New Qubit Suppresses Odd Harmonics by Two Orders of Magnitude

Researchers at the Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Denmark and the Department of Electrical, Computer & Energy Engineering, University of Colorado Boulder, Boulder, CO, USA and the Department of Physics, University of Colorado Boulder, Boulder, CO, USA have developed a new superconducting qubit design that suppresses unwanted signal noise, specifically odd harmonics, by up to two orders of magnitude, as described in their results. This design combines aluminum-oxide tunnel junctions with a gate-tunable InAs/Al nanowire junction to demonstrate control over Cooper pair tunneling. The work demonstrates a simple architecture capable of creating a double-well potential dominated by even harmonics, with minima near ± π/2, and enables supercurrent carried by pairs of Cooper pairs. This advancement provides a new building block for Fourier engineering in superconducting circuits. This is not merely noise reduction, but a fundamentally different approach to achieving it, centered on the behavior of the Cooper pairs themselves.

The team’s design combines two aluminum-oxide tunnel junctions with a gate-tunable InAs/Al nanowire junction forming a SQUID. This configuration allows for reconstruction of the energy-phase relation, the potential landscape governing Cooper pair movement, at 85 gate voltage points. Crucially, at half flux quantum, the odd harmonics of the Josephson potential can be suppressed by up to two orders of magnitude relative to the even harmonics, producing a double-well potential dominated by even harmonics. Conventional Josephson junctions rely on a nearly sinusoidal energy-phase relation, proportional to cosφ, reflecting single Cooper pair tunneling. However, the team’s device introduces higher-order harmonics, cos(kφ), corresponding to correlated tunneling of k Cooper pairs. As the paper explains, “By coherently coupling macroscopic phase variables through the tunneling of Cooper pairs, they provide the nonlinearity underlying various classical and quantum devices,” highlighting the foundational role of these junctions. The researchers demonstrate that tuning the relative weight of odd and even harmonics offers a manageable control parameter. Recent advances in superconducting qubit design are increasingly focused on mitigating the sources of decoherence that limit quantum computation times. This improvement centers on controlling the parity of Cooper pair tunneling, the fundamental process by which electrons move through superconducting circuits. The newly developed design represents a departure from conventional Josephson junction designs. These junctions, typically relying on a nearly sinusoidal energy-phase relation, can exhibit higher-order harmonics that introduce instability and degrade performance. While previous attempts often relied on complex architectures or yielded only small harmonic contributions, this new device offers a compact element with reliably tunable harmonic parity controlled by a single external parameter.

The team utilized spectroscopy across 85 gate voltage points to map the energy-phase relation, demonstrating a robust and predictable response. This platform could enable intrinsically protected qubits, less susceptible to environmental noise and capable of sustaining quantum information for longer durations. InAs/Al Nanowire SQUID & Flux Spectroscopy David Feldstein-Bofill and colleagues at the Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Denmark and the Department of Electrical, Computer & Energy Engineering, University of Colorado Boulder, Boulder, CO, USA and Department of Physics, University of Colorado Boulder, Boulder, CO, USA are developing a new approach to superconducting qubit design, focusing on the precise control of Cooper pair tunneling parity. Their work centers on a novel InAs/Al nanowire junction forming a SQUID, and represents a departure from conventional qubit architectures. This hybrid structure allows for manipulation of the Josephson energy-phase relation, the fundamental nonlinearity driving superconducting circuits. The researchers highlight that this work introduces a compact element where Josephson harmonic parity can be reliably tuned, allowing deliberate control of the energy-phase relation within a single superconducting circuit. At half flux quantum, the odd harmonics of the Josephson potential can be suppressed by up to two orders of magnitude relative to the even harmonics, as stated in the description of their results. They utilized spectroscopy versus flux to reconstruct its energy, phase relation at 85 gate voltage points as part of their method. The ability to tune the balance between even and odd harmonics opens avenues for designing qubits with enhanced stability and coherence, crucial for complex quantum algorithms. Conventional superconducting circuits rely on the predictable, sinusoidal relationship between energy and phase within Josephson junctions, facilitating the coherent tunneling of single Cooper pairs. However, the reality is more nuanced; higher-order harmonics, arising from the correlated tunneling of multiple Cooper pairs, significantly influence circuit behavior and often introduce unwanted noise. Researchers are now demonstrating control over these harmonics, specifically achieving a substantial dominance of even harmonics, a configuration previously requiring complex architectures. Through meticulous spectroscopic analysis across 85 gate voltage points, they reconstructed the energy-phase relation of the device, revealing a remarkable ability to suppress odd harmonics by up to two orders of magnitude relative to the even harmonics. This level of control stems from the device’s design, which leverages the interplay between fixed harmonic content from the tunnel junctions and tunable contributions from the InAs/Al nanowire. A device capable of suppressing unwanted noise in superconducting qubits by two orders of magnitude relative to the even harmonics represents a substantial advance in the pursuit of stable quantum computation. The innovation centers on manipulating the parity of Cooper pair tunneling, a technique that dictates whether an odd or even number of electron pairs participate in the tunneling process. Beyond noise reduction, the ability to sculpt the Josephson potential with such precision has implications for intrinsically protected qubits, where a near-degenerate ground-state manifold promises exponential insensitivity to local noise channels. Source: https://www.nature.com/articles/s41467-026-76160-9 Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: