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Mapping the twist-angle disorder and Landau levels in magic-angle graphene 期刊论文
NATURE, 2020, 581 (7806) : 47-+
作者:  Luck, Katja;  39;Amata, Cassandra
收藏  |  浏览/下载:18/0  |  提交时间:2020/07/03

The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG)(1,2) crucially depend on the interlayer twist angle, theta. Although control of the global theta with a precision of about 0.1 degrees has been demonstrated(1-7), little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip)(8) to obtain tomographic images of the Landau levels in the quantum Hall state(9) and to map the local theta variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resolution of a few moire periods. We find a correlation between the degree of theta disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices-which exhibit correlated states, Landau fans and superconductivity-display considerable local variation in theta of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behaviour. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of theta generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of theta disorder as an unconventional type of disorder enabling the use of twist-angle gradients for bandstructure engineering, for realization of correlated phenomena and for gate-tunable built-in planar electric fields for device applications.


SQUID-on-tip tomographic imaging of Landau levels in magic-angle graphene provides nanoscale maps of local twist-angle disorder and shows that its properties are fundamentally different from common types of disorder.


  
Preparation of cyclohexene isotopologues and stereoisotopomers from benzene 期刊论文
NATURE, 2020, 581 (7808) : 288-+
作者:  Shimazaki, Yuya;  Schwartz, Ido;  Watanabe, Kenji;  Taniguchi, Takashi;  Kroner, Martin;  Imamoglu, Atac
收藏  |  浏览/下载:34/0  |  提交时间:2020/07/03

The hydrogen isotopes deuterium (D) and tritium (T) have become essential tools in chemistry, biology and medicine(1). Beyond their widespread use in spectroscopy, mass spectrometry and mechanistic and pharmacokinetic studies, there has been considerable interest in incorporating deuterium into drug molecules(1). Deutetrabenazine, a deuterated drug that is promising for the treatment of Huntington'  s disease(2), was recently approved by the United States'  Food and Drug Administration. The deuterium kinetic isotope effect, which compares the rate of a chemical reaction for a compound with that for its deuterated counterpart, can be substantial(1,3,4). The strategic replacement of hydrogen with deuterium can affect both the rate of metabolism and the distribution of metabolites for a compound(5), improving the efficacy and safety of a drug. The pharmacokinetics of a deuterated compound depends on the location(s) of deuterium. Although methods are available for deuterium incorporation at both early and late stages of the synthesis of a drug(6,7), these processes are often unselective and the stereoisotopic purity can be difficult to measure(7,8). Here we describe the preparation of stereoselectively deuterated building blocks for pharmaceutical research. As a proof of concept, we demonstrate a four-step conversion of benzene to cyclohexene with varying degrees of deuterium incorporation, via binding to a tungsten complex. Using different combinations of deuterated and proteated acid and hydride reagents, the deuterated positions on the cyclohexene ring can be controlled precisely. In total, 52 unique stereoisotopomers of cyclohexene are available, in the form of ten different isotopologues. This concept can be extended to prepare discrete stereoisotopomers of functionalized cyclohexenes. Such systematic methods for the preparation of pharmacologically active compounds as discrete stereoisotopomers could improve the pharmacological and toxicological properties of drugs and provide mechanistic information related to their distribution and metabolism in the body.


Cyclohexene isotopologues and stereoisotopomers with varying degrees of deuteration are formed by binding a tungsten complex to benzene, which facilitates the selective incorporation of deuterium into any position on the ring.


