In “Errors in the Machine,” Zaira Nazario discusses techniques to correct bugs in quantum computers faster than they can build up. As an early read-in mode (RIM) loader toggler of the PDP-5 minicomputer, which was designed for us at Chalk River Nuclear Laboratories in Ontario 60-odd years ago, I feel like I've stepped into Doctor Who's time machine when reading Nazario's beautiful presentation.

Along these lines, is it possible to separate the entangled qubits physically so that one has two entangled quantum computers? In such a case, this would seem to offer the chance for unhackable communications and navigation, both on Earth and in space. Indeed, if the entangled cohorts respond to each other in a dimension not limited by the speed of light, this could be the technology that eventually allows us to communicate with distant galactic neighbors.

JOHN LENG Flagstaff, Ariz.

NAZARIO REPLIES: Indeed, not only is it possible to have quantum processors physically separated and fully entangled functioning as one larger computer, but this is essential to scale to long-term, error-corrected quantum systems.

Modularity in a quantum system comes in different flavors. Take, for example, superconducting quantum computers. We will be able to parallelize them using classical communication as early as next year. We further envision having separate quantum chips linked inside a single cooling unit to allow gates between distant chips, effectively extending the chip size. We will have longer connections linking quantum processing units (QPUs) in separate cooling units inside a single system to remove bottlenecks in cooling and input/output for qubit control. This will also enable the nonplanar topologies that will allow us to implement more efficient error-correction codes. Finally, as Leng alludes to in his question, longer term, we will connect multiple quantum computers with distributed entanglement among them using technologies that convert the quantum-mechanical signals from microwave to optical frequencies and move them between distant dilution refrigerators via photonic links. At that point, we will have fully entangled clusters of quantum computers in a network resembling the intranet links in supercomputing processors.

This must be accompanied by modularity in software to give us “elastic” computing that quickly expands or decreases computer processing, memory and storage resources to meet demands in real time. Furthermore, run times will weave together QPUs, CPUs (central processing units), GPUs (graphics processing units) and other artificial-intelligence accelerators into a powerful computing fabric that integrates classical and quantum computations, including AI, to speed up the solution of some of the most pressing problems we face as scientists, members of society and business leaders. This computing fabric will be available to anyone in a seamless way, extending through multiple clouds, so that the complexities of the operation or orchestration happening behind the scenes are invisible to them.

Yes, this type of networking could have important implications for computation, communications and navigation. I don't expect quantum networks to beat the speed of light, however.


Designing for Life,” by Carolina Schneider Comandulli, with the Apiwtxa Association, describes lessons in sustainable living from the Indigenous Apiwtxa community in the Amazon basin. I must congratulate the authors on writing such an inspiring article on how there are places that still exist on Earth where humans have figured out how to integrate their society and culture with the ecosystem where they live. It gives ecowarriors everywhere a little slice of hope that ecotopia is possible. I thank the authors for writing it, the editors for including it and Earth for existing right now the way it does so that I was able to read and learn of this culture and place.

ANGELIQUE KAMBEITZ Victoria, British Columbia


In “Holes in the Shield” [May 2021], Daniela Kaufer and Alon Friedman explain how damage to the blood-brain barrier (BBB) may lead to Alzheimer's disease and how animal studies indicate that repairs can make older brains appear young. Because the BBB loses effectiveness in advancing age, the authors note that they have a personal motivation for learning more about it, both being about 50. High levels of stress can cause defects in this important protective filter, and stress is something that an individual can reduce through changes in lifestyle. I hope that the authors or other researchers look into additional ways to protect the BBB that are within the individual's control, such as diet modification, vitamin supplementation and exercise.

STEVE JONES Henderson, Nev.

Kaufer and Friedman describe experiments on mice showing that as the brain ages, the BBB becomes increasingly leaky to substances associated with inflammation and Alzheimer's. And the authors' use of tracer molecules in a study of humans in their mid-20s to mid-70s showed that BBB leakage also increased with age in people. The experiments did not fully prove that a damaged barrier causes Alzheimer's, however.

A noninvasive, ethically acceptable experiment that may help provide conclusive evidence of the role of BBB damage in Alzheimer's might be the following: Using a radioactive tracer, assess for BBB damage in ethnically and racially diverse populations of mentally healthy centenarians. If BBB damage is found in all humans with Alzheimer's, but such damage is absent in cohorts of the oldest mentally healthy humans (or present to no greater extent than it is in a healthy young population), would that not satisfy a criterion for causality?

MARK P. SILVERMAN G. A. Jarvis Professor of Physics, Trinity College


In imploring national leaders to take on racist-driven violence in the U.S. in “Stop Domestic Terrorism” [Science Agenda; April 2021], the editors emphasize, “Building on these steps, we can make it clear that homegrown terror and bigotry are real crimes. With real punishments.”

That last word, “punishments,” in this otherwise necessary editorial left me a little unsettled. I would have preferred “remedies” or similar from a science-based piece. Too often we go back to our religious habit of punishing unwanted behavior instead of finding better ways to counter it.

D. CLOUGH via e-mail


Bold Experiments in Fish Farming,” by Ellen Ruppel Shell, should have said that the system of the proposed major tank facility in Belfast, Me., currently circulates a total of 5,500 gallons of water a minute, not 5,200 gallons.

Voyagers to the Stars,” by Tim Folger [July], should have described NASA shutting down heaters and other nonessential components of the Voyager spacecraft over the past three years, not planning to turn off some of their systems this year.