Researchers from MIT and Lincoln Laboratory have developed a new technique to measure distortions in superconducting quantum circuits, distortions that unexpectedly cause deviations from expected behavior and increase error rates. The ability to pinpoint these distortions is crucial as scientists aim to engineer thousands of quantum circuits with the lowest possible error rate, a necessity for building superconducting quantum computers capable of tackling complex problems like drug discovery and materials development. Their analysis revealed these distortions stem from what are known as second-order harmonic corrections, leading to underperforming circuit architectures. “As we make our quantum computers bigger and we want to have more precise control over the parameters of these devices, identifying and measuring these effects will be important for us to have a precise understanding of how these systems are constructed,” says Max Hays, a research scientist in the Engineering Quantum Systems (EQuS) group of the Research Laboratory of Electronics (RLE) and co-lead author of the paper published in Nature Physics.
Second-Order Harmonic Corrections Limit Quantum Circuit Performance
These subtle deviations from expected circuit behavior directly impact the accuracy of quantum computations, hindering progress toward building powerful, error-free quantum computers. The core of the issue lies in the behavior of Cooper pairs, the charge-carrying electrons in superconducting circuits. Ideally, these pairs tunnel through barriers between circuit elements one at a time, but researchers discovered that pairs can unexpectedly tunnel two at a time, creating second-order harmonic corrections. “If two Cooper pairs tunnel at the same time, then the assumption we used to build our circuit doesn’t apply anymore. We need to fix the circuit so it can handle that,” explains Junghyun Kim, an electrical engineering and computer science graduate student involved in the research. Before correction, however, understanding the source and strength of these distortions is paramount.
To address this, the MIT team fabricated a specialized quantum circuit designed to be highly sensitive to these effects, suppressing single-pair tunneling while allowing two-pair tunneling to proceed. This allowed them to not only detect the presence of second-order harmonic corrections but also to precisely measure their strength and pinpoint their origin. Their analysis revealed that additional inductance from the wires connecting circuit elements, rather than the Josephson junction itself, was the primary source of these distortions. The research, published in Nature Physics, represents a significant advance in the pursuit of scalable and reliable quantum computing.
Josephson Junctions and Cooper Pair Tunneling Explained
Superconducting quantum circuits rely on the unusual behavior of electrons within Josephson junctions to perform calculations, but subtle distortions are increasingly recognized as a significant impediment to building stable, large-scale quantum computers. These junctions, essential for manipulating quantum information, utilize two superconducting wires separated by a nanoscale barrier, allowing charge to flow via Cooper pairs, pairs of electrons acting as single entities. While conventional circuits rely on single electron flow, superconducting circuits leverage the quantum tunneling of these Cooper pairs, a phenomenon crucial for quantum computation. This disrupts the intended single-pair tunneling, impacting the accuracy of complex computations needed for applications like drug discovery and materials development. “This is important because, if we know where the second-order harmonic correction is coming from, we can predict how strong it is likely to be, and use that information to engineer more predictable circuits that will hopefully perform better,” says Max Hays, a research scientist.
This is important because, if we know where the second-order harmonic correction is coming from, we can predict how strong it is likely to be, and use that information to engineer more predictable circuits that will hopefully perform better.
MIT Device Detects and Measures Harmonic Correction Strength
The team’s work addresses the issue of second-order harmonic corrections, unexpected deviations from expected circuit behavior that introduce errors into quantum computations and limit performance. This new technique allows scientists to move beyond simply identifying these distortions and instead precisely measure their strength, offering a pathway to engineer circuits that mitigate their effects. The impetus behind this research lies in the ambitious goal of creating superconducting quantum computers with thousands of circuits, each operating with minimal error, to tackle complex problems in fields like drug discovery and materials science. These distortions arise from Cooper pairs, the fundamental charge carriers in superconducting circuits, occasionally tunneling through barriers in pairs rather than individually, a phenomenon that violates key assumptions used in circuit design. Their analysis pinpointed the source of these distortions not to the Josephson junctions themselves, but to additional inductance originating from the wires connecting these junctions to other circuit elements.
If you try to force more Cooper pairs through, it just doesn’t work. This non-linear effect is extremely important for all our circuits. If we didn’t have that effect, then we wouldn’t be able to control or manipulate any quantum information that we store in these circuits.
Circuit Inductance Identified as Source of Distortions
The pursuit of stable, high-performance quantum computers took a significant step forward with the development of a new measurement technique at MIT and Lincoln Laboratory, allowing researchers to pinpoint the origin of distortions impacting circuit accuracy. These distortions, known as second-order harmonic corrections, subtly alter expected circuit behavior and contribute to increased error rates, a critical challenge in scaling up quantum processing power for complex applications like drug discovery and materials science. The team’s work, published in Nature Physics, directly addresses a fundamental obstacle to building superconducting quantum computers comprised of thousands of precisely engineered circuits. Understanding this source is crucial because the effect limits the performance of circuits designed for single-pair tunneling, a core principle of quantum computation. The ability to not only detect but also precisely measure the strength of these distortions provides a pathway toward deliberately designing circuits that counteract these effects, paving the way for more robust and reliable quantum computation. This detailed understanding of circuit behavior is essential as scientists strive to build increasingly complex quantum systems and maintain precise control over their parameters.
As we make our quantum computers bigger and we want to have more precise control over the parameters of these devices, identifying and measuring these effects is going to be important for us to have a precise understanding of how these systems are constructed. It is always important to keep diving down into the circuit to see if there is an effect you didn’t expect, which impacts how your device is performing.
Max Hays, a research scientist in the Engineering Quantum Systems (EQuS) group of the Research Laboratory of Electronics (RLE) and co-lead author of a paper on this research
