Networked quantum sensors have several applications such as the mapping of magnetic fields. When the magnetic fields are biomagnetic ones, i.e., they contain some private information, the information of from who non-zero magnetic fields occur has to be protected from eavesdroppers. Anonymous quantum sensing keeps it secret by estimating amplitudes of the magnetic fields without disclosing the positions of non-zero magnetic fields. In this paper, we propose an anonymous quantum sensing protocol that is robust against any independent noise in state preparations. To this end, we devise a quantum state verification protocol for a superposition of Greenberger-Horne-Zeilinger and Dicke states and combine it with the original protocol of anonymous quantum sensing. Our verification protocol can decide whether the fidelity between the ideal and actual states is high or low more efficiently than the direct fidelity estimation. Since the original protocol of anonymous quantum sensing cannot correctly estimate the amplitudes of the magnetic fields under state preparation errors, our results would improve the performance of anonymous quantum sensing in realistic situations.

Abstract Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s–1) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

A quantum sensing network is used to simultaneously detect and measure physical quantities, such as magnetic fields, at different locations. However, there is a risk that the measurement data is leaked to the third party during the communication. Many theoretical and experimental efforts have been made to realize a secure quantum sensing network where a high level of security is guaranteed. In this paper, we propose a protocol to estimate statistical quantities of the target fields at different places without knowing individual value of the target fields. We generate an entanglement between L quantum sensors, let the quantum sensor interact with local fields, and perform specific measurements on them. By calculating the quantum Fisher information to estimate the individual value of the magnetic fields, we show that we cannot obtain any information of the value of the individual fields in the limit of large L. On the other hand, in our protocol, we can estimate theoretically any moment of the field distribution by measuring a specific observable and evaluated relative uncertainty of kth ( k=1,2,3,4) order moment. Our results are a significant step towards using a quantum sensing network with security inbuilt.

In the samples collected from the asteroid Ryugu, magnetite displays natural remanent magnetization due to nebular magnetic field, whereas contemporaneously grown iron sulfide does not display stable remanent magnetization. To clarify this counterintuitive feature, we observed their nanoscale magnetic domain structures using electron holography and found that framboidal magnetites have an external magnetic field of 300 A m −1 , similar to the bulk value, and its magnetic stability was enhanced by interactions with neighboring magnetites, permitting a disk magnetic field to be recorded. Micrometer-sized pyrrhotite showed a multidomain magnetic structure that was unable to retain natural remanent magnetization over a long time due to short relaxation time of magnetic-domain-wall movement, whereas submicron-sized sulfides formed a nonmagnetic phase. These results show that both magnetite and sulfide could have formed simultaneously during the aqueous alteration in the parent body of the asteroid Ryugu.

A quantum sensing network is used to simultaneously detect and measure physical quantities, such as magnetic fields, at different locations. However, there is a risk that the measurement data is leaked to the third party during the communication. Many theoretical and experimental efforts have been made to realize a secure quantum sensing network where a high level of security is guaranteed. In this paper, we propose a protocol to estimate statistical quantities of the target fields at different places without knowing individual value of the target fields. We generate an enanglement between $L$ quantum sensors, let the quantum sensor interact with local fields, and perform specific measurements on them. By calculating the quantum Fisher information to estimate the individual value of the magnetic fields, we show that we cannot obtain any information of the value of the individual fields in the limit of large $L$. On the other hand, in our protocol, we can estimate theoretically any moment of the field distribution by measuring a specific observable and evaluated relative uncertainty of $k$-th ($k=1,2,3,4$) order moment. Our results are a significant step towards using a quantum sensing network with security inbuilt.

A lot of attention has been paid to a quantum-sensing network for detecting magnetic fields in different positions. Recently, cryptographic quantum metrology was investigated where the information of the magnetic fields is transmitted in a secure way. However, sometimes, the positions where non-zero magnetic fields are generated could carry important information. Here, we propose an anonymous quantum sensor where an information of positions having non-zero magnetic fields is hidden after measuring magnetic fields with a quantum-sensing network. Suppose that participants are located in different positions and they have quantum sensors. After the quantum sensors are entangled, the participants implement quantum sensing that provides a phase information if non-zero magnetic fields exist, and POVM measurement is performed on quantum sensors. Importantly, even if the outcomes of the POVM measurement is stolen by an eavesdropper, information of the positions with non-zero magnetic fields is still unknown for the eavesdropper in our protocol. Here, tracelessness is guaranteed, and so our protocol is considered as anonymous. In addition, we evaluate the sensitivity of our proposed quantum sensors by using Fisher information when there are at most two positions having non-zero magnetic fields. We show that the sensitivity is finite unless these two (non-zero) magnetic fields have exactly the same amplitude. Our results pave the way for new applications of quantum-sensing network.

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ASTEROID RYUGU. Yuki Kimura1, Takeharu Kato2, Satoshi Anada2, Ryuji Yoshida2, Kazuo Yamamoto2, Toshiaki Tanigaki3, Tetsuya Akashi3, Hiroto Kasai3, Tomoki Nakamura4, Masahiko Sato5, Tomoyo Morita4, Mizuha Kikuiri4, Kana Amano4, Eiichi Kagawa4, Hisayoshi Yurimoto6, Takaaki Noguchi7, Ryuji Okazaki8, Hikaru Yabuta9, Hiroshi Naraoka8, Kanako Sakamoto10, Sei-ichiro Watanabe11, Yuichi Tsuda10, and Shogo Tachibana5,10. 1Hokkaido University, Sapporo 060-0819, Japan (ykimura@lowtem.hokudai.ac.jp), 2Japan Fine Ceramics Center, Nagoya, 4568587, Japan, 3Research & Development Group, Hitachi, Ltd., Hatoyama, Saitama, 350-0395, Japan, 4Tohoku University, Sendai 980-8578, Japan, 5The University of Tokyo, Tokyo 113-0033, Japan, 6Hokkaido University, Sapporo 060-0810, 7Japan, Kyoto University, Kyoto 606-8502, Japan, 8Kyushu University, Fukuoka 819-0395, Japan, 9Hiroshima University, Higashi-Hiroshima 739-8526, Japan, 10ISAS/JAXA, Sagamihara 252-5210, Japan, 11Nagoya University, Nagoya 464-8601, Japan.

