Hasil untuk "Atomic physics. Constitution and properties of matter"

Stefano Longhi

The Mpemba effect (ME)—where hot systems cool faster than colder ones—has intrigued both classical and quantum thermodynamics. Compared to classical systems, quantum systems add complexity due to quantum correlations. Recent works have explored anomalous relaxation and Mpemba-like effects in several quantum systems, considering isolated systems at zero temperature or open systems in contact with reservoirs under Markovian or non-Markovian dynamics. However, these models typically assume an initial unentangled system–bath state, overlooking the role of initial system–environment correlations. Here we propose a type of quantum ME, distinct from the strong ME, originating solely from initial system–bath entanglement. It is shown that the degree of initial entanglement significantly influences the early relaxation dynamics, with certain conditions causing backflow and retarded thermalization. As an example, we investigate the spontaneous emission of a two-level atom in a photonic waveguide at zero temperature, where an initial atom-photon entangled state results in delayed relaxation and a pronounced ME. These findings highlight the crucial role of quantum correlations in thermalization processes and open new avenues for identifying and engineering quantum Mpemba phenomena. Controlling relaxation dynamics through system–environment entanglement may have potential applications in quantum thermal machines, state initialization protocols, and quantum information processing, where precise control over thermalization is essential.

Charles Menil, Brigitte Leridon, Antonella Cavanna et al.

Abstract Co3Sn2S2, a ferromagnetic Weyl semi-metal with Co atoms on a kagome lattice, has generated much recent attention. Experiments have identified a temperature scale below the Curie temperature. Here, we find that this magnet keeps a memory, when not exposed to a magnetic field sufficiently large to erase it. We identify the driver of this memory effect as a small secondary population of spins, whose coercive field is significantly larger than that of the majority spins. The shape of the magnetization hysteresis curve has a threshold magnetic field set by the demagnetizing factor. These two field scales set the hitherto unidentified temperature scale, which is not a thermodynamic phase transition, but a crossing point between meta-stable boundaries. Global magnetization is well-defined, even when it is non-uniform, but drastic variations in local magnetization point to a coarse energy landscape, with the thermodynamic limit not achieved at micrometer length scales.

Maximilian Mengler, Maximilian Tippmann, Thomas Walther

Quantum-key-distribution (QKD) protocols that rely on coincidence events registered with two or more detectors are prone to detector saturation effects due to detector dead times. In particular, systems using avalanche photodiodes may incur a reduction in the achievable secret key rate. We demonstrate a detector gating scheme tuned to mitigate these effects but keep the detectors in an otherwise free-running mode. When a first detector is inactive due to a previous detection, other unfavorable detection events in a second detector are blocked, as they would not contribute to the key but just lead to another dead time. We implement this method by synchronizing the detectors’ dead time in our time-bin entanglement QKD system and demonstrate its feasibility for various fiber link distances and detector settings, increasing the sifted key rate by up to 78% with no significant change in the quantum bit error rate.

Hari Bhandari, Rebecca L. Dally, Peter E. Siegfried et al.

Abstract Kagome lattice magnets are an interesting class of materials as they can host topological properties in their magnetic and electronic structures. YMn6Sn6 is one such compound in which various exotic magnetic and electronic topological properties have been realized. Here, by means of a partial substitution of Sn with an isovalent and slightly smaller atom Ge, we demonstrate the sensitivity of such chemical substitution on the magnetic structure and its influence in the electronic properties. Magnetic structure of YMn6Sn4Ge2 determined by neutron diffraction reveals an incommensurate staggered magnetic spiral with a slightly larger spiral pitch than in YMn6Sn6. This change in magnetic structure influences the Fermi surface enhancing the out-of-plane conductivity. Such a sensitivity to the partial chemical substitution provides a great potential for engineering the magnetic phases and associated electronic properties not only in YMn6Sn6, but also in the large family of 166 rare-earth kagome magnet.

Akos Nagy, Jaime Park, Cindy Zhang et al.

