Nature Physics最新文献

Flat-band moiré graphene systems are a quintessential platform for investigating correlated phases of matter. Various interaction-driven ground states have been proposed, but despite extensive experimental effort, there has been little direct evidence that distinguishes between various phases, in particular near the charge neutrality point. Here we probe the fine details of the density of states and the effects of Coulomb interactions in alternating-twist trilayer graphene by imaging the local thermodynamic quantum oscillations with a nanoscale scanning superconducting quantum interference device. We find that the charging self-energy due to occupied electronic states is most important in explaining the high-carrier-density physics. At half-filling of the conduction flat band, we observe ferromagnetic-driven symmetry breaking, suggesting that it is the most robust mechanism in the hierarchy of phase transitions. Near charge neutrality, where exchange energy dominates over charging self-energy, we find a nematic semimetal ground state, which is theoretically favoured over gapped states in the presence of heterostrain. In this semimetallic phase, the flat-band Dirac cones migrate towards the mini-Brillouin zone centre, spontaneously breaking the threefold rotational symmetry. Our low-field local quantum oscillation technique can be used to explore the ground states of many strongly interacting van der Waals systems.

The rapid development of superconducting quantum hardware is expected to run into substantial restrictions on scalability because error correction in a cryogenic environment has stringent input–output requirements. Classical data centres rely on fibre-optic interconnects to remove similar networking bottlenecks. In the same spirit, ultracold electro-optic links have been proposed and used to generate qubit control signals, or to replace cryogenic readout electronics. So far, these approaches have suffered from either low efficiency, low bandwidth or additional noise. Here we realize radio-over-fibre qubit readout at millikelvin temperatures. We use one device to simultaneously perform upconversion and downconversion between microwave and optical frequencies and so do not require any active or passive cryogenic microwave equipment. We demonstrate all-optical single-shot readout in a circulator-free readout scheme. Importantly, we do not observe any direct radiation impact on the qubit state, despite the absence of shielding elements. This compatibility between superconducting circuits and telecom-wavelength light is not only a prerequisite to establish modular quantum networks, but it is also relevant for multiplexed readout of superconducting photon detectors and classical superconducting logic.

Superconducting quantum processors have made important progress in size and computing potential. However, the practical cryogenic limitations of operating large numbers of superconducting qubits are becoming a bottleneck for further scaling. Due to the low thermal conductivity and the dense optical multiplexing capacity of telecommunications fibre, converting qubit signal processing to the optical domain using microwave-to-optics transduction would substantially relax the strain on cryogenic space and thermal budgets. Here we demonstrate optical readout of a superconducting transmon qubit through an optical fibre connected via a coaxial cable to a fully integrated piezo-optomechanical transducer. Using a demolition readout technique, we achieve a single-shot readout fidelity of 81%. Our results illustrate the benefits of piezo-optomechanical transduction for low-dissipation operation of large quantum processors.

Superconductivity is a common feature of graphite allotropes, having been observed in Bernal bilayers, rhombohedral trilayers and a wide variety of angle-misaligned multilayers. Despite notable differences in the electronic structure of these systems, supporting the graphite on a WSe₂ substrate has been consistently observed to expand the range of the superconductivity in terms of carrier density and temperature. Here we report the observation of two distinct superconducting states in Bernal bilayer graphene with strong proximity-induced Ising spin–orbit coupling. Our quantum oscillation measurements show that, although the normal state of the first superconducting phase is consistent with the single-particle band structure, the second emerges from a nematic normal state with broken rotational symmetry. Both superconductors are robust to in-plane magnetic fields, but neither reach fields expected for spin–valley-locked Ising superconductors. The Fermi surface geometry of the first superconducting phase suggests that the superconductivity is limited by orbital depairing arising from the imperfect layer polarization of the electron wavefunctions. Finally, an analysis of transport and thermodynamic compressibility measurements in the second superconducting phase shows that the proximity to isospin phase boundaries, observed in other rhombohedral graphene allotropes, is probably coincidental, thus constraining theories of the pairing mechanisms in these systems.