New Qubit Circuit Enables Quantum Operations With Higher Accuracy

The advance brings quantum error correction a step closer to reality.

February 1, 2024

Massachusetts Institute of Technology, Cambridge, Massachusetts

This artist rendering shows the researchers’ superconducting qubit architecture, with the fluxonium qubits in red and the blue transmon coupler in between them. (Image: Krantz Nanoart)

In the future, quantum computers may be able to solve problems that are far too complex for today’s most powerful supercomputers. To realize this promise, quantum versions of error correction codes must be able to account for computational errors faster than they occur.

However, today’s quantum computers are not yet robust enough to realize such error correction at commercially relevant scales.

On the way to overcoming this roadblock, MIT researchers demonstrated a novel superconducting qubit architecture that can perform operations between qubits — the building blocks of a quantum computer — with much greater accuracy than scientists have previously been able to achieve.

They utilize a relatively new type of superconducting qubit, known as fluxonium, which can have a lifespan that is much longer than more commonly used superconducting qubits.

Their architecture involves a special coupling element between two fluxonium qubits that enables them to perform logical operations, known as gates, in a highly accurate manner. It suppresses a type of unwanted background interaction that can introduce errors into quantum operations.

This approach enabled two-qubit gates that exceeded 99.9 percent accuracy and single-qubit gates with 99.99 percent accuracy. In addition, the researchers implemented this architecture on a chip using an extensible fabrication process.

“Building a large-scale quantum computer starts with robust qubits and gates. We showed a highly promising two-qubit system and laid out its many advantages for scaling. Our next step is to increase the number of qubits,” says Leon Ding PhD ‘23, who was a physics graduate student in the Engineering Quantum Systems (EQuS) group and is the lead author of a paper on this architecture.

Ding wrote the paper with several industry and university researchers and staff scientists from MIT Lincoln Laboratory. The research appears in Physical Review X.

In a classical computer, gates are logical operations performed on bits (a series of 1s and 0s) that enable computation. Gates in quantum computing can be thought of in the same way: A single qubit gate is a logical operation on one qubit, while a two-qubit gate is an operation that depends on the states of two connected qubits.

Fidelity measures the accuracy of quantum operations performed on these gates. Gates with the highest possible fidelities are essential because quantum errors accumulate exponentially. With billions of quantum operations occurring in a large-scale system, a seemingly small amount of error can quickly cause the entire system to fail.

In practice, one would use error-correcting codes to achieve such low error rates. However, there is a “fidelity threshold” the operations must surpass to implement these codes. Furthermore, pushing the fidelities far beyond this threshold reduces the overhead needed to implement error correcting codes.

For more than a decade, researchers have primarily used transmon qubits in their efforts to build quantum computers. Another type of superconducting qubit, known as a fluxonium qubit, originated more recently. Fluxonium qubits have been shown to have longer lifespans, or coherence times, than transmon qubits.

Coherence time is a measure of how long a qubit can perform operations or run algorithms before all the information in the qubit is lost.

“The longer a qubit lives, the higher fidelity the operations it tends to promote. These two numbers are tied together. But it has been unclear, even when fluxonium qubits themselves perform quite well, if you can perform good gates on them,” Ding says.

For the first time, Ding and his collaborators found a way to use these longer-lived qubits in an architecture that can support extremely robust, high-fidelity gates. In their architecture, the fluxonium qubits were able to achieve coherence times of more than a millisecond, about 10 times longer than traditional transmon qubits.

“Over the last couple of years, there have been several demonstrations of fluxonium outperforming transmons on the single-qubit level,” says Hays. “Our work shows that this performance boost can be extended to interactions between qubits as well.”

This work was performed by Leon Ding in collaboration with other researchers from MIT, and funded in part by the U.S. Army Research Office. For more information, download the Technical Support Package (free white paper) below. TSP-02242