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Scientists are mashing up steamy old thermodynamics and cutting-edge quantum mechanics into a new field they’re calling quantum steampunk. By combining some of the most established 19th-century science with nano-scale quantum technology, researchers say they can solve complex problems that require versatility and skill in both fields.

Writing for

*Scientific American*, theoretical physicist Nicole Yunger Halpern explains that one of the key concepts shared by both kinds of science is entropy. “Entropy quantifies our uncertainty,” she writes. “According to the second law of thermodynamics, the entropy of a closed, isolated system cannot shrink. This fact underlies the reality that time flows in a single direction.”
New technologies like quantum computers and quantum-based information security end up using physics as much as they use the typical programming ideas of traditional barriers like firewalls or complex passwords. Because of that, entropy is back in the picture as a way to measure unknown quantities in these quantum systems.

“Entropy is often thought of as a single entity, but in fact, many breeds of entropy exist in the form of different mathematical functions that describe different situations,” Yunger Halpern explains. She says the overarching idea of entropy and its application as functions and equations means it’s a good model for a variety of other situations, too. Quantum systems down to the nano-scale level can still be closed in the sense of the definition of entropy.

“Suppose we are trying to use entanglement to share information in a certain channel,” Yunger Halpern muses. “We might ask, Is there a theoretical limit to how efficiently we can perform this task? The answer will likely depend on an entropy.” Instead of an idea like bandwidth over a cable, quantum systems deal directly with how electrons are moving. The speeds or capacities can be pure physics.

There’s a more direct and literal way quantum mechanics and thermodynamics must peacefully coexist. Quantum computers rely on that same direct motion of electrons and other particles, which means they generate heat that usually warms the computer to an extent that it stops running correctly. The idea of fault-tolerant quantum computing may posit ways to vent heat from quantum computers and can channel it into energy-producing heat engines.

What Yunger Halpern concludes is that while thermodynamic entropy is the one the public understands best, the next phase of science will pivot on the quantum kind.

“These entropies quantify not only uncertainty but also the efficiency with which we can perform information-processing tasks, like data compression, and thermodynamic tasks, like the powering of a car,” Yunger Halpern writes.

If you want to feel very dizzy and small in the world, think about the microsystems with entropies that nest into larger systems with larger entropies, and so on and so on—all the way up to the scale of the universe itself. Suddenly, the idea of a computer made by monitoring individual electrons one at a time doesn’t seem so far out.

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