Quantum Sensing
Quantum-enhanced sensing is one of the near-term applications for the developing field of quantum technologies. A promising approach to quantum sensing relies on entangling a well-controlled system (the sensor) to the system of interest to detect quanta of excitations in the latter. By entangling a superconducting qubit with a ferrimagnetic crystal, Lachance-Quirion et al. demonstrate that they can detect a quantum of magnetic excitation (a magnon) within the sample. The demonstration of a high-efficiency single-magnon detector will be useful for quantum sensing as well as an active component of hybrid quantum systems and should find a wide range of applications in quantum technologies.
Science , this issue p. [425][1]
[1]: /lookup/doi/10.1126/science.aaz9236
Quantum Sensing
Quantum-enhanced sensing is one of the near-term applications for the developing field of quantum technologies. A promising approach to quantum sensing relies on entangling a well-controlled system (the sensor) to the system of interest to detect quanta of excitations in the latter. By entangling a superconducting qubit with a ferrimagnetic crystal, Lachance-Quirion et al. demonstrate that they can detect a quantum of magnetic excitation (a magnon) within the sample. The demonstration of a high-efficiency single-magnon detector will be useful for quantum sensing as well as an active component of hybrid quantum systems and should find a wide range of applications in quantum technologies.
The recent development of hybrid systems based on superconducting circuits provides the possibility of engineering quantum sensors that exploit different degrees of freedom. Quantum magnonics, which aims to control and read out quanta of collective spin excitations in magnetically ordered systems, provides 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 ferrimagnetic 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.
Magnons, the quanta of collective spin excitations in magnetic materials, may enable functionalities, such as nonreciprocity and transduction in hybrid quantum devices. To assess the potential of such applications, it is necessary to understand magnon dynamics beyond the simple harmonic oscillator regime, where theory predicts effects like population-dependent damping and quantum fluctuations in the form of magnon shot noise. Probing these phenomena requires sensors with high sensitivity and the ability to resolve magnon properties across different excitation regimes. Here, we demonstrate accurate and sensitive detection of magnon population and decay over a wide range of occupation numbers. We use a superconducting qubit to probe magnons in a ferrimagnet over approximately 2000 excitations. Using qubit control and parametrically induced qubit-magnon interactions, we demonstrate few-excitation sensitive detection of magnons with a dynamic range of approximately 30 dB, and are able to accurately resolve their decay with few-ns sensitivity. These capabilities offer a powerful and practical technique for probing magnon dynamics in or beyond the linear regime over a wide range of excitations.
Hybrid systems based on magnons in ferromagnetic materials, such as yttrium iron garnet, have achieved remarkable development in the last decade. These include the coupling of magnons to microwave and optical photons, superconducting qubits, phonons, spins, the center-of-mass motion of a ferromagnet, etc. Here, we review both the experimental and theoretical progress in this field, focusing on the generation of magnonic quantum states and their applications in a broad range of fields. Since the strong coupling is a prerequisite for achieving coherent quantum control of magnons and preparing magnonic quantum states, we start by introducing representative strong-coupling experiments in cavity magnonics, then review a series of protocols for creating various magnonic quantum states, such as Fock, cat, squeezed, and entangled states, and discuss their potential applications in macroscopic quantum studies, quantum information science, quantum sensing, magnonic quantum devices, dark matter detection, and so on. Finally, we summarize the review and give an outlook for the future study of quantum magnonics.
Magnetic nanoparticles have wide applications in various fields, e.g., magnetic recording in information science, magnetic resonance imaging, and magnetic therapy in bio-medicine. In this chapter, we will review recent applications of magnetic nanoparticles or magnetic complexes in the cutting-edge field of quantum sensing, especially in hybrid quantum sensing. Quantum sensing is a quantum technology for ultrasensitive detection, which is particularly useful for sensing weak signals at the nanoscale. The quantum systems that offer unprecedented sensitivity, resolution, and/or flexibility include superconducting qubits, defect centers in solids, atomic vapors, cold atoms, and so on. Usually, they have spin properties and possess excellent magnetic-field sensitivity; but they are insensitive to temperature, electric field, pressure, etc., and even immune to some bio-parameters (e.g., pH and glucose concentration). Recently, hybrid quantum sensing has offered new possibilities. By converting the insensitive or immune parameters into magnetic signals, the range of detectable parameters of quantum sensing can be broadened significantly. In this chapter, we first give a brief introduction to diamond quantum sensing, then review the applications of magnetic nanoparticles or magnetic complexes in diamond quantum sensing and nano-magnetism, and finally, discuss the potential applications and challenges for hybrid diamond quantum sensing.
Hybrid quantum devices expand the tools and techniques available for quantum sensing in various fields. Here, we experimentally demonstrate quantum sensing of a steady-state magnon population in a magnetostatic mode of a ferrimagnetic crystal. Dispersively coupling the magnetostatic mode to a superconducting qubit allows for the detection of magnons using Ramsey interferometry with a sensitivity on the order of 10^{-3} magnons/sqrt[Hz]. The protocol is based on dissipation as dephasing via fluctuations in the magnetostatic mode reduces the qubit coherence proportionally to the number of magnons.
Abstract Sensing and metrology are crucial in both fundamental science and practical applications. They meet the constant demand for precise data, enabling more dependable assessments of theoretical models’ validity. Sensors, now a common feature in many fields, play a vital role in applications like gravity imaging, geology, navigation, security, timekeeping, spectroscopy, chemistry, magnetometry, healthcare, and medicine. The advancements in quantum technologies have sparked interest in employing quantum systems as sensors, offering enhanced capabilities and new possibilities. This article describes the optimization of the quantum-enhanced sensing of magnetic fluxes with a Kitaev phase estimation algorithm based on frequency tunable transmon qubits. It provides the optimal flux biasing point for sensors with different qubit transition frequencies and gives an estimation of decoherence rates and achievable sensitivity. The use of 2- and 3-qubit entangled states are compared in simulation with the single-qubit case. The flux sensing accuracy reaches 10 − 8 ⋅ Φ 0 and scales inversely with time, which proves the speed-up of sensing with high ultimate accuracy.