Error-detected coherence metrology of a dual-rail encoded fixed-frequency multimode superconducting qubit (2506.15420v1)

Abstract: Amplitude damping is a dominant source of error in high performance quantum processors. A promising approach in quantum error correction is erasure error conversion, where errors are converted into detectable leakage states. Dual-rail encoding has been shown as a candidate for the conversion of amplitude-damping errors; with unique sensitivities to noise and decoherence sources. Here we present a dual-rail encoding within a single fixed-frequency superconducting multimode transmon qubit. The three island, two junction device comprises two transmonlike modes with a detuning of 0.75-1 GHz, in a coaxial circuit QED architecture. We show the logical bit-flip and phase-flip error rates are more than one order of magnitude lower than the physical error rates, and demonstrate stability and repeatability of the architecture through an extended measurement of three such devices. Finally, we discuss how the error-detected subspace can be used for investigations into the fundamentals of noise and decoherence in fixed-frequency transmon qubits.

• The paper introduces a novel dual-rail encoding method that converts amplitude damping errors into detectable erasure states in a fixed-frequency multimode superconducting qubit.
• The paper shows that postselecting data with an innovative end-of-line readout improves coherence metrics, reducing bit-flip and phase-flip errors by over an order of magnitude.
• The paper outlines future directions for fault-tolerant quantum computing by mitigating differential noise and refining quantum error correction strategies.

Overview of Error-detected Coherence Metrology of Dual-rail Encoded Superconducting Qubits

The paper focuses on advancements in superconducting qubit technology, specifically using dual-rail encoding to address amplitude damping errors within quantum processors. This approach holds promise for improving quantum error correction (QEC) by converting errors into detectable leakage states, thus enhancing fault tolerance.

Dual-rail Encoding and Device Architecture

The authors describe a novel implementation of dual-rail encoding using a fixed-frequency superconducting multimode transmon qubit, known as a dimon device. This device is constructed within a coaxial circuit quantum electrodynamics (QED) architecture, which holds potential for hardware efficiency due to its minimal physical footprint. The coaxial setup comprises a three-island, two-junction multimode transmon, generating two transmonlike modes with distinct detuning. The logical qubit states are encoded within a single-excitation subspace, effectively converting amplitude damping into detectable erasure errors. The end-of-line (EOL) readout method implemented in this system enables error detection that is crucial for the conversion to erasure errors.

Coherence Metrics and Error Detection

The paper extensively analyzes the error-detected coherence metrics of the logical qubits. It demonstrates that postselecting results to remove amplitude damping errors enhances logical coherence metrics, notably showing improvements in bit-flip and phase-flip error rates by over an order of magnitude compared to the physical qubits. The research asserts the stability and repeatability of these logical operations over extended periods, with significant data collected across multiple devices. This advancement suggests that the dual-rail encoded dimon qubit (DDQ) can effectively stabilize the logical qubit against errors that typically plague conventional transmon qubit systems.

Implications for Noise and Decoherence Studies

This dual-rail encoding introduces a unique sensitivity profile to noise and decoherence mechanisms. While traditional noise sources like charge noise and quasiparticle tunneling are mitigated due to the encoding flexibility, differential noise sources, such as those affecting only one mode of the qubit, remain a challenge. The observed improvements in logical qubit coherence times suggest the DDQ could serve as an analytical tool for dissecting the subtilities of noise and decoherence in quantum circuits. Furthermore, the paper illustrates how noise spectrums, especially low-frequency fluctuations potentially tied to two-level systems (TLS), affect both frequency stability and coherence.

Future Directions

Looking ahead, insights gained from this research could direct the evolution of quantum computing architectures toward greater fault tolerance through targeted error correction strategies. The paper's approach to converting amplitude-damping errors into erasure errors via dual-rail encoding sets a precedent for enhancing the reliability and scalability of quantum processors. Continued exploration into the mitigation of TLS effects and other differential noise sources would be essential for optimizing the performance of such qubits. Moreover, coupling these findings with advances in quantum integrated circuits could yield even more robust implementations of QEC in superconducting systems.

In conclusion, this paper propels the understanding of error-detected coherence metrology within quantum systems, providing a clear pathway for the utilization of dual-rail encoded qubits in practical quantum computing applications. As the field progresses, these findings could facilitate the realization of a fault-tolerant quantum computer capable of performing complex computations with high fidelity.