  
Feedback generates a second receptive field in neurons of the visual cortex 期刊论文
NATURE, 2020
作者:  Shi, Enzheng;  Yuan, Biao;  Shiring, Stephen B.;  Gao, Yao;  Akriti;  Guo, Yunfan;  Su, Cong;  Lai, Minliang;  Yang, Peidong;  Kong, Jing;  Savoie, Brett M.;  Yu, Yi;  Dou, Letian
收藏  |  浏览/下载:54/0  |  提交时间:2020/07/03

Animals sense the environment through pathways that link sensory organs to the brain. In the visual system, these feedforward pathways define the classical feedforward receptive field (ffRF), the area in space in which visual stimuli excite a neuron(1). The visual system also uses visual context-the visual scene surrounding a stimulus-to predict the content of the stimulus(2), and accordingly, neurons have been identified that are excited by stimuli outside their ffRF(3-8). However, the mechanisms that generate excitation to stimuli outside the ffRF are unclear. Here we show that feedback projections onto excitatory neurons in the mouse primary visual cortex generate a second receptive field that is driven by stimuli outside the ffRF. The stimulation of this feedback receptive field (fbRF) elicits responses that are slower and are delayed in comparison with those resulting from the stimulation of the ffRF. These responses are preferentially reduced by anaesthesia and by silencing higher visual areas. Feedback inputs from higher visual areas have scattered receptive fields relative to their putative targets in the primary visual cortex, which enables the generation of the fbRF. Neurons with fbRFs are located in cortical layers that receive strong feedback projections and are absent in the main input layer, which is consistent with a laminar processing hierarchy. The observation that large, uniform stimuli-which cover both the fbRF and the ffRF-suppress these responses indicates that the fbRF and the ffRF are mutually antagonistic. Whereas somatostatin-expressing inhibitory neurons are driven by these large stimuli, inhibitory neurons that express parvalbumin and vasoactive intestinal peptide have mutually antagonistic fbRF and ffRF, similar to excitatory neurons. Feedback projections may therefore enable neurons to use context to estimate information that is missing from the ffRF and to report differences in stimulus features across visual space, regardless of whether excitation occurs inside or outside the ffRF. By complementing the ffRF, the fbRF that we identify here could contribute to predictive processing.


Feedback projections onto neurons of the mouse primary visual cortex generate a second excitatory receptive field that is driven by stimuli outside of the classical feedforward receptive field, with responses mediated by higher visual areas.


  
Spin squeezing of 10(11) atoms by prediction and retrodiction measurements 期刊论文
NATURE, 2020, 581 (7807) : 159-+
作者:  Lan, Jun;  Ge, Jiwan;  Yu, Jinfang;  Shan, Sisi;  Zhou, Huan;  Fan, Shilong;  Zhang, Qi;  Shi, Xuanling;  Wang, Qisheng;  Zhang, Linqi;  Wang, Xinquan
收藏  |  浏览/下载:30/0  |  提交时间:2020/07/03

The measurement sensitivity of quantum probes using N uncorrelated particles is restricted by the standard quantum limit(1), which is proportional to 1/root N. This limit, however, can be overcome by exploiting quantum entangled states, such as spin-squeezed states(2). Here we report the measurement-based generation of a quantum state that exceeds the standard quantum limit for probing the collective spin of 10(11) rubidium atoms contained in a macroscopic vapour cell. The state is prepared and verified by sequences of stroboscopic quantum non-demolition (QND) measurements. We then apply the theory of past quantum states(3,4) to obtain spin state information from the outcomes of both earlier and later QND measurements. Rather than establishing a physically squeezed state in the laboratory, the past quantum state represents the combined system information from these prediction and retrodiction measurements. This information is equivalent to a noise reduction of 5.6 decibels and a metrologically relevant squeezing of 4.5 decibels relative to the coherent spin state. The past quantum state yields tighter constraints on the spin component than those obtained by conventional QND measurements. Our measurement uses 1,000 times more atoms than previous squeezing experiments(5-10), with a corresponding angular variance of the squeezed collective spin of 4.6 x 10(-13) radians squared. Although this work is rooted in the foundational theory of quantum measurements, it may find practical use in quantum metrology and quantum parameter estimation, as we demonstrate by applying our protocol to quantum enhanced atomic magnetometry.


  
Ionic solids from common colloids 期刊论文
NATURE, 2020, 580 (7804) : 487-+
作者:  Delord, T.;  Huillery, P.;  Nicolas, L.;  Hetet, G.
收藏  |  浏览/下载:24/0  |  提交时间:2020/07/03

Oppositely charged colloidal particles are assembled in water through an approach that allows electrostatic interactions to be precisely tuned to generate macroscopic single crystals.


