Quantum computers hold the potential to tackle complex issues beyond the reach of classical machines, such as simulating intricate molecular interactions for drug and materials development. However, creating a superconducting quantum computer suitable for practical use requires scientists to engineer thousands of circuits with minimal error rates. To aid in this, MIT and Lincoln Laboratory researchers have developed a method to measure a property that can unexpectedly cause deviations in a superconducting quantum circuit’s behavior.
Their study identified second-order harmonic corrections as the source of these deviations, which lead to underperforming circuits. The MIT team created a device to detect these corrections, determine their origin, and measure their strength accurately. This technique may enable the deliberate design of circuits to counteract these deviations, which is crucial for larger and more complex quantum systems, where such effects can be more pronounced.
“As quantum computers grow and we seek more precise control over their parameters, understanding these effects is vital,” explains Max Hays, a research scientist in MIT’s Engineering Quantum Systems group. Hays co-authored a paper on the research with Junghyun Kim, an EECS graduate student, and others, including senior author William D. Oliver. The research appears in Nature Physics.
Superconducting circuits in quantum computers rely on Josephson junctions, which transfer information using two closely spaced superconducting wires with a nanometer-scale barrier. In these junctions, electrons form Cooper pairs that can “quantum tunnel” through the barrier, a process usually involving one pair at a time, enabling quantum computation.
Occasionally, two Cooper pairs tunnel simultaneously, a phenomenon known as second-order harmonic correction, which can degrade circuit performance. “If two Cooper pairs tunnel together, our circuit assumptions fail, and we must adjust the circuit,” says Kim. To address this, researchers built a quantum circuit sensitive to these effects, suppressing single-pair tunneling while allowing two-pair tunneling.
This setup allowed them to detect and measure the strength of second-order harmonic corrections and identify their sources. They discovered that additional inductance from connecting wires, rather than junction dynamics, was the main cause. Understanding the source enables the design of more predictable circuits that perform better.
Looking ahead, the researchers aim to design experiments to better predict device performance in the presence of second-order harmonic corrections and investigate other potential sources of such corrections. This work is funded by multiple U.S. agencies and foundations.
Original Source: news.mit.edu
