Quantum computers could someday solve pressing problems that are too convoluted for classical computers, such as modeling complex molecular interactions to streamline drug discovery and materials development. To build a superconducting quantum computer that is large and resilient enough for real-world applications, scientists must precisely engineer thousands of quantum circuits so they perform operations with the lowest possible error rate. To help scientists design more predictable circuits, researchers from MIT and Lincoln Laboratory developed a technique to measure a property that can unexpectedly cause a superconducting quantum circuit to deviate from its expected behavior. Their analysis revealed the source of these distortions, known as second-order harmonic corrections, leading to underperforming circuit architectures.
The MIT researchers fabricated a device to detect second-order harmonic corrections, identify their origin, and precisely measure their strength. This technique could help scientists deliberately design quantum circuits that can counteract the effects of these deviations. This is especially important in larger and more complicated quantum circuits, where the negative impact of second-order harmonic corrections can be amplified. Max Hays, a research scientist in the Engineering Quantum Systems (EQuS) group, stated, "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."
In a quantum computer utilizing superconducting circuits, Josephson junctions are critical elements that enable the transfer and manipulation of information. These devices utilize two superconducting wires that are brought very close together, with a nanometer-scale barrier between them. The electric charge in Josephson junctions is carried by electrons, which form Cooper pairs in a superconducting circuit. These Cooper pairs can "quantum tunnel" through the barrier, transporting current from one wire to the other. However, sometimes Cooper pairs can unexpectedly tunnel two at a time, known as a second-order harmonic correction, which limits the performance of circuits configured for single-pair tunneling. To address this, scientists need to know the source and strength of these distortions.
The MIT researchers fabricated a quantum circuit to be very sensitive to these effects, designed to suppress the quantum tunneling process of single Cooper pairs while allowing two-pair tunneling to continue. This enables them to detect the presence of second-order harmonic corrections and measure their strength. They can also use this circuit to pinpoint the source of these harmonics, which helps researchers identify the best way to correct for them. The researchers found that the additional inductance from wires in the circuit, rather than the dynamics of the junction, was the actual source of the second-order harmonics.
In the future, they aim to design experiments that more accurately predict device performance during second-order harmonic corrections and explore other sources of these corrections. This work is partially funded by the U.S. Department of Energy, the U.S. Co-design Center for Quantum Advantage, and other organizations.
Blogger's Review: This research provides crucial technical support for enhancing the reliability of quantum computer circuits. By effectively identifying and correcting second-order harmonic corrections, it significantly improves circuit performance, which has profound implications for the development of quantum computers and establishes a solid foundation for future related studies.