Domain walls (DWs) on magnetic racetracks are at the core of the field of spintronics, providing a basic element for classical information processing. Here, we show that mobile DWs also provide a blueprint for large-scale quantum computers. Remarkably, these DW qubits showcase exceptional versatility, serving not only as stationary qubits, but also performing the role of solid-state flying qubits that can be shuttled in an ultrafast way. We estimate that the DW qubits are long-lived because they can be operated at sweet spots to reduce potential noise sources. Single-qubit gates are implemented by moving the DW, and two-qubit entangling gates exploit naturally emerging interactions between different DWs. These gates, sufficient for universal quantum computing, are fully compatible with current state-of-the-art experiments on racetrack memories. Further, we discuss possible strategies for qubit readout and initialization, paving the way toward future quantum computers based on mobile topological textures on magnetic racetracks.

Layered trihalides, mainly formed by metal elements and halogens, are promising candidates for spintronic devices and van der Waals heterostructures, but are rarely found in nonmetal halides. Motivated by the purpose of searching for the compelling layered configuration in a covalent system, we performed systemically high-pressure experiments to explore the crystal structures in typical ${\mathrm{PBr}}_3$ with diamond anvil cell technique. At room temperature, ${\mathrm{PBr}}_3$ crystallizes into a molecular crystal with orthorhombic symmetry above 0.9 GPa and is found to persist up to at least 43.9 GPa as confirmed by in situ Raman and x-ray diffraction measurements. Strikingly, a novel layered phase with a space group $P{2}_1/c$ is observed at $\sim 27.0\mathrm{GPa}$ and $\sim 1800$ K, which is characterized by quasi-six-coordination of P atoms with nearby Br atoms. In addition, first-principles calculations indicate that the unusual coordination of the P atom in $P{2}_1/c$ phase is highly correlated with the pressure-induced generation of pnictogen bonds. Our current findings not only expand the understanding of the phase diagram of ${\mathrm{PBr}}_3$, but also show an interesting transformation from molecular crystal to an extended layered phase achieved in a broad range of nonmetal trihalides.

The noisy-storage model of quantum cryptography allows for information-theoretically secure two-party computation based on the assumption that a cheating user has at most access to an imperfect, noisy quantum memory, whereas the honest users do not need a quantum memory at all. In general, the more noisy the quantum memory of the cheating user, the more secure the implementation of oblivious transfer, which is a primitive that allows universal secure two-party and multiparty computation. For experimental implementations of oblivious transfer, one has to consider that also the devices held by the honest users are lossy and noisy, and error correction needs to be applied to correct these trusted errors. The latter are expected to reduce the security of the protocol, since a cheating user may hide themselves in the trusted noise. Here we leverage entropic uncertainty relations to derive tight bounds on the security of oblivious transfer with a trusted and untrusted noise. In particular, we discuss noisy storage and bounded storage, with independent and correlated noise.

We present a study on the transport and material properties of aluminum spanning from ambient to warm dense matter conditions using a machine-learned interatomic potential (ML-IAP). Prior research has utilized ML-IAPs to simulate phenomena in warm dense matter, but these potentials have often been calibrated for a narrow range of temperatures and pressures. In contrast, we train a single ML-IAP over a wide range of temperatures, using density functional theory molecular dynamics (DFT-MD) data. Our approach overcomes the computational limitations of DFT-MD simulations, enabling us to study the transport and material properties of matter at higher temperatures and longer time scales. We demonstrate the ML-IAP transferability across a wide range of temperatures using molecular dynamics by examining the ionic part of thermal conductivity, shear viscosity, self-diffusion coefficient, sound velocity, and structure factor of aluminum up to about 60000 K, where we find good agreement with previous theoretical data.

We investigate the excitation spectrum and compressibility of a dipolar Bose-Einstein condensate in an infinite tube potential in the parameter regime where the transition between superfluid and supersolid phases occurs. Our study focuses on the density range in which crystalline order develops continuously across the transition. Above the transition the superfluid shows a single gapless excitation band, phononic at small momenta and with a roton at a finite momentum. Below the transition, two gapless excitations branches (three at the transition point) emerge in the supersolid. We examine the two gapless excitation bands and their associated speeds of sound in the supersolid phase. Our results show that the speeds of sound and the compressibility are discontinuous at the transition, indicating a second-order phase transition. These results provide valuable insights into the identification of supersolid phenomena in dipolar quantum gases and the relationship to supersolidity in spin-orbit coupled gases.

Fermi gases and liquids display an excitation spectrum that is simply connected, ensuring closed Fermi surfaces. In strongly correlated systems such as the cuprate superconductors, the existence of open sheets of Fermi surface known as Fermi arcs indicate a distinctly different topology of the spectrum with no equivalent in Fermi-liquid theory. Here, we demonstrate a generic mechanism by which correlation effects in fermionic systems can change the topology of the spectrum. Using diagrammatic Monte Carlo simulations, we demonstrate the existence of disconnected and multiply connected excitation spectra in the attractive Hubbard model in the BCS-BEC crossover regime. These topologically nontrivial spectra are a prerequisite for Fermi arcs.

