Nature Physics最新文献

The local equilibration time of quantum many-body systems has been conjectured to satisfy a Planckian bound, so that it always exceeds some value on the order of ℏ/T, where T is the temperature of the system. Here we provide a sharp and universal definition of the local equilibration timescale, and show that it is bounded below by the strong-coupling scale of diffusive fluctuations, which can be expressed in terms of familiar transport parameters. When applied to conformal field theories at a finite temperature, this result produces the Planckian bound. Moreover, this fluctuation bound applies to any local thermalizing system. We study its implication for correlated insulators, metals and disordered fixed points, where it can be used to establish a lower bound on diffusivity in terms of specific heat. Finally, we discuss how the local equilibration time can be directly measured in experiments.

The fascinating hypothesis that supercooled water may segregate into two distinct liquid phases, each with unique structures and densities, was first posited in 1992. This idea, initially based on numerical analyses with the ST2 water-like empirical potential, challenged the conventional understanding of water’s phase behaviour at the time and has since intrigued the scientific community. Over the past three decades, advancements in computational modelling—particularly through the advent of data-driven many-body potentials rigorously derived from first principles and augmented by the efficiency of neural networks—have greatly enhanced the accuracy of molecular simulations, enabling the exploration of the phase behaviour of water with unprecedented realism. Our study leverages these computational advances to probe the elusive liquid–liquid transition in supercooled water. Microsecond-long simulations with chemical accuracy, conducted over several years, provide compelling evidence that water indeed exists in two discernibly distinct liquid states at low temperature and high pressure. By pinpointing a realistic estimate for the location of the liquid–liquid critical point at ~198 K and ~1,250 atm, our study not only advances the current understanding of water’s anomalous behaviour but also establishes a basis for experimental validation. Importantly, our simulations indicate that the liquid–liquid critical point falls within temperature and pressure ranges that could potentially be experimentally probed in water nanodroplets, opening up the possibility for direct measurements.

Correction to: Nature Physics https://doi.org/10.1038/s41567-022-01704-x, published online 18 August 2022.

Silicon-based qubits are often made by trapping individual electrons in quantum dots defined by electric gates. Quantum information can then be stored using the spin states of the electrons. However, the nuclei of the surrounding atoms also have spin degrees of freedom that couple to the electron-spin qubits and cause decoherence. The emergence of a nuclear-spin dark state has been predicted to reduce this coupling during dynamic nuclear polarization, when the electrons in the quantum dot drive the nuclei in the semiconductor into a decoupled state. Here we report the formation of a nuclear-spin dark state in a gate-defined silicon double quantum dot. We show that, as expected, the transverse electron–nuclear coupling rapidly diminishes in the dark state, and that this state depends on the synchronized precession of the nuclear spins. Moreover, the dark state significantly reduces the relaxation rate between the singlet and triplet electronic spin states. This nuclear-spin dark state has potential applications as a quantum memory or in quantum sensing, and might enable increased polarization of nuclear-spin ensembles.