Entanglement-based single-shot detection of a single magnon with a superconducting qubit (1910.09096v1)

Abstract: The recent development of hybrid systems based on superconducting circuits has opened up the possibility of engineering sensors of quanta of different degrees of freedom. Quantum magnonics, which aims to control and read out quanta of collective spin excitations in magnetically-ordered systems, furthermore provides unique opportunities for advances in both the study of magnetism and the development of quantum technologies. Using a superconducting qubit as a quantum sensor, we report the detection of a single magnon in a millimeter-sized ferromagnetic crystal with a quantum efficiency of up to~$0.71$. The detection is based on the entanglement between a magnetostatic mode and the qubit, followed by a single-shot measurement of the qubit state. This proof-of-principle experiment establishes the single-photon detector counterpart for magnonics.

• The paper introduces an entanglement-based approach that enables single-shot detection of individual magnons in a hybrid quantum system.
• It employs strong dispersive qubit-magnon coupling in a tri-mode setup, achieving a readout fidelity of 0.9 and a quantum efficiency of 0.71.
• The study provides a detailed error analysis and suggests enhancements in decoherence and device integration for scalable quantum technologies.

Overview: Entanglement-Based Single-Shot Detection of a Single Magnon with a Superconducting Qubit

The paper presents an exploration into advanced quantum sensing methodologies, focusing specifically on the detection of individual magnons using a superconducting qubit. This work sits at the intersection of condensed matter physics and quantum information science, exploring the field of quantum magnonics—a branch concerned with the manipulation and detection of magnons, the quantized spin-wave excitations in magnetically-ordered materials.

Methodology and System Architecture

The authors detail a hybrid quantum system model comprising a spherical ferrimagnetic crystal of yttrium iron garnet (YIG), a transmon-type superconducting qubit, and a three-dimensional microwave cavity. This tri-mode system includes the Kittel mode, representing uniform magnetostatic excitations; the qubit, which acts as a quantum sensor; and a microwave cavity mode. Enhancing the system’s interactivity, the strong coupling of these elements allows for high-fidelity control and readout operations, essential for effective quantum sensing.

A critical aspect of the paper is the exploitation of a strong dispersive qubit-magnon interaction, where the qubit's frequency is shifted in response to the presence of magnons. Such interactions allow the team to realize a single-magnon detector through conditional operations that induce qubit excitations contingent on the magnonic state.

Significant Results

The authors report a quantum efficiency of 0.71, showcasing the sensitivity and precision of their detection mechanism. They demonstrate an innovatively high single-shot readout fidelity of 0.9 without requiring near-quantum-limited amplifiers. These results are significant as they represent the first instance of measuring single magnons with a single-shot protocol analogous to single-photon detection in optics.

Error Analysis and Limitations

Explicit in their investigation, the paper presents an error budget addressing the various sources of error—including qubit initialization, decoherence, readout errors, and entanglement fidelity. Notably, qubit decoherence is identified as the dominant limiting factor, with a dark-count probability recorded at 0.24. The authors propose that with expected improvements in coherence times and experimental conditions, more refined magnon detection efficiencies and reduced dark-count probabilities are achievable.

Theoretical and Practical Implications

The single-magnon detector opens new trajectories for advancements in both fundamental studies of magnetism and practical quantum technologies. It introduces an analog to optical single-photon advisors within the quantum magnonics field, potentially facilitating the development of magnon-based quantum transducers and quantum communication technologies.

Moreover, this detection capability may assist in exploring weak magnon excitation phenomena, such as those potentially initiated by galactic axions—a topic of interest within quantum cosmology and astrophysics.

Future Prospects

The authors suggest that enhancing the decoherence times and optimizing existing detection protocols could substantially advance this research frontier. Furthermore, developing planar devices integrating such detection technologies would align with the demands of emerging magnon spintronic applications, potentially enabling scalable and efficient quantum devices for next-generation information technologies.

The paper achieves a nuanced balance between theoretical abstraction and empirical validation, charting a course for further exploration into hybrid quantum systems and their applications in quantum information science.