State-of-the-art noisy digital quantum computers can only execute short-depth quantum circuits. Variational algorithms are a promising route to unlock the potential of noisy quantum computers since the depth of the corresponding circuits can be kept well below hardware-imposed limits. Typically, the variational parameters correspond to virtual $R_Z$ gate angles, implemented by phase changes of calibrated pulses. By encoding the variational parameters directly as hardware pulse amplitudes and durations, we succeed in further shortening the pulse schedule and overall circuit duration. This decreases the impact of qubit decoherence and gate noise. As a demonstration, we apply our pulse-based variational algorithm to the calculation of the ground state of different hydrogen-based systems (${\mathrm{H}}_2, {\mathrm{H}}_3$, and ${\mathrm{H}}_4$) using IBM cross-resonance-based hardware. We observe a reduction in schedule duration of up to $5\times$ compared to cnot-based Ansätze, while also reducing the measured energy. In particular, we observe a sizable improvement of the minimal energy configuration of ${\mathrm{H}}_3$ compared to a cnot-based variational form. Finally, we discuss possible future developments including error-mitigation schemes and schedule optimizations, which will enable further improvements of our approach, paving the way towards the simulation of larger systems on noisy quantum devices.

Dual-comb spectroscopy (DCS) is a powerful technique for broadband spectroscopy with high precision. High-frequency resolution requires long data acquisition times, limiting the temporal resolution in time-resolved measurements. We overcome this limitation by engineering the interaction between the sample under test and the DCS pulsed laser. The DCS interferogram is measured in steps with every step comprising a different number of pulses that interact with the sample. The sample's complex properties (absorption and phase) are extracted from the Fourier transform of the interferogram as a function of the number of pulses; this maps the temporal evolution of the excited state population. A two-dimensional spectrum is generated from which the system time evolution is deduced. We benchmark this method by measuring the two-dimensional spectrum of a room-temperature rubidium vapor. The measured population dynamics of the excited state show a square dependence on the number of interacting pulses due to the coherent accumulation of population. Rabi oscillations are observed under intense excitation conditions. This is the first demonstration of DCS with high frequency and high temporal resolutions (which is given by the inverse of the repetition rate of the comb laser) without invoking pump-probe spectroscopy, combining the pulsed laser spectral and temporal properties. This method allows one to detect simultaneously the kinetics of different chemical species and hence the pathway for chemical reactions.

Motivated by long-range dispersal in ecological systems, we formulate and apply a general strong-disorder renormalization group (SDRG) framework to describe one-dimensional disordered contact processes with heavy-tailed (such as power law, stretched exponential, and log-normal) dispersal kernels, widely used in ecology. The focus is on the close-to-critical scaling of the order parameters, including the commonly used density, as well as the less known persistence, which is nonzero in the inactive phase. Our analytic and numerical results obtained by SDRG schemes at different levels of approximation reveal that the more slowly decaying dispersal kernels lead to faster-vanishing densities as the critical point is approached. The persistence, however, shows an opposite tendency: the broadening of the dispersal makes its decline sharper at the critical point, becoming discontinuous for the extreme case of power-law dispersal. The SDRG schemes presented here also describe the quantum phase transition of random transverse-field Ising chains with ferromagnetic long-range interactions, the density corresponding to the magnetization of that model.

Non-Gaussian states of light, which are essential in fault-tolerant and universal optical quantum computation, are typically generated by a heralding scheme using photon detectors. Recently, it has been theoretically shown that the large timing jitter of the photon detectors deteriorates the purity of the generated non-Gaussian states [T. Sonoyama et al., Phys. Rev. A 105, 043714 (2022)]. Here, we circumvent this problem by time-gated photon detection and generate non-Gaussian states with Wigner negativity using continuous-wave light. We use a fast optical switch for time gating to effectively improve the timing jitter of a photon-number-resolving detector based on a transition edge sensor from 50 ns to 10 ns. As a result, we generate non-Gaussian states with Wigner negativity of −$0.011\pm 0.004$ $(\hslash =1)$, which cannot be observed without the time-gated photon detection method. These results confirm the effect of the timing jitter on non-Gaussian state generation experimentally for the first time. The proposed time-gated photon detection method would be useful for high-purity non-Gaussian state preparation in optical quantum information processing.

Two-level systems (TLSs) coupled to waveguides are a fundamental paradigm for light-matter interactions and quantum networks. We introduce and experimentally demonstrate a method to tune the interaction between a TLS, implemented as a flux qubit, and a transmission line waveguide from a decoupled state to a coupling strength that is a significant fraction of the TLS transition frequency, near the ultrastrong coupling regime. The coupling, controlled via magnetic flux, is described by a normalized coupling strength $\alpha$ that is measured to range between $6.2\times {10}^{-5}$ and $2.19\times {10}^{-2}$, with larger attainable maximum values predicted by a circuit model of the device. This system enables future investigations in the dynamics of the spin-boson model, microwave photonics, and relativistic quantum information.

