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<jats:title>Abstract</jats:title><jats:p>When atoms are placed into liquids, their optical spectral lines corresponding to the electronic transitions are greatly broadened compared to those of single, isolated atoms. This linewidth increase can often reach a factor of more than a million, obscuring spectroscopic structures and preventing high-resolution spectroscopy, even when superfluid helium, which is the most transparent, cold and chemically inert liquid, is used as the host material<jats:sup>1–6</jats:sup>. Here we show that when an exotic helium atom with a constituent antiproton<jats:sup>7–9</jats:sup> is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 10<jats:sup>6</jats:sup>, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation<jats:sup>7</jats:sup>. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses<jats:sup>9</jats:sup>. The sharp spectral lines may enable the detection of cosmic-ray antiprotons<jats:sup>10,11</jats:sup> or searches for antideuterons<jats:sup>12</jats:sup> that come to rest in liquid helium targets.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Interacting many-electron problems pose some of the greatest computational challenges in science, with essential applications across many fields. The solutions to these problems will offer accurate predictions of chemical reactivity and kinetics, and other properties of quantum systems<jats:sup>1–4</jats:sup>. Fermionic quantum Monte Carlo (QMC) methods<jats:sup>5,6</jats:sup>, which use a statistical sampling of the ground state, are among the most powerful approaches to these problems. Controlling the fermionic sign problem with constraints ensures the efficiency of QMC at the expense of potentially significant biases owing to the limited flexibility of classical computation. Here we propose an approach that combines constrained QMC with quantum computation to reduce such biases. We implement our scheme experimentally using up to 16 qubits to unbias constrained QMC calculations performed on chemical systems with as many as 120 orbitals. These experiments represent the largest chemistry simulations performed with the help of quantum computers, while achieving accuracy that is competitive with state-of-the-art classical methods without burdensome error mitigation. Compared with the popular variational quantum eigensolver<jats:sup>7,8</jats:sup>, our hybrid quantum-classical computational model offers an alternative path towards achieving a practical quantum advantage for the electronic structure problem without demanding exceedingly accurate preparation and measurement of the ground-state wavefunction.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Oxidation can deteriorate the properties of copper that are critical for its use, particularly in the semiconductor industry and electro-optics applications<jats:sup>1–7</jats:sup>. This has prompted numerous studies exploring copper oxidation and possible passivation strategies<jats:sup>8</jats:sup>. In situ observations have, for example, shown that oxidation involves stepped surfaces: Cu<jats:sub>2</jats:sub>O growth occurs on flat surfaces as a result of Cu adatoms detaching from steps and diffusing across terraces<jats:sup>9–11</jats:sup>. But even though this mechanism explains why single-crystalline copper is more resistant to oxidation than polycrystalline copper, the fact that flat copper surfaces can be free of oxidation has not been explored further. Here we report the fabrication of copper thin films that are semi-permanently oxidation resistant because they consist of flat surfaces with only occasional mono-atomic steps. First-principles calculations confirm that mono-atomic step edges are as impervious to oxygen as flat surfaces and that surface adsorption of O atoms is suppressed once an oxygen face-centred cubic (fcc) surface site coverage of 50% has been reached. These combined effects explain the exceptional oxidation resistance of ultraflat Cu surfaces.</jats:p>