From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces(1-4). On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equilibrium structures such as clusters and gels(5-7). Although various systems have been engineered to grow binary crystals(8-11), native surface charge in aqueous conditions has not been used to assemble crystalline materials. Here we form ionic colloidal crystals in water through an approach that we refer to as polymer-attenuated Coulombic self-assembly. The key to crystallization is the use of a neutral polymer to keep particles separated by well defined distances, allowing us to tune the attractive overlap of electrical double layers, directing particles to disperse, crystallize or become permanently fixed on demand. The nucleation and growth of macroscopic single crystals is demonstrated by using the Debye screening length to fine-tune assembly. Using a variety of colloidal particles and commercial polymers, ionic colloidal crystals isostructural to caesium chloride, sodium chloride, aluminium diboride and K4C60 are selected according to particle size ratios. Once fixed by simply diluting out solution salts, crystals are pulled out of the water for further manipulation, demonstrating an accurate translation from solution-phase assembly to dried solid structures. In contrast to other assembly approaches, in which particles must be carefully engineered to encode binding information(12-18), polymer-attenuated Coulombic self-assembly enables conventional colloids to be used as model colloidal ions, primed for crystallization.


  
Coupling of Indo-Pacific climate variability over the last millennium 期刊论文
NATURE, 2020
作者:  Chow, Brian W.;  Nunez, Vicente;  Kaplan, Luke;  Granger, Adam J.;  Bistrong, Karina;  Zucker, Hannah L.;  Kumar, Payal;  Sabatini, Bernardo L.;  Gu, Chenghua
收藏  |  浏览/下载:57/0  |  提交时间:2020/05/13

Coral records indicate that the variability of the Indian Ocean Dipole over the last millennium is strongly coupled to variability in the El Nino/Southern Oscillation and that recent extremes are unusual but not unprecedented.


The Indian Ocean Dipole (IOD) affects climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean(1-4). The frequency and intensity of positive IOD events increased during the twentieth century(5) and may continue to intensify in a warming world(6). However, confidence in predictions of future IOD change is limited by known biases in IOD models(7) and the lack of information on natural IOD variability before anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is strong and unambiguous, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare before 1960. However, the most extreme event on record (1997) is not unprecedented, because at least one event that was approximately 27 to 42 per cent larger occurred naturally during the seventeenth century. We further show that a persistent, tight coupling existed between the variability of the IOD and the El Nino/Southern Oscillation during the last millennium. Indo-Pacific coupling was characterized by weak interannual variability before approximately 1590, which probably altered teleconnection patterns, and by anomalously strong variability during the seventeenth century, which was associated with societal upheaval in tropical Asia. A tendency towards clustering of positive IOD events is evident in our reconstruction, which-together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling-may have implications for improving seasonal and decadal predictions and managing the climate risks of future IOD variability.


  
Universal quantum logic in hot silicon qubits 期刊论文
NATURE, 2020, 580 (7803) : 355-+
作者:  Li, Jia;  Yang, Xiangdong;  Liu, Yang;  Huang, Bolong;  Wu, Ruixia;  Zhang, Zhengwei;  Zhao, Bei;  Ma, Huifang;  Dang, Weiqi;  Wei, Zheng;  Wang, Kai;  Lin, Zhaoyang;  Yan, Xingxu;  Sun, Mingzi;  Li, Bo;  Pan, Xiaoqing;  Luo, Jun;  Zhang, Guangyu;  Liu, Yuan;  Huang, Yu;  Duan, Xidong;  Duan, Xiangfeng
收藏  |  浏览/下载:62/0  |  提交时间:2020/07/03

