These unintended interactions between qubits, even when idle, can introduce errors that accumulate during computation, impacting the fidelity of results. “In superconducting processors, ZZ crosstalk is one of the most significant sources of systematic errors,” the company reports, as physicists work to characterize and mitigate these effects through techniques like decoupling and pulse sequences. Understanding these interactions is essential for calibrating quantum gates and improving performance as processors grow in complexity.
Joint-Ramsey Experiments: Pairwise Characterization Limitations
Conventional methods for characterizing interactions between qubits struggle to keep pace with increasing processor sizes, as the number of required experiments grows quadratically with the number of qubits. Horizon Quantum Computing addressed this limitation by applying multiplexed protocols to joint-Ramsey experiments, a gold standard for ZZ crosstalk measurement, and demonstrated a significant reduction in measurement settings on a 100-qubit processor, the company says.
The company’s approach cut the number of settings from approximately 5,000, typical of pairwise measurements, to just 7, representing a more than 700-fold reduction in complexity. This scaling advantage stems from a logarithmic relationship between measurement settings and qubit count, promising to maintain efficiency even as quantum processors expand to the million-qubit scale and beyond.
The logarithmic scaling of Horizon Quantum Computing’s protocol offers a path toward characterizing even extremely large processors with a manageable number of measurements. A million-qubit processor would require only 20 settings, while a billion-qubit processor would need just 30. Beyond reducing the sheer number of measurements, this approach also dramatically decreases the time required for characterization, a critical factor given that processors are unavailable for computation during these diagnostic procedures.
On a 1,000-qubit chip with 100-microsecond coherence times, conventional pairwise methods would take approximately 694 hours to characterize ZZ crosstalk using 10,000 shots per joint-Ramsey experiment, but Horizon Quantum Computing’s multiplexed approach completed the same task in just 50 seconds, with similar precision, according to the company. Recently, the team implemented their multiplexed processor characterization on the Rigetti Cepheus-1-108Q processor to validate its accuracy against conventional pairwise measurements. Randomly selected qubit pairs were subjected to both methods, allowing for a direct comparison of results.
While the multiplexed protocol proved effective, the analysis revealed limitations when dealing with qubits exhibiting anomalously low coherence times or high readout errors. “For qubits with anomalously low coherence time or high readout error, there is significant uncertainty in the computed coupling from both direct or multiplexed experiments, leading to some qubits appearing to have many strong long-range interactions,” the team reports. These problematic qubits introduce uncertainty into the computed coupling, potentially leading to the appearance of spurious long-range interactions, and require further investigation to refine the characterization process.
Horizon Quantum Computing’s Logarithmic Multiplexing Protocol
Horizon Quantum Computing’s approach to characterizing quantum processors reduces the computational burden by using logarithmic scaling, enabling analysis of larger systems with fewer measurement settings. This efficiency stems from combining multiple characterization experiments into a single run, minimizing the overall time and resources needed to map out qubit interactions.
The company reports that this method allows for a more than 700-fold reduction in settings for a 100-qubit processor, directly addressing a key bottleneck in scaling quantum hardware. The benefits of this logarithmic scaling become even more pronounced as qubit counts increase, promising to enable characterization of processors far beyond current capabilities.
Beyond the reduction in settings, the protocol also dramatically decreases the time needed for analysis, freeing up valuable processor time for actual computations while the characterization is underway. To validate the results, the researchers performed direct pairwise measurements on randomly selected qubit pairs, confirming strong agreement between the multiplexed protocol and the standard method. The team’s work highlights the importance of software-driven characterization in advancing quantum hardware performance, offering a pathway to more efficient and scalable processor analysis.
“Advancing quantum hardware performance through software-driven characterization,” is how Horizon Quantum Computing frames the work, emphasizing the role of innovative algorithms in overcoming the challenges of building larger and more complex quantum systems. The company’s multiplexed protocol demonstrates how carefully designed software can help characterize and optimize quantum processors.
Qubit Processor Validation with Reduced Measurement Settings
Characterizing crosstalk, unwanted interactions between qubits, is essential to ensuring accurate gate operations and developing control strategies that minimize unwanted interactions as a processor scales. The company’s approach shifts from assessing each qubit pair individually to a multiplexed method where the number of required measurement settings scales logarithmically with qubit count, a significant departure from conventional techniques. This allows for a far more efficient assessment of processor-wide interactions, important for mitigating errors and improving fidelity.
The conventional method of assessing qubit interactions, such as through T1 and T2 characterization, provides precise data but becomes impractical at scale, demanding resources that grow with the square of the number of qubits. The protocol addresses this limitation by combining multiple characterization experiments into fewer runs, reducing the burden on quantum hardware and accelerating the validation process.
By gaining a better understanding of where these interactions are occurring, physicists can use decoupling techniques to mitigate the effects of crosstalk and reduce errors. The team demonstrated that crosstalk, including both ZZ and control crosstalk, can occur not only between neighboring qubits but also between more distant qubits across the processor, the company says.
They also found that applying an echoing sequence of pulses can significantly reduce ZZ crosstalk by dividing the processor into groups of qubits with maximal coupling. The protocol allows for the characterization of interactions between multiple pairs in a single experiment if they share a common driven qubit, streamlining the diagnostic process. The ability to characterize crosstalk efficiently is vital for improving quantum hardware performance, and by identifying and mitigating these unwanted interactions, physicists can enhance the fidelity of computations and reduce the accumulation of errors.
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