Letters in 2020

The cores of neutron stars could be made of hadronic matter or quark matter. By combining first-principles calculations with observational data, evidence for the presence of quark matter in neutron star cores is found.

The Rydberg-atom superhet, based on microwave-dressed Rydberg atoms and a tailored electromagnetically induced transparency spectrum, allows SI-traceable measurements of microwave electric fields with unprecedented sensitivity.

The localization properties of waves in the quasiperiodic chains described by the Aubry–André model and Fibonacci model are investigated. Passing from one model to the other, the system develops a cascade of delocalization transitions.

Boundary-localized bulk eigenstates given by the non-Hermitian skin effect are observed in a non-reciprocal topological circuit. A fundamental revision of the bulk–boundary correspondence in an open system is required to understand the underlying physics.

A careful study of quantum oscillations of single crystals of the cuprate superconductor YBCO placed under a magnetic field reveals a sawtooth behaviour that is reminiscent of two-dimensional electronic systems—in turn suggesting the existence of a so-called ‘hard antinodal gap’ in this system.

Transmitting the time signal and generating the secure key with the same carrier photon improves the security of a satellite-based quantum-secure time transfer protocol, which uses two-way quantum key distribution.

Error-transparent quantum gates that can tolerate certain error during the execution of quantum operations have been demonstrated. Substantial improvement of the gate fidelity sheds lights on large-scale universal quantum computation.

Coupling of the quadrupole moment of an electron in a triple quantum dot to photons has been predicted to be a good platform for reducing the effect of charge noise on the decoherence time of a qubit. Here, the authors create such a coupling.

Electron spins in solid usually relax their energy through the coupling with phonons in the host lattice. By using the coupling to microwave photons in a cavity as an alternative relaxation path, it is demonstrated that spins can be cooled below the lattice temperature.

α-RuCl₃ is a promising candidate for realizing the Kitaev quantum spin liquid, but the physics governing its magnetic behaviour remain elusive. Resonant elastic X-ray scattering data now set unambiguous constraints on the leading terms in the Hamiltonian.

A passive, heralded and high-fidelity quantum memory network node has been realized, which connects simultaneously to two quantum channels provided by orthogonally aligned optical fibre cavities coupled with a single atom.

Isotopes with an odd number of neutrons are usually slightly smaller in size than their even-neutron neighbours. In charge radii of short-lived copper isotopes, a reduction of this effect is observed when the neutron number approaches fifty.

Compton scattering experiments off helium atoms for photon energies close to the ionization threshold reveal that electrons are not only emitted in the direction of the momentum transfer but also backwards.

Moiré engineering has rapidly gained currency as a means to manipulate electronic states of matter in van der Waals heterostructures. Now, the feat is achieved in epitaxially grown oxide heterostructures, thus opening up fresh opportunities for strongly correlated electronic systems.

In one-dimensional quantum magnets, complex bound states of magnetic excitations known as Bethe strings have long been predicted. Now, a detailed neutron scattering study of SrCo₂V₂O₈ reveals their magnetic-field-dependent dispersion relation.

What happens to topological materials when their electrons are strongly interacting is an open question. Shao and others demonstrate that ZrSiSe is a material that can address this as it has a topological band structure and non-trivial correlations.

A trapped quantum gas and optical microscopy are simultaneously employed to measure the nematicity of an iron-based superconductor. This demonstrates the potential of quantum gases to be used for scanning microscopy of quantum materials.

The quark–gluon plasma, in which quarks and gluons are deconfined, is a transient state created in collisions of heavy nuclei. By defining an effective temperature, this temperature and the system’s entropy density and speed of sound are determined.

Placing two Bernal-stacked graphene bilayers on top of each other with a small twist angle gives correlated states. As the band structure can be tuned by an electric field, this platform is a more varied setting to study correlated electrons.