Scientists Reversed a Law of Physics by Scrambling Cause and Effect

3 min readHere’s what you’ll learn when you read this story:Scientists just discovered that the second law of thermodynamics can be massaged by using a qubit as a quantum switch.This implies that engineers could one day build a system that can keep itself cool while performing work, which could have massive implications.This idea was simulated in real life, using a photon beam and a series of switches.An international team of scientists has described a truly bizarre scenario: two quantum chambers sending heat energy from a cooler side toward a warmer side. In other words, the opposite of how heat generally flows. To accomplish their goal, the team capitalized on quantum physics’ most iconic quality—superposition, where a system can exist in two states simultaneously. Superposition creates a dynamic called indefinite causal order (ICO), where a hidden third thing can act on the two reservoirs without being certain of their contents, so it never actually knows which ‘side’ was hot and which was cold. The team’s research appears now in the journal Physical Review Letters.Explorations like this aim to bring quantum and classical physics closer to a unified theory of gravity. For instance, while ICO explains some aspects of this finding, the scientists say their setup also works with classical causality. Journalist Chaitanya Gupta explains the idea at play in an article published in Physics World:“Imagine two chambers of gas side by side, separated by a permeable membrane. Now imagine a small agent, or “demon”, standing next to the partition. What if the demon allows cold gas particles to move into the hotter chamber, but stops the hot gas particles from moving to the colder chamber?”To make sure this setup doesn’t violate the second law of thermodynamics, the demon must occasionally have its memory erased, which restores the overall amount of entropy (disorder) in the system, according to the math. The idea of an interfering demon like this is generally known as a quantum switch, which allows the path of causality to be set or chosen by a qubit (quantum bit), the smallest unit of quantum computing.Traditional computing is ultimately a series of many millions of (non-quantum) switches, which take the form of 0s and 1s that represent the flow of electrical current. A quantum switch does the same in a quantum computing scenario, but it can also theoretically be both 0 and 1 at the same time, enabling superposition.But what about the real world? To bring these mathematically sound ideas to life, the group set up an experiment using light—splitting a beam to realize the idea of superposition, then arranging the two different sub-beams to represent the ordering of the two reservoirs of particles. This setup matched the expected theoretical values at more than 99% fidelity.These findings apply to both the quantum switch in superposition and a specified causal order, but the scientists explain that the quantum switch version may be more flexible for use in “free space photonics”—a scenario where the flow is not going through a fiber optic cable or other solid medium. And without a required medium, scientists could direct beams like this over enormous distances without the added logistics (and overhead cost) of miles of wire or fiber.Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.