In this article, we study a Grover-type method for quadratic unconstrained binary optimization (QUBO) problems. For an <inline-formula><tex-math notation="LaTeX">$n$</tex-math></inline-formula>-dimensional QUBO problem with <inline-formula><tex-math notation="LaTeX">$m$</tex-math></inline-formula> nonzero terms, we construct a marker oracle for such problems with a tunable parameter, <inline-formula><tex-math notation="LaTeX">$\Lambda \in [ 1, m ] \cap \mathbb {Z}$</tex-math></inline-formula>. At <inline-formula><tex-math notation="LaTeX">$d \in \mathbb {Z}_+$</tex-math></inline-formula> precision, the oracle uses <inline-formula><tex-math notation="LaTeX">$O (n + \Lambda d)$</tex-math></inline-formula> qubits and has total depth of <inline-formula><tex-math notation="LaTeX">$O (\frac{m}{\Lambda } \log _{2} (n) + \log _{2} (d))$</tex-math></inline-formula> and a non-Clifford depth of <inline-formula><tex-math notation="LaTeX">$O (\frac{m}{\Lambda })$</tex-math></inline-formula>. Moreover, each qubit is required to be connected to at most <inline-formula><tex-math notation="LaTeX">$O (\log _{2} (\Lambda + d))$</tex-math></inline-formula> other qubits. In the case of a maximum graph cuts, as <inline-formula><tex-math notation="LaTeX">$d = 2 \left\lceil \log _{2} (n) \right\rceil$</tex-math></inline-formula> always suffices, the depth of the marker oracle can be made as shallow as <inline-formula><tex-math notation="LaTeX">$O (\log _{2} (n))$</tex-math></inline-formula>. For all values of <inline-formula><tex-math notation="LaTeX">$\Lambda$</tex-math></inline-formula>, the non-Clifford gate count of these oracles is strictly lower (at least by a factor of <inline-formula><tex-math notation="LaTeX">$\sim 2$</tex-math></inline-formula>) than previous constructions. Furthermore, we introduce a novel fixed-point Grover adaptive search for QUBO problems, using our oracle design and a hybrid fixed-point Grover search, motivated by the works of Boyer et al. (1988) and Li et al. (2019). This method has better performance guarantees than previous Grover adaptive search methods. Some of our results are novel and useful for any method based on the fixed-point Grover search. Finally, we give a heuristic argument that, with high probability and in <inline-formula><tex-math notation="LaTeX">$O (\frac{\log _{2} (n)}{\sqrt{\epsilon }})$</tex-math></inline-formula> time, this adaptive method finds a configuration that is among the best <inline-formula><tex-math notation="LaTeX">$\epsilon 2^{n}$</tex-math></inline-formula> ones.

Charanpreet Singh, Sk Jamaluddin, Subhadip Pradhan et al.

Abstract Owing to geometrical frustration in the kagome lattice, Mn3Sn displays a 120° in-plane triangular antiferromagnetic order, a manifestation of exchange interaction within the Heisenberg model. Here, we show the formation of a tunable noncoplanar magnetic ground state stabilized by higher-order exchange interactions in electron-doped Mn3Sn samples. Our density Functional Theory calculations reveal that the higher-order exchange induces a partial out-of-plane alignment of the Mn moments, resulting in a canted magnetic state, further experimentally confirmed by neutron diffraction study along with 60 T magnetic and Hall resistivity measurements. Interestingly, we find a large scalar spin chirality-induced Hall signal depending on the degree of non-coplanarity of the Mn moments. Additionally, we demonstrate simultaneous manipulation of two-component order-parameter in the system, where the two Hall signals can be independently manipulated. The present study explores the quantum phenomena associated with the coexistence of multiple magnetic orders and their prospective use in spintronic devices.

Sofia Sanz, Géza Giedke, Daniel Sánchez-Portal et al.

Here, we analyze the electron transport properties of a device formed of two crossed graphene nanoribbons with zigzag edges (ZGNRs) in a spin state with total magnetization different from zero. While the ground state of ZGNRs has been shown to display antiferromagnetic ordering between the electrons at the edges, for wide ZGNRs—where the localized spin states at the edges are decoupled and the exchange interaction is close to zero—in the presence of relatively small magnetic fields, the ferromagnetic (FM) spin configuration can become the state of lowest energy due to the Zeeman effect. In these terms, by comparing the total energy of a periodic ZGNR as a function of the magnetization per unit cell, we obtain the FM-like solution of the lowest energy for the perfect ribbon, the corresponding FM-like configuration of the lowest energy for the four-terminal device formed of crossed ZGNRs, and the critical magnetic field needed to excite the system to this spin configuration. By performing transport calculations, we analyze the role of the distance between layers and the crossing angle of this device in the electrical conductance, at small gate voltages. The problem is approached employing the mean-field Hubbard Hamiltonian in combination with non-equilibrium Green’s functions. We find that ZGNR devices subject to transverse magnetic fields may acquire a high-spin configuration that ensures a metallic response and tunable beam-splitting properties, making this setting promising for studying electron quantum optics with single-electron excitations.

