In this paper we generalize the quantum Brownian motion (QBM) to include the momentum-dependent system-environment couplings. The resulting Hamiltonian in the particle number representation is given by $H_{t}ot=\hslash {\omega }_{\mathrm{S}}{a}^{†}a+{\sum }_k\hslash {\omega }_k{b}_k^{†}{b}_k+{\sum }_k\hslash (V_k{a}^{†}{b}_k+V_k^{*}{b}_k^{†}a)+{\sum }_k\hslash (W_k{a}^{†}{b}_k^{†}+W_k^{*}{b}_{k}a)$. The conventional QBM model corresponds to the special case $W_k=V_k$. The generalized QBM can capture more physical applications because the single-particle transition and the two-particle pair production between the system and the environment represent two very different physical processes that usually do not have the same coupling strengths. We discuss the physical realizations of the generalized QBM in different physical systems, and derive its exact master equation for both the initial decoupled states and initial correlated states. The Hu-Paz-Zhang master equation of the conventional QBM model is reproduced as a special case. We find that the renormalized system Hamiltonian after tracing out all the environmental states induced naturally a momentum-dependent potential, which shows the consistency of including the momentum-dependent coupling in the QBM Hamiltonian. In the Hu-Paz-Zhang master equation, such a renormalized potential is misplaced so that the correct renormalization Hamiltonian has not been found. With the exact master equation for both the initial decoupled and initial correlated states, the issues about the initial jolt are also reexamined. We find that the so-called “initial jolt,” which has been thought to be an artificial effect due to the use of the initial decoupled system-environment states, has nothing to do with the initial decoupled state. The exact master equation for the generalized QBM also has potential applications to photonics quantum computing.

The speedup of heavy numerical tasks by quantum computing is now actively investigated in various fields including data analysis in physics and astronomy. In this paper, we propose a quantum algorithm for matched filtering in gravitational wave (GW) data analysis based on the previous work by Gao et al. [Phys. Rev. Research 4, 023006 (2022)]. Our approach uses the quantum algorithm for Monte Carlo integration for the signal-to-noise ratio (SNR) calculation instead of the fast Fourier transform used in Gao et al. and searches signal templates with high SNR by quantum amplitude amplification. In this way, we achieve an exponential reduction of the qubit number compared with Gao et al.'s algorithm, keeping a quadratic speedup over classical GW matched filtering with respect to the template number.

We present a method for network-capable quantum computing that relies on holographic spin-wave excitations stored collectively in ensembles of qubits. We construct an orthogonal basis of spin waves in a one-dimensional array and show that high-fidelity universal linear controllability can be achieved using only phase shifts, applied in both momentum and position space. Neither single-site addressability nor high single-qubit cooperativity is required, and the spin waves can be read out with high efficiency into a single cavity mode for quantum computing and networking applications. We describe how to establish linear quantum processing using a lambda scheme in a rubidium-atom system and calculate the expected experimental operational fidelities due to fundamental and technical errors. We derive efficient methods to achieve linear controllability in both a single-ensemble and dual-ensemble configuration. Finally, we propose to use the spin-wave processor for continuous-variable quantum information processing and present a scheme to generate large dual-rail cluster states useful for deterministic computing.

Frustrated magnets with a strong spin-lattice coupling can show rich magnetic phases and the associated fascinating phenomena. A promising platform is the breathing pyrochlore magnet ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ with localized $S=3/2{\mathrm{Cr}}^{3+}$ ions, which is proposed to be effectively viewed as an $S=6$ Heisenberg antiferromagnet on the face-centered-cubic lattice. Here we unveil that ${\mathrm{CuInCr}}_4{\mathrm{S}}_8$ exhibits a complex magnetic phase diagram with a small phase pocket ($A$ phase) by means of magnetization, magnetostriction, magnetocapacitance, and magnetocaloric-effect measurements in pulsed high magnetic fields of up to 60 T. Remarkably, the appearance of $A$ phase is accompanied by anomalous magnetostrictive and magnetocapacitive responses, suggesting the emergence of helimagnetism in contrast to the neighboring commensurate magnetic phases. Besides, the high-entropy nature is confirmed in the high-temperature side of $A$ phase. These features are potentially related to a thermal fluctuation-driven multiple-$q$ state caused by the magnetic frustration, which has been theoretically predicted but yet experimentally undiscovered in insulating magnets.

