Using the principle of quantum entanglement, scientists at the University of Tokyo’s Research Center for Advanced Science and Technology (RCAST) demonstrated the coupling of a millimeter-size magnetic sphere with a sensor and showed that the sensor could determine if a single magnetic excitation was present in the sphere. A millimeter-size sphere of yttrium iron garnet was placed in the same resonant cavity as a superconducting Josephson junction qubit, which acted as the sensor. Because of the coupling of the sphere to the resonant cavity, and in turn, the coupling of the cavity to the qubit, the qubit could only be excited by an electromagnetic pulse if no magnetic excitations were present in the sphere. Using the superconducting qubit as a quantum sensor, the researchers demonstrated the detection of a single magnon (a quantum of magnetic excitation) in the yttrium iron garnet. The detection was based on the entanglement between a magnetostatic mode and the qubit, followed by a single-shot measurement of the qubit state. Schematic of modes of interest in the single-magnon detector. The uniformly processing mode of collective spin excitations in the ferromagnetic crystal, called Kittel mode, is coherently coupled to a superconducting qubit through a microwave cavity mode. Courtesy of Dany Lachance-Quirion. Such a high-efficiency single-magnon detector could be useful for quantum sensing and could be an active component of hybrid quantum systems. “By using single-shot detection instead of averaging, we were able to make our device both highly sensitive and very fast,” professor Yasunobu Nakamura said. “This research could open the way for sensors powerful enough to help with the search for theoretical dark-matter particles called axions.” The research was published in Science (www.doi.org/10.1126/science.aaz9236).