Kentaro Tamura, Yohichi Suzuki, Rudy Raymond et al.

Relaxation is a common way for dealing with combinatorial optimization problems. Quantum random-access optimization (QRAO) is a quantum-relaxation-based optimizer that uses fewer qubits than the number of bits in the original problem by encoding multiple variables per qubit using quantum random-access code (QRAC). Reducing the number of qubits will alleviate physical noise (typically, decoherence), and as a result, the quality of the binary solution of QRAO may be robust against noise, which is, however, unknown. In this article, we numerically demonstrate that the mean approximation ratio of the (3, 1)-QRAC Hamiltonian, i.e., the Hamiltonian utilizing the encoding of three bits into one qubit by QRAC, is less affected by noise compared with the conventional Ising Hamiltonian used in the quantum annealer and the quantum approximate optimization algorithm. Based on this observation, we discuss a plausible mechanism behind the robustness of QRAO under depolarizing noise. Finally, we assess the number of shots required to estimate the values of binary variables correctly under depolarizing noise and show that the (3, 1)-QRAC Hamiltonian requires less shots to achieve the same accuracy compared with the Ising Hamiltonian.

Shu Lok Tsang, Maxwell T. West, Sarah M. Erfani et al.

Quantum machine learning (QML) has received increasing attention due to its potential to outperform classical machine learning methods in problems, such as classification and identification tasks. A subclass of QML methods is quantum generative adversarial networks (QGANs), which have been studied as a quantum counterpart of classical GANs widely used in image manipulation and generation tasks. The existing work on QGANs is still limited to small-scale proof-of-concept examples based on images with significant downscaling. Here, we integrate classical and quantum techniques to propose a new hybrid quantum&#x2013;classical GAN framework. We demonstrate its superior learning capabilities over existing quantum techniques by generating <inline-formula><tex-math notation="LaTeX">$28 \times 28$</tex-math></inline-formula> pixels grayscale images without dimensionality reduction or classical pre/postprocessing on multiple classes of the standard Modified National Institute of Standards and Technology (MNIST) and Fashion MNIST datasets, which achieves comparable results to classical frameworks with three orders of magnitude less trainable generator parameters. To gain further insight into the working of our hybrid approach, we systematically explore the impact of its parameter space by varying the number of qubits, the size of image patches, the number of layers in the generator, the shape of the patches, and the choice of prior distribution. Our results show that increasing the quantum generator size generally improves the learning capability of the network. The developed framework provides a foundation for future design of QGANs with optimal parameter set tailored for complex image generation tasks.

Mostafizur Rahaman Laskar, Subhadeep Mondal, Amit Kumar Dutta

Toeplitz matrix reconstruction algorithms (TMRAs) are one of the central subroutines in array processing for wireless communication applications. The classical TMRAs have shown excellent accuracy in the spectral estimation for both uncorrelated and coherence sources in the recent era. However, TMRAs incorporate the classical eigenvalue decomposition technique for estimating the eigenvalues of the Toeplitz-structured covariance matrices that demand very high computational complexity for large arrays. We demonstrate a low-complexity quantum simulation framework exploiting the structured Hamiltonian of Toeplitz and circulant variants. In this framework, we consider two approaches for the estimation of the eigenvalue spectrum of a given Toeplitz-structured matrix: first, an analytical framework with Jordan form-based sparse-decomposition of a dense-Toeplitz matrix, and second, an approximation method for the conversion of a Toeplitz matrix into a circulant matrix embedding quantum subroutines. We have also compared the efficacy of the proposed algorithms with standard Hamiltonian simulation and quantum phase estimation techniques for different quantum time resolutions and gate complexities. We show quantum gate-complexity analysis for our proposed algorithms. Considering the large dimensions of the Toeplitz matrix, we have employed random matrix theory in deriving the error bounds for the estimated eigenvalues. The numerical results are obtained partly in a classical computer and in an IBM quantum simulator.