We propose an estimation method for quantum measurement tomography (QMT) based on semidefinite programming (SDP) and discuss how it may be employed to detect experimental imperfections, such as shot noise and/or faulty preparation of the input states on near-term quantum computers. Moreover, if the positive operator-valued measure (POVM) we aim to characterize is informationally complete, we put forward a method for self-consistent tomography, i.e., for recovering a set of input states and POVM effects that is consistent with the experimental outcomes and does not assume any a priori knowledge about the input states of the tomography. Contrary to many methods that have been discussed in the literature, our approach does not rely on additional assumptions such as low noise or the existence of a reliable subset of input states.

We establish fundamental upper bounds on the amount of secret key that can be extracted from quantum Gaussian states by using local Gaussian operations, local classical processing, and public communication. For one-way public communication or when two-way public communication is allowed but Alice and Bob first perform destructive local Gaussian measurements, we prove that the key is bounded by the Rényi-2 Gaussian entanglement of formation $E_{F,2}^{\mathrm{G}}$. The saturation of this inequality for pure Gaussian states provides an operational interpretation of the Rényi-2 entropy of entanglement as the secret key rate of pure Gaussian states accessible with Gaussian operations and one-way communication. In the general setting of two-way communication and arbitrary interactive protocols, we argue that $2E_{F,2}^{\mathrm{G}}$ still serves as an upper bound on the extractable key. We conjecture that the factor of 2 is spurious, suggesting that $E_{F,2}^{\mathrm{G}}$ coincides with the secret key rate of Gaussian states under Gaussian measurements and two-way public communication. We use these results to prove a gap between the secret key rates obtainable with arbitrary versus Gaussian operations. This gap is observed for states produced by sending one half of a two-mode squeezed vacuum through a pure loss channel, in the regime of sufficiently low squeezing or sufficiently high transmissivity. Finally, for a wide class of Gaussian states, including all two-mode states, we prove a recently proposed conjecture on the equality between $E_{F,2}^{\mathrm{G}}$ and the Gaussian intrinsic entanglement. The unified entanglement quantifier emerging from such an equality is then endowed with a direct operational interpretation as the value of a quantum teleportation game.

Recently, intriguing physical properties have been unraveled in anisotropic semiconductors in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry. The atomic chain is the ultimate limit in material downscaling for electronics—a frontier for establishing an entirely new field of one-dimensional quantum materials. Electronic and structural properties of chain-like InTe are essential for a better understanding of device applications such as thermoelectrics. Here, we use scanning tunneling microscopy/scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations to image the in-plane structural anisotropy directly in tetragonal InTe. As results, we report the direct observation of one-dimensional ${\mathrm{In}}^{1+}$ chains in InTe. We demonstrate that InTe exhibits a bandgap of about $0.40\pm 0.02$ eV located at the M point of the Brillouin zone. Additionally, line defects are observed in our sample and were attributed to ${\mathrm{In}}^{1+}$ chain vacancy along the $c$-axis—a general feature in many other TlSe-like compounds. Our STS and DFT results prove that the presence of ${\mathrm{In}}^{1+}$ induces a localized gap state, located near the valence band maximum. This acceptor state is responsible for the high intrinsic p-type doping of InTe that we also confirm using angle-resolved photoemission spectroscopy.

Fast and accurate current-driven manipulation of magnetic domain walls (DWs) is crucial for the realization of high-performance spintronic devices. Conventional methods to observe current-induced DW motion (CIDWM) require the accumulative measurement of magnetization dynamics, which is in general incompatible with stochastic magnetization dynamics. We demonstrate stroboscopic time-resolved imaging of CIDWM in a Pt/GdFeCo wire by electrically synchronizing the optical probe pulse and current injection pulse. The observed CIDWM dynamics unveil a time-varying DW velocity during the pulse, which can largely be attributed to the gradual temperature change due to Joule heating and an enhanced DW velocity at the angular momentum compensation temperature. The stroboscopic observation of DW dynamics was also utilized to investigate the contributions of inertial forces in CIDWM. Our method provides access to the spatial and stochastic magnetization dynamics in sophisticated structures driven by external stimuli and should be applicable to a wide range of current-driven phenomena.

We study the problem of parameter estimation in time series stemming from general stochastic processes, where the outcomes may exhibit arbitrary temporal correlations. In particular, we address the question of how much Fisher information is lost if the stochastic process is compressed into a single histogram, known as the empirical distribution. As we show, the answer is nontrivial due to the correlations between outcomes. We derive practical formulas for the resulting Fisher information for various scenarios, from generic stationary processes to discrete-time Markov chains to continuous-time classical master equations. The results are illustrated with several examples.