Pound-drever-hall Method Achieves Stable Quantum-Qubit Readout over 2 Hours, Enabling Robust Phase Measurement

Achieving practical quantum computation demands increasingly sophisticated methods for reading out the states of individual qubits, and a team led by Ibukunoluwa Adisa, Won Chan Lee, and Kevin C. Cox from the University of Maryland and DEVCOM Army Research Laboratory now presents a significant advance in this area. They demonstrate an ultrastable readout technique for superconducting qubits, adapting the Pound-Drever-Hall method, originally developed for stabilising lasers, to the quantum realm. This innovative approach overcomes a major challenge in quantum measurement, namely sensitivity to microwave phase drift, and achieves remarkable phase stability over extended periods. The results show that this method not only maintains fidelity but also potentially enhances the signal strength compared to conventional readout techniques, paving the way for more reliable and scalable quantum systems.

requires the implementation of many parallel qubit readouts. This work presents an ultrastable superconducting-qubit readout method using the multi-tone self-phase-referenced Pound-Drever-Hall (PDH) technique, originally developed for use with optical cavities. The team benchmarks PDH readout of a single transmon qubit, employing room-temperature heterodyne detection of all tones to reconstruct the PDH signal. Results demonstrate that PDH qubit readout is insensitive to microwave phase drift, displaying 0. 73° phase stability over two hours, and capable of single-shot readout even in the presence of phase errors exceeding the phase shift induced by the qubit state.

Synthetic PLL Readout with Phase Sensitivity

This research details a novel qubit readout technique employing a synthetic Phase-Locked Loop (PLL) and Phase-Sensitive Detection (PSD), specifically a synthetic Pound-Drever-Hall (PDH) technique. The core innovation lies in adapting the traditional PDH method, renowned for its sensitivity in frequency stabilization, to the challenging realm of qubit measurement. Superconducting qubits, the quantum bits implemented using superconducting circuits, are the target of this readout method, and accurate qubit readout is a critical step in any quantum computation or information processing task. The system utilizes cryogenic amplification, employing a Josephson Parametric Amplifier (JPA) to boost the weak readout signals at extremely low temperatures. This approach overcomes hurdles presented by directly implementing traditional PDH with superconducting qubits, enabling high-fidelity qubit state discrimination.

Stable Qubit Readout Using PDH Technique

Scientists have developed an ultrastable method for reading the state of superconducting qubits, employing a technique originally used for stabilizing lasers, known as the multi-tone self-phase-referenced Pound-Drever-Hall (PDH) technique. This work demonstrates a readout method insensitive to microwave phase drift, maintaining phase stability for over two hours, and capable of accurately determining the qubit’s state even with significant phase errors. Experiments reveal that traditional heterodyne readout is susceptible to fluctuations caused by temperature drifts or residual phase-locking errors, while PDH readout eliminates these sensitivities by using the sideband as a phase reference. The team further demonstrated single-shot phase stability, successfully recovering a usable signal even with significant carrier-phase errors, a significant achievement for accurate qubit measurement.

This was accomplished by preserving separability between qubit states due to carrier-sideband phase coherence. A key innovation is the reconstruction of a “scissors phase,” orthogonal to errors in both modulation and detection timing, as well as generator phase, providing robustness against common sources of error in microwave setups. Tests prove that even with deliberately induced timing offsets, the scissors phase maintains clear separation of ground and excited states. Furthermore, the team discovered that the PDH signal benefits from intrinsic heterodyne gain, potentially allowing for higher readout signal-to-noise ratios. Quantitative benchmarking confirms that the system can tolerate substantial sideband amplitude without triggering measurement-induced state transitions (MIST), paving the way for improved qubit readout performance.

Stable Qubit Readout via Laser Stabilisation

This research demonstrates a new approach to reading information from superconducting qubits, employing a technique originally developed for stabilizing lasers, the multi-tone self-phase-referenced Pound-Drever-Hall (PDH) technique. The team successfully adapted PDH to the quantum microwave domain, achieving stable qubit readout without reliance on complex cryogenic detectors. Results show the PDH readout is robust against microwave phase drift, maintaining stability for extended periods and enabling single-shot readout even with significant phase errors. Importantly, the method does not induce unwanted changes in the qubit’s state during measurement, and offers the potential for substantial signal enhancement compared to traditional readout methods. The researchers demonstrated that the PDH scheme tolerates high power levels in the sideband tones without introducing measurement errors, suggesting a potential gain in signal quality. Future work focuses on creating detectors optimized for amplifying the subtle signals generated by the PDH technique, which could lead to even higher signal-to-noise ratios and more efficient qubit readout, promising to facilitate the development of scalable and cost-effective architectures for superconducting quantum computing.