Physical review letters最新文献

We theoretically study the quantum spin Hall insulator (QSHI) in a perpendicular magnetic field. In the noninteracting case, the QSHI with space inversion and/or uniaxial spin rotation symmetry undergoes a topological transition into a normal insulator phase at a critical magnetic field B_{c}. The exciton condensation in the lowest Landau levels is triggered by Coulomb interactions in the vicinity of B_{c} at low temperature and spontaneously breaks the inversion and the spin rotation symmetries. We propose that the electron spin resonance spectroscopy with the ac magnetic field also aligned in the perpendicular direction can directly probe the exciton condensation order. Our results should apply to QSHIs such as the InAs/GaSb quantum wells and monolayer transition-metal dichalcogenides.

The emerging field of free-electron quantum optics enables electron-photon entanglement and holds the potential for generating nontrivial photon states for quantum information processing. Although recent experimental studies have entered the quantum regime, rapid theoretical developments predict that qualitatively unique phenomena only emerge beyond a certain interaction strength. It is thus pertinent to identify the maximal electron-photon interaction strength and the materials, geometries, and particle energies that enable one to approach it. We derive an upper limit to the quantum vacuum interaction strength between free electrons and single-mode photons, which illuminates the conditions for the strongest interaction. Crucially, we obtain an explicit energy selection recipe for electrons and photons to achieve maximal interaction at arbitrary separations and identify two optimal regimes favoring either fast or slow electrons over those with intermediate velocities. We validate the limit by analytical and numerical calculations on canonical geometries and provide near-optimal designs indicating the feasibility of strong quantum interactions. Our findings offer fundamental intuition for maximizing the quantum interaction between free electrons and photons and provide practical design rules for future experiments on electron-photon and electron-mediated photon-photon entanglement. They should also enable the evaluation of key metrics for applications such as the maximum power of free-electron radiation sources and the maximum acceleration gradient of dielectric laser accelerators.

We show how a driven-dissipative cavity coupled to a collective ensemble of atoms can dynamically generate metrologically useful spin-squeezed states. In contrast to other dissipative approaches, we do not rely on complex engineered dissipation or input states, nor do we require tuning the system to a critical point. Instead, we utilize a strong symmetry, a special type of symmetry that can occur in open quantum systems and emerges naturally in systems with collective dissipation, such as superradiance. This symmetry preserves coherence and allows for the accumulation of an atom number-dependent Berry phase which in turn creates spin-squeezed states via emergent one axis twisting dynamics. This work shows that it is possible to generate entanglement in an atom-cavity resonant regime with macroscopic optical excitations of the system, going beyond the typical dispersive regime with negligible optical excitations often utilized in current cavity-QED experiments.

We define and study a long-range version of the xx model, arising as the free-fermion point of the xxz-type Haldane-Shastry (HS) chain. It has a description via nonunitary fermions, based on the free-fermion Temperley-Lieb algebra, and may also be viewed as an alternating gl(1|1) spin chain. Even and odd lengths behave very differently; we focus on odd length. The model is integrable, and we explicitly identify two commuting Hamiltonians. While nonunitary, their spectrum is real by PT symmetry. One Hamiltonian is chiral and quadratic in fermions, while the other is parity invariant and quartic. Their one-particle spectra have two linear branches, realizing a massless relativistic dispersion on the lattice. The appropriate fermionic modes arise from "quasi-translation" symmetry, which replaces ordinary translation symmetry. The model exhibits exclusion statistics, like the isotropic HS chain, with even more "extended symmetry" and larger degeneracies.

We consider the certification of temporal quantum correlations using the pseudo-density operator (PDO), an extension of the density matrix to the time domain, where negative eigenvalues are key indicators of temporal correlations. Conventional methods for detecting these correlations rely on PDO tomography, which often involves excessive redundant information and requires exponential resources. In this work, we develop an efficient protocol for temporal correlation detection by virtually preparing the PDO within a single time slice and estimating its second-order moments using randomized measurements. Through sample complexity analysis, we demonstrate that our protocol requires only a constant number of measurement bases, making it particularly advantageous for systems utilizing ensemble average measurements, as it maintains constant runtime complexity regardless of the number of qubits. We experimentally validate our protocol on a nuclear magnetic resonance platform, a typical thermodynamic quantum system, where the experimental results closely align with theoretical predictions, confirming the effectiveness of our protocol.

We experimentally demonstrate similarity laws for capacitive radio-frequency (rf) plasmas, showing that two rf discharges are scale-invariant in geometrically similar systems in which the gas pressure, gap dimension, and driving frequency are proportionally tuned. Spatiotemporal distributions of the excitation rate are measured based on phase-resolved optical emission spectroscopy, and the tendencies of the excitation dynamics scaling with control parameters are presented and agree well with particle-in-cell simulations. Furthermore, similarity-based scaling networks are established, which extensively correlate the discharge states (i.e., the initial, intermediate, and similarity states), enabling an effective strategy for determining scaling relations with fewer experiments. The framework of the scaling networks is interpreted based on the kinetic Boltzmann equation coupled with Poisson's equation. The present work reveals the nature of discharge similarity and provides an additional knob for the exploration of upscaled rf plasma sources for industrial applications, such as large-area etching facilities.

The doped Hubbard model is one of the paradigmatic platforms to engineer exotic quantum many-body states, including charge-ordered states, strange metals, and unconventional superconductors. While undoped and doped correlated phases have been experimentally realized in a variety of twisted van der Waals materials, experiments in monolayer materials, and in particular 1T transition metal dichalcogenides, have solely reached the conventional insulating undoped regime. Correlated phases in monolayer two-dimensional materials have much higher associated energy scales than their twisted counterparts, making doped correlated monolayers an attractive platform for high temperature correlated quantum matter. Here, we demonstrate the realization of a doped Mott phase in a van der Waals dichalcogenide 1T-NbSe_{2} monolayer. The system is electron doped due to electron transfer from a monolayer van der Waals substrate via proximity, leading to a correlated triangular lattice with both half-filled and fully filled sites. We analyze the distribution of the half-filled and filled sites and show the arrangement is unlikely to be controlled by disorder alone, and we show that the presence of competing nonlocal many-body correlations would account for the charge correlations found experimentally. Our results establish 1T-NbSe_{2} as a potential monolayer platform to explore correlated doped Mott physics in a frustrated lattice.