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Traffic is the expression of human purpose.
--Tom Vanderbilt, Traffic

On residential streets, engineers attempt to control car speeds by installing traffic-calming devices . You probably know all about the venerable speed bump , but these days you’re more likely to drive over the lower and wider speed hump or the even wider speed table . Many people refer to these generally as sleeping policemen , a richly evocative, perhaps even poetic locution.

In 1861, when the few steam-powered automobiles around were known as road locomotives (or light locomotives), the British Parliament established the world’s first speed limit of 10 miles per hour (16 kilometers per hour). In 1865 a revised law slashed it to 4 mph in the country and 2 mph in towns and villages. Apparently alarmed by even these modest speeds, Parliament also decreed the world’s first traffic-­calming devices: each vehicle had to be preceded by a person walking 60 yards (about 55 meters) ahead and waving a red flag to warn others of the vehicle’s approach.

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Quantum Error Correction: Time to Make It Work

If technologists can’t perfect it, quantum computers will never be big

13 min read
Quantum Error Correction: Time to Make It Work
Chad Hagen

Dates chiseled into an ancient tombstone have more in common with the data in your phone or laptop than you may realize. They both involve conventional, classical information, carried by hardware that is relatively immune to errors. The situation inside a quantum computer is far different: The information itself has its own idiosyncratic properties, and compared with standard digital microelectronics, state-of-the-art quantum-computer hardware is more than a billion trillion times as likely to suffer a fault. This tremendous susceptibility to errors is the single biggest problem holding back quantum computing from realizing its great promise.

Fortunately, an approach known as quantum error correction (QEC) can remedy this problem, at least in principle. A mature body of theory built up over the past quarter century now provides a solid theoretical foundation, and experimentalists have demonstrated dozens of proof-of-principle examples of QEC. But these experiments still have not reached the level of quality and sophistication needed to reduce the overall error rate in a system.

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