Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero energy end states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) is the etiological agent responsible for the global COVID-19 (coronavirus disease 2019) outbreak. The main protease of SARS-CoV-2, Mpro, is a key enzyme that plays a pivotal role in mediating viral replication and transcription. We designed and synthesized two lead compounds (11a and 11b) targeting Mpro. Both exhibited excellent inhibitory activity and potent anti–SARS-CoV-2 infection activity. The x-ray crystal structures of SARS-CoV-2 Mpro in complex with 11a or 11b, both determined at a resolution of 1.5 angstroms, showed that the aldehyde groups of 11a and 11b are covalently bound to cysteine 145 of Mpro. Both compounds showed good pharmacokinetic properties in vivo, and 11a also exhibited low toxicity, which suggests that these compounds are promising drug candidates.

Review of some recent work ]A multi-input multi-output (MIMO) radar system, unlike standard phased-array radar, can transmit, via its antennas, multiple probing signals that may be correlated or uncorrelated with each other. While the companion article by Blum et al., to appear in the November issue, exploited the diversity offered by widely separated transmit/receive antenna elements, we focus on the merits of the waveform diversity allowed by transmit and receive antenna arrays containing elements that are colocated. For the latter type of MIMO radar systems, we provide an overview of recent results showing that the waveform diversity enables the MIMO radar superiority in several fundamental aspects, including: 1) significantly improved parameter identifiability, 2) direct applicability of adaptive arrays for target detection and parameter estimation, and 3) much enhanced flexibility for transmit beampattern design. Specifically, we show that 1) the maximum number of targets that can be uniquely identified by the MIMO radar is up to M t times that of its phased-array counterpart, where M t is the number of transmit antennas, 2) the echoes due to targets at different locations can be linearly independent of each other, which allows the [ Jian Li and Petre Stoica ] MIMO Radar with Colocated AntennasDigital Object Identifier 10. 1109 /MSP.2007 direct application of many adaptive techniques to achieve high resolution and excellent interference rejection capability, and 3) the probing signals transmitted via its antennas can be optimized to obtain several transmit beampattern designs with superior performance. For example, the covariance matrix of the probing signal vector transmitted by the MIMO radar can be optimized to maximize the power around the locations of the targets of interest and also to minimize the cross-correlation of the signals reflected back to the radar by these targets, thereby significantly improving the performance of adaptive MIMO radar techniques. Additionally, we demonstrate the advantages of several MIMO transmit beampattern designs, including a beampattern matching design and a minimum sidelobe beampattern design, over their phased-array counterparts. In conclusion, the two articles in this issue show that MIMO radar is a fertile research ground that merits further investigation, including reaping the full benefits of both types of diversity covered in the two articles.