Researchers from Laboratoire de Physique de l’Ecole Normale Supérieure and Google Quantum AI measure a qubit charge matrix element over 100 times smaller than that of the circuit’s unprotected plasmon transition, placing the dielectric limit on the qubit lifetime above 10 ms. The team’s design uses a 13.6 MHz cos 2φ transmon qubit, a superconducting circuit where Cooper-pair parity symmetry protects against dielectric loss. This work demonstrates that the remaining limitation stems from the qubit’s sensitivity to flux noise within its interferometer loop, a factor predicted to be reduced with larger superinductances.
Cooper-Pair Parity Protection Extends Qubit Lifetime
A qubit architecture using Cooper-pair tunneling demonstrates a charge matrix element exceeding 100 times smaller than that of an unprotected plasmon transition, effectively extending the potential qubit lifetime beyond 10 milliseconds. This design, a 13.6 MHz cos 2φ transmon operating in the soft-transmon regime, uses an island shunted to ground via an interferometer of Josephson junctions and superinductances to favor paired Cooper-pair tunneling.
Symmetry-based selection rules inherent in this approach create distinct symmetry sectors within the qubit’s spectrum, shielding it from dielectric loss, a common source of qubit instability, and allowing for coherent control and single-shot readout. The circuit’s design actively bounds potential noise channels, serving as a diagnostic tool; any coherence below predicted levels indicates an unidentified loss mechanism.
Measurements reveal a relaxation time of 70 microseconds, a value predicted by the 1/f flux noise within the interferometer loop that currently limits dephasing, measured at = 2.5 microseconds. This finding confirms that Cooper-pair parity symmetry effectively protects against the targeted dielectric loss, pinpointing flux noise as the primary remaining limitation. The interferometer’s loop area and layout are key factors influencing its susceptibility to flux noise, with researchers suggesting gradiometric designs or fluxoid quantization as potential mitigation strategies.
The experiment’s success lies in encoding the qubit in states of opposite Cooper-pair parity, a technique that uses fundamental symmetries to enhance stability. This approach differs from other qubit designs, such as GKP qubits using translational symmetry or fluxonium qubits relying on fluxon parity, demonstrating a diverse range of methods for achieving qubit protection.
The ability to operate in the soft transmon regime while preserving coherent control and single-shot readout represents a step toward realizing more robust and scalable quantum computing architectures. Further improvements are expected through reducing the interferometer loop areas and implementing flux-resilient designs, potentially extending coherence times still further.
cos(2φ) Transmon Qubit Experimental Realization
Achieving a dielectric loss limit exceeding 10 milliseconds, the design of a superconducting qubit results from a specific circuit architecture and operating regime. This qubit operates in a “soft transmon” regime, balancing the need for a manageable frequency with minimized sensitivity to charge noise, a common instability factor in superconducting qubits. This approach allows for a qubit frequency small enough to mitigate charge noise, while still enabling practical control and measurement capabilities.
The energy splitting of the qubit’s doublet, representing a tunneling energy, directly corresponds to the charge dispersion, a key characteristic of this design. While deeper operation within the transmon regime typically eliminates charge sensitivity, it also risks diminishing the qubit frequency and increasing susceptibility to flux fluctuations. The “soft transmon” regime, however, offers a compromise, allowing for both charge noise mitigation and functional control.
13.6 MHz Qubit Operation in the Soft-Transmon Regime
The qubit architecture achieves a dielectric limit on lifetime exceeding 10 milliseconds, a result enabled by a design that prioritizes Cooper-pair parity symmetry. This symmetry, inherent in the circuit’s structure, shields the qubit from dielectric loss, a common instability factor in superconducting systems, by favoring the tunneling of Cooper-pair pairs. Operation relies on a finite asymmetry of the KITE element, approximately 3%, which provides residual coupling to the charge line, enabling qubit manipulation despite the low frequency. 5 microseconds).
Dielectric Loss Reduction via Symmetry in the Circuit
This extended coherence time stems from a circuit topology that uses Cooper-pair parity symmetry, effectively shielding the qubit from a common source of error. While dielectric loss has been successfully mitigated, the interferometer’s sensitivity to flux noise now represents the primary obstacle to further extending qubit lifetime. Researchers have identified specific KITE parameters, the circuit element facilitating Cooper-pair tunneling, that reduce flux sensitivity by two orders of magnitude without compromising the dielectric loss protection.
This optimization suggests a clear pathway for future improvements, with the potential to push flux-noise-limited coherence into the millisecond range. Combining these refined parameters with strategies like smaller loop areas, gradiometric designs, or fluxoid locking could further enhance stability and performance. The device itself is fabricated using a 2D coplanar waveguide geometry and measured in a standard cQED setup, anchored to the 10 mK plate of a dilution refrigerator.
KITE Loop Design Enables Cooper-Pair Tunneling
This performance stems from a cos(2φ) transmon qubit operating in the soft-transmon regime, a design where Cooper-pair parity symmetry actively shields against dielectric loss, a common source of instability in superconducting circuits. 6 MHz qubit. This architecture differs from conventional approaches by implementing a slight asymmetry within the KITE, described as an additional single Cooper-pair tunneling amplitude, which allows for driving the protected qubit transition.
Detailed parameter selection within the KITE is critical, mapping directly to the Hamiltonian governing the qubit’s behavior. While dielectric loss has been effectively suppressed, the experiment reveals that flux noise within the interferometer loop now limits dephasing, resulting in a measured T2E of 2. Researchers predict that increasing the superinductances within the interferometer will further reduce the impact of flux noise without compromising the protective effect against dielectric loss.
Looking ahead, the team suggests exploring novel materials that could enable quartet tunneling without a flux loop, such as twisted superconductors. This circuit combines characteristics of both heavy fluxonium qubits, known for low-lying transitions and high anharmonicity, and Cooper-pair boxes, sensitive to quasiparticle tunneling.
Charge Matrix Element Minimization for Lifetime Extension
This level of protection stems from the qubit’s architecture, which favors the tunneling of Cooper-pair pairs and uses Cooper-pair parity symmetry to shield against dielectric loss. While the KITE element’s asymmetry limits the degree of suppression, the measured performance confirms the effectiveness of the parity symmetry protection.
The measured relaxation time, , of 70 μs is fully predicted by this flux noise, demonstrating that the dielectric loss targeted by the design has been successfully mitigated. Achieving 83% single-shot fidelity with a 5 μs integration time further validates the qubit’s performance, initializing the doublet at an effective temperature of 400 μK.
The underlying principle is to encode information in charge states with non-overlapping supports, minimizing the impact of dielectric loss, which is proportional to the dielectric loss tangent and inversely proportional to the charge matrix element. This work demonstrates that the protection of the qubit charge degree of freedom is not merely theoretical, but achievable through careful circuit design and material selection. The experiment’s findings provide a clear pathway for future improvements, focusing on minimizing flux noise and optimizing KITE asymmetry to further extend qubit coherence times.
Hamiltonian (H_(cos2φ)) Defines cos(2φ) Qubit Behavior
The Hamiltonian governing this cos(2φ) qubit, described as + cos(2φ), dictates behavior distinct from standard superconducting qubits due to its inherent symmetry. This symmetry manifests as qubit states residing within unique sectors of the energy spectrum, a characteristic that contributes to extended coherence times. These signals, arising from combined digital and physical detunings, demonstrate precise control over the qubit’s state.
👉 More information
🗞 Experimental realization of a $cos(2φ)$ transmon qubit
✍️ Erwan Roverc'h et al.
🧠 ArXiv: https://arxiv.org/html/2603.13114v2
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