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# Volume 18 Issue 1, January 2022

## Entangled with Cherenkov

Despite their relevance for quantum technology, photon-pair sources are difficult to control. A theoretical proposal shows how photon pairs can be created from vacuum fluctuations in time-dependent systems, potentially enabling heralded single-photon frequency combs.

See Sloan et al. and Talebi

Image: Haley Park. Cover Design: Allen Beattie

## Editorial

• ### A year of Guided OA

The Guided Open Access pilot we are trialling with five other journals in the Nature Portfolio will continue into 2022. We highlight some of the main lessons we’ve learned so far.

Editorial

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## Thesis

• ### Pinning down the proton

• Mark Buchanan
Thesis
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## News & Views

• ### To measure a magnon population

Magnons are collective spin excitations that can propagate over long distances — an attractive trait for information-transfer technologies — but we need to better understand their thermodynamic properties. A platform using graphene may hold the key.

• Matteo Carrega
• Stefan Heun
News & Views
• ### Nonlinear effects get into shape

Nonlinear optical effects are by default weak but they can be enhanced by sculpting the resulting spectrally periodic pulses from a fibre laser into an optimal shape.

• Thibaut Sylvestre
News & Views
• ### Topological excitons

A condensate of excitons was theoretically conjectured in the 1960s but has been challenging to pinpoint experimentally. Evidence has now emerged that it could be the ground state of tungsten ditelluride, a rich topological material.

• Vitor M. Pereira
News & Views
• ### An improved recipe for error detection

The reliability of quantum computers depends on the correction of noise-induced errors, which requires additional resources. Experiments on superconducting qubits have now demonstrated the capabilities of a less-demanding scheme for error detection.

• Morten Kjaergaard
News & Views
• ### Freeform fabrication

• Nina Meinzer
News & Views
• ### OK Computer

Promising machine learning techniques can deduce the properties of merging black holes from gravitational wave signals a million times faster than current state-of-the-art methods.

• Rory Smith
News & Views
• ### A fast push to photon pairs

Solid-state sources of entangled photons with tailored properties are key elements for integrated quantum computing. Refractive-index perturbations propagating faster than the speed of light may offer a practical approach for generating entangled photon pairs.

• Nahid Talebi
News & Views

• Bart Verberck
News & Views
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## Review Articles

• ### Optomechanics for quantum technologies

Interaction with light can be used to precisely control motional states. This Review surveys recent progress in the preparation of non-classical mechanical states and in the application of optomechanical platforms to specific tasks in quantum technology.

• Shabir Barzanjeh
• André Xuereb
• Eva M. Weig
Review Article
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## Letters

• ### Quantum anomalous Hall effect with a permanent magnet defines a quantum resistance standard

Accurate measurements of the ohm require high magnetic fields to support the quantum Hall effect. Now, high precision is achieved by using the quantum anomalous Hall effect in a low magnetic field, making the measurement much more accessible.

• Yuma Okazaki
• Takehiko Oe
• Nobu-Hisa Kaneko
Letter
• ### Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures

Stacking and twisting two-dimensional materials has led to the observation of a variety of electronic phenomena. Now, magnetic behaviour that is distinct from anything seen in individual layers is induced by a moiré pattern in double bilayer chromium triiodide.

• Hongchao Xie
• Xiangpeng Luo
• Liuyan Zhao
Letter
• ### Thermodynamics of free and bound magnons in graphene

Although magnons in the quantum Hall regime of graphene have been detected, their thermodynamic properties have not yet been measured. Now, a local probe technique enables the detection of the magnon density and chemical potential.

• Andrew T. Pierce
• Yonglong Xie
• Amir Yacoby
Letter Open Access
• ### Topological charge density waves at half-integer filling of a moiré superlattice

Topological states that are created from strong electron–electron interactions at half-integer superlattice fillings are observed at zero magnetic field.

• H. Polshyn
• Y. Zhang
• A. F. Young
Letter
• ### Bulk and edge properties of twisted double bilayer graphene

Twisted double bilayer graphene is predicted to be a topological insulator under certain conditions. Simultaneous bulk and edge measurements now show metallic transport with a bulk bandgap, suggestive of this prediction.

