Researchers at MIT and Lincoln Laboratory have developed a technique to identify and measure a subtle source of error in superconducting quantum circuits, known as second-order harmonic corrections. These distortions can cause quantum circuits to deviate from their expected behavior, limiting the performance and scalability of quantum computers.
Superconducting quantum computers rely on delicate circuits that use Josephson junctions—components where two superconducting wires are separated by a thin barrier. In these junctions, charge is transported by paired electrons called Cooper pairs, which typically tunnel through the barrier one pair at a time. This controlled single-pair tunneling is fundamental to quantum computation.
However, the MIT team found that sometimes two Cooper pairs can tunnel simultaneously, a phenomenon termed second-order harmonic correction. This effect is not accounted for in existing quantum circuit designs, leading to circuit underperformance.
Measuring and pinpointing second-order harmonics
To study these distortions, the researchers fabricated a quantum device specifically engineered to suppress single pair tunneling while allowing two-pair tunneling. This design enabled precise detection and measurement of second-order harmonic corrections.
The team also identified the principal source of these distortions. While previous work had suggested that second-order harmonics arise from intrinsic junction dynamics, MIT researchers demonstrated that additional inductance in the circuit’s wiring—resistance to changes in electric current flow—was responsible in their devices. Understanding this external source allows for better prediction and engineering of circuits that minimize such errors.
Implications for scaling quantum computers
As quantum computers grow larger and more complex, controlling and mitigating sources of error like second-order harmonic corrections becomes increasingly important. According to Max Hays, co-lead author and research scientist at MIT’s Engineering Quantum Systems group, identifying these unexpected effects is critical for improving device precision and performance.
Ongoing research aims to refine the predictions of device behavior in the presence of these corrections, explore additional sources of second-order harmonics, and assess their impact under varying fabrication conditions.
Background
Quantum computers have the potential to solve problems beyond the reach of classical computers, including modeling complex molecular interactions for drug development and materials science. Superconducting circuits, one of the leading quantum computing platforms, use Josephson junctions to manipulate quantum information through controlled tunneling of Cooper pairs.
Errors arising from unexpected tunneling effects like second-order harmonic corrections pose challenges to building reliable quantum hardware at scale. The MIT research, published in Nature Physics, advances understanding of these errors and offers a path toward more predictable and robust superconducting quantum circuits.
This research was supported by the U.S. Department of Energy, the U.S. Co-design Center for Quantum Advantage, the U.S. Air Force, the Korea Foundation for Advanced Studies, and MIT’s Intelligence Community Postdoctoral Research Fellowship Program.
Sources
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