Thermally activated delayed fluorescence (TADF) is the internal conversion of triplet excitons into singlet excitons via reverse intersystem crossing. TADF can significantly enhance the efficiency of organic light-emitting diodes (OLEDs). In order for a chromophore to display TADF the energy difference between its lowest singlet and lowest triplet states, $S_1$ and $T_1$, should be as small as possible. This requirement is facilitated by spatial separation between the frontier orbitals. Computer simulations based on time-dependent density functional theory (TDDFT) have been used extensively to predict the excited state properties of TADF chromophores. However, the accuracy of TDDFT largely depends on the choice of exchange-correlation functional. Here, we present a benchmark study of the performance of TDDFT based on different classes of hybrid functionals for 16 TADF chromophores consisting of different donor and acceptor moieties. We find that only the range-separated double hybrid functionals, $\omega \mathrm{B}2\mathrm{PLYP}$ and $\omega \mathrm{B}2\mathrm{GP}\text{−}\mathrm{PLYP}$, provide qualitatively correct predictions of the relative singlet excitation energies of different molecules, the spectral composition of excited states, and the energy ordering of intramolecular charge-transfer versus valence excited states. Therefore, we recommend using these functionals to assess prospective TADF chromophores. Nevertheless, further development is needed to improve the quantitative performance of TDDFT. These findings are important for our ability to computationally screen and design candidate TADF chromophores and advance the development of highly efficient OLEDs.

We study the role of electron-electron interactions near integer and abelian fractional quantum Hall (QH) transitions using composite fermion (CF) representations. Interaction effects are encapsulated in CF theories as gauge fluctuations. Without gauge fluctuations, the CF system realizes a ‘dual’ representation of the noninteracting QH transition. With gauge fluctuations, the system is governed by a gauged nonlinear sigma model (NLSM) with a $\theta \mathrm{term}$. While the transition is described by a strong-coupling fixed point of the NLSM, we are nevertheless able to deduce two of its properties. With $1/r$ interactions, (1) the transition has a dynamical exponent $z=1$, and (2) all transitions are “superuniversal”: fractional and integer QH transitions are in the same universality class. With short-range interactions, $z=2$ and the fate of superuniversality remains unclear.

The ${\mathbb{Z}}_2$ topological metals $R{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($R$ = K, Rb, Cs) with a layered kagome structure provide a unique opportunity to investigate the interplay between charge order, superconductivity, and topology. Here, we report muon-spin relaxation/rotation ($\mu \mathrm{SR}$) measurements performed on $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ across a broad temperature range, in order to uncover the nature of the charge density wave order and superconductivity in this material. From zero-field $\mu \mathrm{SR}$, we find that spontaneous magnetic fields appear below 50 K, which is well below the charge density wave transition ($T^{*}\sim 93$ K). We show that these spontaneous fields are dynamic in nature making it difficult to associate them with a hidden static order. The superconducting state of $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ is found to preserve time-reversal symmetry, and the transverse-field $\mu \mathrm{SR}$ results are consistent with a superconducting state that has two fully open gaps.

Theoretical work has shed light on the phase behavior of idealized mixtures of many components with random interactions. However, typical mixtures interact through particular physical features, leading to a structured, nonrandom interaction matrix of lower rank. Here, we develop a theoretical framework for such mixtures and derive mean-field conditions for thermodynamic stability and critical behavior. Irrespective of the number of components and features, this framework allows for a generally lower-dimensional representation in the space of features and proposes a principled way to coarse-grain multicomponent mixtures as binary mixtures. Moreover, it suggests a way to systematically characterize different series of critical points and their codimensions in mean-field. Since every pairwise interaction matrix can be expressed in terms of features, our work is applicable to a broad class of mean-field models.

Superposition of trajectories, which modify quantum evolutions by superposing paths through interferometry, has been utilized to enhance various quantum communication tasks. However, little is known about its impact from the viewpoint of open quantum systems. Thus we examine this subject from the perspective of system-environment interactions. We show that the superposition of multiple trajectories can result in quantum state freezing, suggesting a space-time dual to the quantum Zeno effect. Moreover, nontrivial Dicke-like super(sub)radiance can be triggered without utilizing multiatom correlations.

Despite extensive research efforts, few quantum algorithms for classical optimization demonstrate a realizable quantum advantage. The utility of many quantum algorithms is limited by high requisite circuit depth and nonconvex optimization landscapes. We tackle these challenges by introducing a variational quantum algorithm that benefits from two innovations: multibasis graph encodings using single-qubit expectation values and nonlinear activation functions. Our technique results in increased observed optimization performance and a factor-of-two reduction in requisite qubits. While the classical simulation of many qubits with traditional quantum formalism is impossible due to its exponential scaling, we mitigate this limitation with exact circuit representations using factorized tensor rings. In particular, the shallow circuits permitted by our technique, combined with efficient factorized tensor-based simulation, enable us to successfully optimize the MaxCut of the 512-vertex DIMACS library graphs on a single GPU. By improving the performance of quantum optimization algorithms while requiring fewer quantum resources and utilizing shallower, more error-resistant circuits, we offer tangible progress for variational quantum optimization.