• Yimeng Wang
• Jonah Herzog-Arbeitman
• Emanuel Tutuc
Letter
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## Articles

• ### Precise determination of the $$B_{\mathrm{s}}^0$$–$$\overline B_{\mathrm{s}}^0$$ oscillation frequency

The LHCb collaboration reports an improved measurement of the oscillation frequency of mesons consisting of a bottom quark and strange quark, which is then combined with previous results.

• R. Aaij
• C. Abellán Beteta
• G. Zunica
Article Open Access
• ### Spectrally periodic pulses for enhancement of optical nonlinear effects

The nonlinear optical effects underlying many applications are typically weak, but linear dispersion engineering allows the generation of pulses comprising equidistant frequency components, which enhances the effective nonlinearity.

• Joshua P. Lourdesamy
• Antoine F. J. Runge
• C. Martijn de Sterke
Article
• ### Controlling two-photon emission from superluminal and accelerating index perturbations

Despite their relevance for quantum technology, photon-pair sources are difficult to control. A theoretical proposal shows how photon pairs can be created from vacuum fluctuations in time-dependent systems, potentially enabling heralded single-photon frequency combs.

• Jamison Sloan
• Nicholas Rivera
• Marin Soljačić
Article
• ### Measuring the capabilities of quantum computers

Evaluations of quantum computers across architectures need reliable benchmarks. A class of benchmarks that can directly reflect the structure of any algorithm shows that different quantum computers have considerable variations in performance.

• Timothy Proctor
• Kenneth Rudinger
• Robin Blume-Kohout
Article
• ### Logical-qubit operations in an error-detecting surface code

Large-scale quantum computers will manipulate quantum information encoded in error-corrected logical qubits. A complete set of operations has now been realized on a logical qubit with error detection.

• J. F. Marques
• B. M. Varbanov
• L. DiCarlo
Article
• ### Evidence for a monolayer excitonic insulator

Insulating states that are formed because of pairing between electrons and holes are known to exist in engineered bilayer structures in high magnetic fields. Now evidence suggests they can occur in a monolayer crystal at zero field.

• Yanyu Jia
• Pengjie Wang
• Sanfeng Wu
Article
• ### Evidence for equilibrium exciton condensation in monolayer WTe2

Exciton condensation has been observed in various three-dimensional (3D) materials. Now, monolayer WTe2—a 2D topological insulator—also shows the phenomenon. Strong electronic interactions allow the excitons to form and condense at high temperature.

• Bosong Sun
• Wenjin Zhao
• David H. Cobden
Article Open Access
• ### Degree-preserving network growth

Network models rarely fix the number of connections of each node during evolution, despite this being needed in real-world applications. Addressing this need, a new approach can grow scale-free networks without preferential attachment.

• Shubha R. Kharel
• Tamás R. Mezei
• Zoltan Toroczkai
Article
• ### Resolving catastrophic error bursts from cosmic rays in large arrays of superconducting qubits

Cosmic rays flying through superconducting quantum devices create bursts of excitations that destroy qubit coherence. Rapid, spatially resolved measurements of qubit error rates make it possible to observe the evolution of the bursts across a chip.

• Matt McEwen
• Lara Faoro
• Rami Barends
Article
• ### Bayesian parameter estimation using conditional variational autoencoders for gravitational-wave astronomy

A method for estimating the source properties of gravitational-wave events shows a speed-up of six orders of magnitude over established approaches. This is a promising tool for follow-up observations of electromagnetic counterparts.

• Hunter Gabbard
• Chris Messenger
• Roderick Murray-Smith
Article
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## Amendments & Corrections

• ### Author Correction: Casimir spring and dilution in macroscopic cavity optomechanics

• J. Mummery
• J. M. Pate
• M. E. Tobar
Author Correction
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## Measure for Measure

• ### The heat of the moment

To celebrate the International Year of Basic Sciences for Sustainable Development, James Gallagher tells the story of the British thermal unit, a unit for heat.

• James Gallagher
Measure for Measure
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