Sang-Wook Cheong, Fei-Ting Huang

Abstract Ferromagnetism can be characterized by various distinct phenomena such as non-zero magnetization (inducing magnetic attraction/repulsion), diagonal piezomagnetism, nonreciprocal circular dichroism (such as Faraday effect), odd-order (including linear) anomalous Hall effect, and magneto-optical Kerr effect. We identify all broken symmetries requiring each of the above phenomena, and also the relevant magnetic point groups (MPGs) with those broken symmetries. All ferromagnetic point groups, relevant for ferromagnets, ferrimagnets, and weak ferromagnets, can certainly exhibit all these phenomena, including non-zero magnetization. Some of the true antiferromagnets, which are defined as magnets with MPGs that do not belong to ferromagnetic point groups, can display these phenomena through magnetization induced by external perturbations such as applied current, light illumination, and uniaxial stress, which preserve the combined symmetry of spatial inversion together with time reversal. Such MPGs are identified for each external perturbation. Since high-density and ultrafast spintronic technologies can be enabled by antiferromagnets, our findings will be essential guidance for future magnetism-related science as well as technology.

J. Luo, Z. Zhao, Y. Z. Zhou et al.

Abstract AV3Sb5 (A = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this system. Here, we report 51V nuclear magnetic resonance (NMR) and 121/123Sb nuclear quadrupole resonance (NQR) studies on kagome-metal CsV3Sb5. Below the CDW transition temperature T CDW ~ 98 K, an abrupt change of spectra was observed, indicating that the transition is of the first order. By further analyzing the spectra, we find that the CDW order is commensurate. And most remarkably, the obtained experimental results suggest that the charge modulation of the CDW order is of star-of-David pattern and accompanied by an additional charge modulation in bulk below T * ~ 40 K. Our results revealing the unconventional CDW order provide new insights into AV3Sb5.

R.A. Barve, B.S. Panigrahi, N. Suriyamurthy et al.

V. A. Babenko, V. N. Pavlovych

The main peculiarities of ignition and development of self-sustaining nuclear chain reaction (SCR) in fuel-containing masses (FCM) of Chernobyl "Ukryttya" were studied for the case of varying velocity of water incoming into the system or its outcoming. On the basis of analysis and numerical solution of the corresponding system of differential equations for the main characteristics of the system, it was shown that the variations of water inflow could lead to very sufficient and various changes in SCR development comparing to possible modes at constant velocities of water inflow. In particular, the calculations show that the neutron bursts with great amplitude could take place in the system under definite sufficiently reasonable physical conditions. It was also shown that the increase of velocity of water inflow into the FCM in the mode of constant oscillations can lead to transition into "beyond critical" state which is the subcritical state with big quantity of water.

Institute for Nuclear Research

Brief biography and scientific achievements of Petro Grygorovych Lytovchenko in relation with his 80-th anniversary.

Claudia Ratti

Yu. V. Khomutinin, V. O. Kashparov , Ukrainian Institute of Agricultural Radiology of NUBiP of Ukraine, Kyiv, Ukraine

Problem of optimization of sampling procedure for evaluating the median of specific activity and accumulation coefficients of the 137Cs and 90Sr for the populations of different species of fish lived in the pond was observed. Estimates of the geometric standard deviation of the specific activity (1.2 ÷ 1.9) and accumulation coefficients (1.8 ÷ 2.3) of radionuclides for different species of fish were obtained. Minimum number of samples required for evaluating the median of the specific activity and corresponding accumulation coefficients of 137Cs and 90Sr with desired relative error was determined. In order to obtain the median value of the specific activity of 137Cs with relative error δ = 20 % and confidence level of p = 0.95 at the time of harvest the following numbers of fish samples should be selected for the activity measurement: 16 - 20 samples of pike, perch, sunder, rudd and grass carp; 10 - 13 samples of catfish, bream, tench, carassius, pelecus cultratus; 8-9 samples of bream, roach, carp (common carp), bighead carp; and 5 samples of chub.

L.C Gomes, J.D Walecka, V.F Weisskopf

B. O. Kerbikov

O. M. Shevtsova

Low temperature crystal properties with superconducting inclusions of different types have been studied. In an approach of small sizes of inclusions (the inclusion radius is less than the coherence length/the penetration depth) the magnetoresistance theory of crystal with two different types of inclusions was developed and magnetization as magnetic field function was calculated.