This illustration uses a layered sculpture to interpret a phenomenon that can cause a quantum circuit to perform differently than expected, increasing the error in computations. MIT researchers developed a method to detect and precisely measure the strength of these distortions.
Image Credit: Amy Pan and Sampson Wilcox
(CC BY-NC-ND 3.0)
Scientific Frontline: Extended "At a Glance" Summary: Quantum Circuit Reliability via Harmonic Detection
The Core Concept: A novel diagnostic technique enables the detection and precise measurement of "second-order harmonic corrections," a non-linear distortion that causes superconducting quantum circuits to deviate from expected operational behaviors.
Key Distinction/Mechanism: Functional superconducting circuits rely on Cooper pairs of electrons quantum tunneling through a Josephson junction barrier one pair at a time. Second-order harmonic corrections occur when two pairs tunnel simultaneously. This two-pair tunneling, driven by additional inductance from connective wiring rather than the junction's intrinsic dynamics, bypasses the circuit's intended single-pair limitations.
Major Frameworks/Components:
- Josephson Junctions: Critical circuit elements consisting of two superconducting wires separated by a nanometer-scale barrier, enabling the transfer and manipulation of quantum information.
- Cooper Pairs: Paired charge-carrying electrons that transport current via quantum tunneling.
- Second-Order Harmonic Corrections: The specific distortion caused by the simultaneous multi-pair tunneling effect.
- Series Inductance: The tendency of wires to oppose changes in electric current flow, identified as the primary source of these harmonic distortions in the tested devices.
Branch of Science: Quantum Physics, Electrical Engineering, and Computer Science.
Future Application: This diagnostic methodology allows for the deliberate engineering of highly predictable, resilient superconducting architectures. These improvements are necessary for scaling up quantum computers to handle complex tasks like modeling molecular interactions for drug discovery and advanced materials development.
Why It Matters: As quantum computers expand in size and complexity, the negative impact of second-order harmonic corrections is amplified. Identifying and counteracting these unexpected deviations is essential for minimizing computational error rates and constructing viable, large-scale quantum systems.
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 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, which lead 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 counteract the effects of these deviations.
This capability is especially important in larger and more complicated quantum circuits, where the negative impact of second-order harmonic corrections can be amplified.
“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,” 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 study.
Hays was joined by co-lead author Junghyun Kim, an electrical engineering and computer science (EECS) graduate student in the EQuS group; senior author William D. Oliver, the Henry Ellis Warren (1894) Professor of EECS and professor of physics, leader of the EQuS group, director of the Center for Quantum Engineering, and associate director of RLE; and others at MIT and Lincoln Laboratory. The research appears in Nature Physics.
A Pair-Wise Problem
In a quantum computer that utilizes superconducting circuits—one of many potential computing platforms—Josephson junctions are critical elements that enable the transfer and manipulation of information. These devices utilize two superconducting wires brought very close together, with a nanometer-scale barrier between them. As in a traditional circuit, the electric charge in Josephson junctions is carried by electrons.
In a superconducting circuit, however, charge-carrying electrons pair up, forming what are called Cooper pairs. These Cooper pairs can “quantum tunnel” through the barrier between the two wires, transporting current from one wire to the other.
Cooper pairs can usually tunnel only one pair at a time, a key property that makes quantum computation possible.
“If you try to force more Cooper pairs through, it just doesn’t work. This nonlinear 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,” Hays explains.
Occasionally, Cooper pairs can unexpectedly squeeze through the barrier two at a time, an effect known as a second-order harmonic correction. This limits the performance of a quantum circuit configured to allow only single-pair tunneling.
“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,” Kim says.
Before they can fix the circuit, however, scientists must know the source and strength of these distortions.
To obtain this information, the MIT researchers fabricated a quantum circuit to be highly sensitive to these effects. Essentially, the device is designed to suppress the quantum tunneling process of single Cooper pairs while allowing the two-pair tunneling process to continue.
In this way, they can detect the presence of second-order harmonic corrections and precisely measure their strength.
Straight to the Source
They can also use this circuit to pinpoint the source of these harmonics, helping researchers identify the best way to correct for them.
There are two potential sources of second-order harmonics: one source is intrinsic to the dynamics of the Josephson junction, and the other is caused by the wires connecting the junction to other circuit elements.
While prior research indicated the second-order harmonics could be due to the dynamics of the junction, the MIT researchers found that additional inductance—the tendency to oppose changes in the flow of electric current—from wires in the circuit was the actual source in their devices.
“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,” Hays says.
In the future, the researchers aim to design experiments that more accurately predict how a device will perform when second-order harmonic corrections occur. They also plan to study other sources of second-order harmonic corrections and whether those sources could negatively impact a circuit under different fabrication conditions.
Funding: This work is funded, in part, by the US Department of Energy, the US Co-design Center for Quantum Advantage, the US Air Force, the Korea Foundation for Advanced Studies, and the Intelligence Community Postdoctoral Research Fellowship Program at MIT.
Published in journal: Nature Physics
Title: Higher-order harmonics in Josephson tunnel junctions due to series inductance
Authors: Junghyun Kim, Max Hays, Ilan T. Rosen, Junyoung An, Helin Zhang, Aranya Goswami, Kate Azar, Jeffrey M. Gertler, Bethany M. Niedzielski, Mollie E. Schwartz, Terry P. Orlando, Jeffrey A. Grover, Kyle Serniak, and William D. Oliver
Source/Credit: Massachusetts Institute of Technology | Adam Zewe
Reference Number: qs051226_01