Quantum computation requires many qubits that can be coherently controlled and coupled to each other(1). Qubits that are defined using lithographic techniques have been suggested to enable the development of scalable quantum systems because they can be implemented using semiconductor fabrication technology(2-5). However, leading solid-state approaches function only at temperatures below 100 millikelvin, where cooling power is extremely limited, and this severely affects the prospects of practical quantum computation. Recent studies of electron spins in silicon have made progress towards a platform that can be operated at higher temperatures by demonstrating long spin lifetimes(6), gate-based spin readout(7) and coherent single-spin control(8). However, a high-temperature two-qubit logic gate has not yet been demonstrated. Here we show that silicon quantum dots can have sufficient thermal robustness to enable the execution of a universal gate set at temperatures greater than one kelvin. We obtain single-qubit control via electron spin resonance and readout using Pauli spin blockade. In addition, we show individual coherent control of two qubits and measure single-qubit fidelities of up to 99.3 per cent. We demonstrate the tunability of the exchange interaction between the two spins from 0.5 to 18 megahertz and use it to execute coherent two-qubit controlled rotations. The demonstration of '  hot'  and universal quantum logic in a semiconductor platform paves the way for quantum integrated circuits that host both the quantum hardware and its control circuitry on the same chip, providing a scalable approach towards practical quantum information processing.


  
Experimental demonstration of memory-enhanced quantum communication 期刊论文
NATURE, 2020
作者:  Quinn, Robert A.;  Melnik, Alexey, V;  Vrbanac, Alison;  Fu, Ting;  Patras, Kathryn A.;  Christy, Mitchell P.;  Bodai, Zsolt;  Belda-Ferre, Pedro;  Tripathi, Anupriya;  Chung, Lawton K.;  Downes, Michael;  Welch, Ryan D.;  Quinn, Melissa;  Humphrey, Greg;  Panitchpakdi, Morgan;  Weldon, Kelly C.;  Aksenov, Alexander;  da Silva, Ricardo;  Avila-Pacheco, Julian;  Clish, Clary;  Bae, Sena;  Mallick, Himel;  Franzosa, Eric A.;  Lloyd-Price, Jason;  Bussell, Robert;  Thron, Taren;  Nelson, Andrew T.;  Wang, Mingxun;  Leszczynski, Eric;  Vargas, Fernando;  Gauglitz, Julia M.;  Meehan, Michael J.;  Gentry, Emily;  Arthur, Timothy D.;  Komor, Alexis C.;  Poulsen, Orit;  Boland, Brigid S.;  Chang, John T.;  Sandborn, William J.;  Lim, Meerana;  Garg, Neha;  Lumeng, Julie C.;  Xavier, Ramnik J.;  Kazmierczak, Barbara, I;  Jain, Ruchi;  Egan, Marie;  Rhee, Kyung E.;  Ferguson, David;  Raffatellu, Manuela;  Vlamakis, Hera;  Haddad, Gabriel G.;  Siegel, Dionicio;  Huttenhower, Curtis;  Mazmanian, Sarkis K.;  Evans, Ronald M.;  Nizet, Victor;  Knight, Rob;  Dorrestein, Pieter C.
收藏  |  浏览/下载:67/0  |  提交时间:2020/07/03

The ability to communicate quantum information over long distances is of central importance in quantum science and engineering(1). Although some applications of quantum communication such as secure quantum key distribution(2,3) are already being successfully deployed(4-7), their range is currently limited by photon losses and cannot be extended using straightforward measure-and-repeat strategies without compromising unconditional security(8). Alternatively, quantum repeaters(9), which utilize intermediate quantum memory nodes and error correction techniques, can extend the range of quantum channels. However, their implementation remains an outstanding challenge(10-16), requiring a combination of efficient and high-fidelity quantum memories, gate operations, and measurements. Here we use a single solid-state spin memory integrated in a nanophotonic diamond resonator(17-19) to implement asynchronous photonic Bell-state measurements, which are a key component of quantum repeaters. In a proof-of-principle experiment, we demonstrate high-fidelity operation that effectively enables quantum communication at a rate that surpasses the ideal loss-equivalent direct-transmission method while operating at megahertz clock speeds. These results represent a crucial step towards practical quantum repeaters and large-scale quantum networks(20,21).


A solid-state spin memory is used to demonstrate quantum repeater functionality, which has the potential to overcome photon losses involved in long-distance transmission of quantum information.


