Nature Physics最新文献

Quantum interconnects facilitate entanglement distribution between non-local computational nodes in a quantum network. For superconducting processors, microwave photons are a natural means to mediate this distribution. However, many existing architectures limit node connectivity and directionality. In this work, we construct a chiral quantum interconnect between two nominally identical modules in separate microwave packages. Our approach uses quantum interference to emit and absorb microwave photons on demand and in a chosen direction between these modules. We optimize our protocol using model-free reinforcement learning to maximize the absorption efficiency. By halting the emission process halfway through its duration, we generate remote entanglement between modules in the form of a four-qubit W state with approximately 62% fidelity in each direction, limited mainly by propagation loss. This quantum network architecture enables all-to-all connectivity between non-local processors for modular and extensible quantum simulation and computation.

The recent observation of the fractional quantum anomalous Hall effect in moiré fractional Chern insulators provides an opportunity to investigate zero magnetic field anyons. One approach for potentially realizing non-abelian anyons is to engineer higher flat Chern bands that mimic higher Landau levels. We investigate the interaction, topology and ferromagnetism of the second moiré miniband in twisted MoTe₂ bilayers. At half-filling of the second miniband, we observed spontaneous ferromagnetism and an incipient Chern insulator state. The Chern numbers of the top two moiré flat bands exhibited opposite signs for twist angles above 3.1° but had the same sign near 2.6°, consistent with theoretical predictions. In the 2.6° device, increasing the magnetic field induced a topological phase transition due to band-crossing between opposite valleys, resulting in an emergent state with Chern number C = −2. Additionally, an insulating state at half-filling of the second valley-polarized band indicates that a charge-ordered state is favoured over the fractional Chern insulator state. These findings lay a foundation for understanding the higher flat Chern bands, which are crucial for the discovery of non-abelian fractional Chern insulators.

Strong interactions between electrons in two-dimensional systems in the presence of a high magnetic field give rise to fractional quantum Hall states that host quasiparticles with a fractional charge and fractional exchange statistics. Here we demonstrate high-resolution scanning tunnelling microscopy and spectroscopy of fractional quantum Hall states in ultra-clean Bernal-stacked bilayer graphene devices. Spectroscopy measurements show sharp excitations that have been predicted to emerge when electrons fractionalize into bound states of quasiparticles. We found energy gaps for candidate non-abelian fractional states that are larger by a factor of five than those in other related systems, for example, semiconductor heterostructures, and this suggests that bilayer graphene is an ideal platform for manipulating these quasiparticles and for creating topological quantum bits. We also found previously unobserved fractional states in our very clean graphene samples.

Superconductivity serves as a basis for non-trivial quantum phenomena and devices, but they often require artificial control of the superconducting gap. In real space, there are various ways to tailor the superconducting gap, such as by introducing interfaces and defects. However, it is challenging to manipulate the superconducting gap in momentum space. Here we demonstrate that the superconducting gap of NbSe₂ monolayers on graphene can be modified at specific momenta by changing the twist angle between the layers. Our spectroscopic-imaging-based scanning tunnelling microscopy experiments reveal the interference patterns of Bogoliubov quasiparticles that are twisted with respect to NbSe₂ and graphene lattices. We find that these chiral interference patterns originate from the twist-dependent sextet of regions in momentum space in which the Fermi surfaces of the NbSe₂ monolayer and graphene overlap. This finding not only broadens our understanding of superconductivity in twisted bilayer systems but also opens up possibilities for designing artificial superconducting materials and devices with tunable properties.

Topological flat bands formed in two-dimensional lattice systems offer an opportunity to study fractional phases of matter in the absence of an external magnetic field. Examples include fractional quantum anomalous Hall effects and fractional topological insulators. Recently, fractional quantum anomalous Hall effects have been experimentally realized in both twisted bilayer MoTe₂ and rhombohedral-stacked multilayer graphene on hexagonal boron nitride. These studies focus mainly on the first moiré flat band, but there is a possibility that non-Abelian states could occur in the second moiré band. Here we present a systematic transport study of twisted bilayer MoTe₂ devices, focusing on the second moiré band. We observe ferromagnetism in the second moiré band, and a Chern insulator state driven by out-of-plane magnetic fields at a filling factor of three holes per moiré unit cell. Between fillings of 2.2 and 2.7 holes per moiré unit cell, we observe a finite temperature resistivity minimum with a 1/T scaling law at low temperatures and a large out-of-plane negative magnetoresistance. Applying an out-of-plane electric field can induce quantum phase transitions at both integer and fractional filling factors. Our studies lay the groundwork for realizing tunable topological states and other unexpected magnetic phases beyond the first moiré flat band based in twisted MoTe₂.

Altermagnetism is a recently discovered unconventional magnetic phase that is characterized by time-reversal symmetry breaking and spin-split band structures in materials with zero net magnetization. Recently, spin-polarized band structures and a vanishing net magnetization were observed in semiconductors MnTe and MnTe₂, confirming this unconventional magnetic order. Metallic altermagnets offer advantages for exploring physical phenomena related to low-energy quasiparticle excitations and for applications in spintronics because the finite electrical conductivity of metals allows direct manipulation of the spin current through the electric field. We demonstrate that KV₂Se₂O is a metallic room-temperature altermagnet with d-wave spin-momentum locking. Our experiments probe the magnetic and electronic structures of this compound and reveal a highly anisotropic spin-polarized Fermi surface and the emergence of a spin-density-wave order in the altermagnetic phase. These characteristics suggest that KV₂Se₂O could be a helpful platform for high-performance spintronic devices and for studying many-body effects coupled with unconventional magnetism.

The thermopower of a clean two-dimensional electron system is directly proportional to the entropy per charge carrier and can probe strongly interacting quantum phases such as fractional quantum Hall liquids. In particular, thermopower is a valuable parameter to probe the quasiparticle statistics that give rise to excess entropy in certain even-denominator fractional quantum Hall states. Here we demonstrate that the magneto-thermopower detection of fractional quantum Hall states is more sensitive than resistivity measurements. We do this in the context of Bernal-stacked bilayer graphene and highlight several even-denominator states at a relatively low magnetic field. These capabilities of thermopower measurements support the interest in fractional quantum Hall states for finding quasiparticles with non-Abelian statistics and elevate bilayer graphene as a promising platform for achieving this.