Calculations of the superfluid density in the inner crust of neutron stars using different approaches are in strong disagreement, which causes a debate on the accountability of pulsar glitches based on superfluidity. Using a simple unified model, we study the dependence on approximation of the superfluid density in a periodic potential. In comparison with the Hartree-Fock-Bogoliubov theory, which treats the effects of the band gap and the pairing gap on equal footing, we examine a Hartree-Fock-BCS-type (HF-BCS-type) approximation in which the former is preferentially incorporated and another approximation in which the latter is preferentially incorporated. We find that, when the pairing gap and the band gap are comparable as in the inner crust of neutron stars, they need to be treated on equal footing, and the HF-BCS approximation can considerably underestimate the superfluid density even if the pairing gap is much smaller than the Fermi energy. Our result suggests that the validity of the HF-BCS approximation for evaluating the superfluid density in neutron star crusts is questionable.

Recent progress in noisy intermediate-scale quantum (NISQ) hardware shows that quantum devices may be able to tackle complex problems even without error correction. However, coherent errors due to the increased complexity of these devices is an outstanding issue. They can accumulate through a circuit, making their impact on algorithms hard to predict and mitigate. Iterative algorithms like quantum imaginary time evolution are susceptible to these errors. This article presents the combination of both noise tailoring using randomized compiling and error mitigation with purification. We also show that cycle benchmarking gives an estimate of the reliability of the purification. We apply this method to the quantum imaginary time evolution of a transverse field Ising model and report an energy estimation error and a ground-state infidelity both below 1%. Our methodology is general and can be used for other algorithms and platforms. We show how combining noise tailoring and error mitigation will push forward the performance of NISQ devices.

Robust spin injection and detection in antiferromagnetic thin films is a prerequisite for the exploration of antiferromagnetic spin dynamics and the development of nanoscale antiferromagnet-based spintronic applications. Previous studies have shown spin injection and detection in antiferromagnet/nonmagnetic metal bilayers; however, spin injection in these systems has been found effective at cryogenic temperatures only. Here, we experimentally demonstrate sizable interfacial spin transport in a hybrid antiferromagnet/ferromagnet system, consisting of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and permalloy, which remains robust up to the room temperature. We examine our experimental data within a spin diffusion model and find evidence for the important role of interfacial magnon pumping in the signal generation. The results bridge spin-orbitronic phenomena of ferromagnetic metals with antiferromagnetic spintronics and demonstrate an advancement toward antiferromagnetic spin-torque devices.

We have calculated the directed flow $v_1$ and charge-dependent directed flow $\mathrm{\Delta }{v}_1$ of pions and protons in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV by solving the coupled Boltzmann-Maxwell equations self-consistently. Our numerical results show that $v_1$ for pions and protons are all negative in the positive midrapidity region and have similar behavior and magnitude. In contrast we find a quite different behavior in $\mathrm{\Delta }{v}_1$ for pions and protons. The difference lies in that $\mathrm{\Delta }{v}_1$ for protons mainly comes from pressure gradients of the medium, while the dominant contribution to $\mathrm{\Delta }{v}_1$ for pions is from electromagnetic fields. Our results indicate that the effect of the electric field will slightly exceed that of the magnetic field and lead to a small negative slope of $\mathrm{\Delta }{v}_1$ for pions.

We investigate symmetry-preserving gapped boundary of $(2+1)$-dimensional $\left[\right(2+1\left)\mathrm{D}\right]$ topological phases with global symmetry, which can be either bosonic or fermionic. We develop a general algebraic description for gapped boundary condition for symmetry-enriched or fermionic topological phases, extending the framework of Lagrangian algebra anyon for bosonic phases without symmetry. We then focus on application to the case with U(1) symmetry. We derive new obstructions to symmetry-preserving gapped boundary for U(1)${}^f$-symmetric $(2+1)\mathrm{D}$ fermionic topological phases, which are beyond chiral central charge $c_{-}$ and electric Hall conductivity ${\sigma }_H$. These obstructions are given by a simple Gauss-Milgram-type formula valid for supermodular category and regarded as a higher version of $c_{-}$ and ${\sigma }_H$.

With the ever-increasing energy need to process big data, the realization of low-power computing technologies, such as superconducting logic and memories, has become a pressing issue. Developing fast and nonvolatile superconducting memory elements, however, remains a challenge. Superconductor-ferromagnet hybrid devices offer a promising solution, as they combine ultrafast manipulation of spins with dissipationless readout. Here, we present a type of nonvolatile Josephson junction memory that utilizes the bistable magnetic texture of a single mesoscopic ferromagnet. We use micromagnetic simulations to design an ellipse-shaped planar junction structured from a Co/Nb bilayer. The ellipse can be prepared as uniformly magnetized or as a pair of vortices at zero applied field. The two states yield considerably different critical currents, enabling reliable electrical readout of the element. We describe the mechanism that controls the critical current by applying numerical calculations to quantify the local stray field from the ferromagnet, which shifts the superconducting interference pattern. Our approach presents a route towards realizing superconducting memory applications by combining micromagnetic modeling with bistable spin-textured junctions.