  
Recurrent interactions in local cortical circuits 期刊论文
NATURE, 2020, 579 (7798) : 256-+
作者:  Liu, Yang;  Nguyen, Phong T.;  Wang, Xun;  Zhao, Yuting;  Meacham, Corbin E.;  Zou, Zhongju;  Bordieanu, Bogdan;  Johanns, Manuel;  Vertommen, Didier;  Wijshake, Tobias;  May, Herman;  Xiao, Guanghua;  Shoji-Kawata, Sanae;  Rider, Mark H.
收藏  |  浏览/下载:15/0  |  提交时间:2020/07/03

Most cortical synapses are local and excitatory. Local recurrent circuits could implement amplification, allowing pattern completion and other computations(1-4). Cortical circuits contain subnetworks that consist of neurons with similar receptive fields and increased connectivity relative to the network average(5,6). Cortical neurons that encode different types of information are spatially intermingled and distributed over large brain volumes(5-7), and this complexity has hindered attempts to probe the function of these subnetworks by perturbing them individually(8). Here we use computational modelling, optical recordings and manipulations to probe the function of recurrent coupling in layer 2/3 of the mouse vibrissal somatosensory cortex during active tactile discrimination. A neural circuit model of layer 2/3 revealed that recurrent excitation enhances sensory signals by amplification, but only for subnetworks with increased connectivity. Model networks with high amplification were sensitive to damage: loss of a few members of the subnetwork degraded stimulus encoding. We tested this prediction by mapping neuronal selectivity(7) and photoablating(9,10) neurons with specific selectivity. Ablation of a small proportion of layer 2/3 neurons (10-20, less than 5% of the total) representing touch markedly reduced responses in the spared touch representation, but not in other representations. Ablations most strongly affected neurons with stimulus responses that were similar to those of the ablated population, which is also consistent with network models. Recurrence among cortical neurons with similar selectivity therefore drives input-specific amplification during behaviour.


Computational modelling, imaging and single-cell ablation in layer 2/3 of the mouse vibrissal somatosensory cortex reveals that recurrent activity in cortical neurons can drive input-specific amplification during behaviour.


  
Coherent laser spectroscopy of highly charged ions using quantum logic 期刊论文
NATURE, 2020, 578 (7793) : 60-+
作者:  Oh, Myoung Hwan;  Cho, Min Gee;  Chung, Dong Young;  Park, Inchul;  Kwon, Youngwook Paul;  Ophus, Colin;  Kim, Dokyoon;  Kim, Min Gyu;  Jeong, Beomgyun;  Gu, X. Wendy;  Jo, Jinwoung;  Yoo, Ji Mun;  Hong, Jaeyoung;  McMains, Sara;  Kang, Kisuk;  Sung, Yung-Eun;  Alivisatos, A. Paul;  Hyeon, Taeghwan
收藏  |  浏览/下载:72/0  |  提交时间:2020/07/03

Precision spectroscopy of atomic systems(1) is an invaluable tool for the study of fundamental interactions and symmetries(2). Recently, highly charged ions have been proposed to enable sensitive tests of physics beyond the standard model(2-5) and the realization of high-accuracy atomic clocks(3,5), owing to their high sensitivity to fundamental physics and insensitivity to external perturbations, which result from the high binding energies of their outer electrons. However, the implementation of these ideas has been hindered by the low spectroscopic accuracies (of the order of parts per million) achieved so far(6-8). Here we cool trapped, highly charged argon ions to the lowest temperature reported so far, and study them using coherent laser spectroscopy, achieving an increase in precision of eight orders of magnitude. We use quantum logic spectroscopy(9,10) to probe the forbidden optical transition in Ar-40(13+) at a wavelength of 441 nanometres and measure its excited-state lifetime and g-factor. Our work unlocks the potential of highly charged ions as ubiquitous atomic systems for use in quantum information processing, as frequency standards and in highly sensitive tests of fundamental physics, such as searches for dark-matter candidates(11) or violations of fundamental symmetries(2).


The precision of laser spectroscopy of highly charged ions is improved by eight orders of magnitude by cooling trapped, highly charged ions and using quantum logic spectroscopy, thereby enabling tests of fundamental physics.