a fundamentally new coupler architecture that addresses the largest source of error in previous IBM processors: always-on ZZ crosstalk between neighbouring qubits.
"Eagle and Osprey used fixed-frequency transmons with cross-resonance gates. Every qubit was permanently coupled to its neighbours, creating a constant background of ZZ interactions that limited circuit depth. Heron replaces this with tunable couplers that switch the interaction on and off — reducing residual ZZ by more than 100×."
The Tunable Coupler Architecture
Heron uses a direct microwave interaction between fixed-frequency qubits mediated by a tunable coupler element. When the coupler is biased to its operating point, it enables fast two-qubit gates (~100 ns). When idle, the coupling is suppressed to below 1 kHz — compared to 10–100 kHz residual ZZ in previous generations.
| Processor | Year | Qubits | 2Q Gate Error | Residual ZZ |
|---|---|---|---|---|
| Eagle r3 | 2023 | 127 | ~0.3% | ~50 kHz |
| Osprey | 2022 | 433 | ~0.5% | ~80 kHz |
| Heron r1 | 2023 | 133 | ~0.1% | <500 Hz |
| Heron r2 | 2024 | 133 | ~0.05% | <200 Hz |
IBM's 2033 Roadmap
IBM has published an aggressive roadmap targeting a 100,000-qubit "Quantum Centric Supercomputer" by 2033. The architecture uses a modular design where multiple Heron-class chips are connected via quantum communication links, with classical HPC resources tightly integrated for error correction decoding. Key milestones include a 1,000-logical-qubit system by 2028 and demonstration of quantum advantage on a commercially relevant problem by 2026.
Key Takeaways
- Heron's tunable coupler architecture reduces residual ZZ crosstalk by over 100× compared to Eagle, enabling much deeper circuits.
- IBM is pursuing a modular, multi-chip path to scale — rather than building ever-larger monolithic processors.
- The 100,000-qubit target by 2033 is ambitious but grounded in a credible architectural roadmap.
- Heron's 2Q gate fidelity of 99.95% is now competitive with the best trapped-ion processors.