Hiroto Kasai1,2,∗ Yuki Takeuchi, Hideaki Hakoshima, Yuichiro Matsuzaki2,† and Yasuhiro Tokura1‡ 1 Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan 2 Research Center for Emerging Computing Technologies, National institute of Advanced Industrial Science and Technology (AIST), Central2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan and 3 NTT Communication Science Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

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Magnetic particles with a hollow structure have arisen as an important class of nanomagnets because of a large pore volume and higher surface-to-volume ratio compared with the same-sized solid particles. The hollow structure results in unique magnetic features such as enhanced surface exchange bias, spin freezing, and preferential stability of a magnetic vortex. Despite a recent growing understanding of sub-100 nm hollow spherical magnetic nanoparticles, magnetic properties of larger-sized hollow particles were not currently understood in detail. Here, we report results of observations of magnetic microstructures for 420 nm-sized hollow Fe3O4 spherical particles with an electron holography imaging technique, where a magnetic-vortex formation is inferred from bulk measurements. We directly observe a magnetic vortex in a remanence state with magnetization circularly oriented within the shell and the reduced stray field. Micromagnetic simulations demonstrate an increasing stability of a vortex for a hollow sphere and the formation of a field-induced curling double vortex with a pair of clockwise and counterclockwise vortices. This double vortex structure is not confirmed for the solid counterpart, and its stability enhances with decreasing the shell thickness. The present work provides useful knowledge in designing magnetic particles, where a hollow structure and a magnetic vortex are key factors for high-performance biomedical applications.

Using cryogenic Lorentz transmission electron microscopy, we investigate the temperature evolution of magnetic structures in single crystal samples of K2CuF4 with a magnetic phase transition at 6.1 K. This material is known as one of the candidates for a two-dimensional (2D) XY magnet that may exhibit a Berezinskii-Kosterlitz-Thouless (BKT) phase transition. A fine magnetic stripe pattern was found with a period of about 120 nm in a direction along the c axis below 7.3K in a thin sample with the c axis in the plane. Magnetic columns of vortices and antivortices with a separation of about three micrometers were observed below 6K in a c-plane sample approximately 150nm in thickness. The formation of two different types of magnetic structures at different threshold temperatures is likely to be consistent with a picture of two-step dimensional crossover in spin and real spaces previously derived from magnetization and neutron experiments in K2CuF4. These results indicate how the 2D XY character of K2CuF4 is incorporated in three-dimensional magnetic structures. Based on the experimental observations, we discuss the lengthscale of film thickness appropriate for expanding the 2D XY regime and generating the robust BKT excitations. We expect our study to be an important step in realizing the BKT phase transition in a real magnetic system.

The local gradient of electrostatic potential in the Pt gate of a Si-metal oxide semiconductor field effect transistor has been visualized using a phase reconstruction method with through-focus images of transmission electron microscopy based on the transport-of-intensity equation, which was quantitatively corrected by electron holography. The potential change by hydrogen adsorption in a specimen was detected by taking the difference between H2-adsorbed and H2-not-adsorbed phase images. We found that the potential gradient was localized at Pt interfaces and that the direction of the polarization was Pt inward. The direct visualization suggests that the potential gradient originated from hydrogen dipoles generated and located at the Pt interfaces.

Information transfer of a 1-MV field-emission transmission electron microscope (TEM) was improved by reducing mechanical vibrations and improving the stability of an acceleration voltage. The resulting mechanical stability was estimated from lattice fringes with an obtained spacing of 19.6 pm under achromatic conditions. This value corresponds to a vibration amplitude of <19.6 pm. The stability of the acceleration voltage was improved by reducing thermal noises in the power supply. As a result, 39.2-pm-spacing linear lattice fringes were obtained under chromatic conditions. This indicates that 25.5 nm(-1) information transfer was accomplished in the 1 MV field-emission TEM.

One of the long standing problems affecting electron holography, such as lateral coherence limitation, has been solved by the split illumination method with a specially customized transmission electron microscope (TEM). The customized TEM has single [1] or double [2] biprisms in a condenser optical system. Conventional TEM, constructed for electron holography, however, does not have any biprisms in the condenser system. The problem, therefore, still remains unsolved in practice. In order to reduce the difficulties of conventional electron holography, an “accumulated reconstruction” method depending on the principle of two-wave interferometry was developed.

REBaCuO (RE: rare earth) bulk superconductors, whose microstructures can be controlled by quench and melt growth (QMG) processing, have a high critical current density Jc, which makes them well suited for use in practical applications. Through a combination of electron holography and a focused ion-beam technique, we succeeded in visualizing a fluxon pinned by an insulating particle in a QMG bulk Y-Ba-Cu-O superconductor. We also clarified the magnetic flux distribution in the vicinity of the insulating particles at different temperatures in a uniform external magnetic field.