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Sensor Uses Quantum Entanglement to Detect Magnetic Excitation

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TOKYO, Feb. 21, 2020 — 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.

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). 
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Published: February 2020
Glossary
quantum
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
quantum entanglement
Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become correlated to such an extent that the state of one particle instantly influences the state of the other(s), regardless of the distance separating them. This means that the properties of each particle, such as position, momentum, spin, or polarization, are interdependent in a way that classical physics cannot explain. When particles become entangled, their individual quantum states become inseparable,...
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