Physicists at Penn State University have devised a quantum equivalent of a famous 19th-century thought experiment, capable of creating order out of an array of randomly distributed atoms. Such highly ordered blocks of atoms could one day be used as quantum gates in a quantum computer. The physicists reported their findings last week in Nature.
The second law of thermodynamics—which holds that entropy (disorder) always increases in a closed system—is one of the most unyielding laws of physics. So much so that Arthur Eddington famously proclaimed, “If your theory is found to be against the second law of thermodynamics… there is nothing for it but to collapse in deepest humiliation.”
Demonic thought experiment
Around 1870, James Clerk Maxwell (father of the famous equations for electromagnetism every physics student learns in college) devised a thought experiment offering a potential loophole. He envisioned a tiny imp capable of creating order out of disorder in a closed container filled with gas. The imp accomplished this by making heat flow from a cold compartment to a hot one in apparent violation of the second law. The two compartments would be separated by a wall with a shutter covering a pinhole just large enough for a gas molecule to pass through.
Maxwell’s hypothetical imp would perch atop the dividing wall and open and close the shutter at will. Gas molecules would generally be highly disordered (high entropy), in the sense that they have roughly the same average speed and temperature, and would therefore be close to equilibrium. So there would not be much energy available for “work“—defined in physics as the force over a given distance (W=fd).
Because the atoms that make up the molecules are constantly in motion, there will be small fluctuations over time. Whenever the demon spots a molecule moving a bit faster near the pinhole in the right (cold) compartment, it will open the shutter and let it pass through to the left (hot) compartment. It does the same for any slow-moving gas molecules in the left compartment, allowing them to pass into the colder right compartment. So the molecules in the left compartment get hotter and hotter, while the ones in the right get colder and colder, in an apparent reversal of entropy. Once you have that temperature difference, you basically have a heat pump capable of performing useful work.
“Physicists have come up with clever experiments to bring some version of the demon to the laboratory.”
Statistically, of course, it’s well nigh impossible to sort and separate billions of single molecules by speed or temperature—a bit like throwing a glass of water into the sea and thinking you can get the exact same glass of water back, down to the last identical molecule. In principle, a huge amount of additional energy would be required. Maxwell’s demon supplies that extra energy, so the thought experiment is not a truly closed system, and there’s no violation of the second law.
In recent years, physicists have come up with some pretty clever experiments to bring some version of the demon to the laboratory. For instance, Scottish scientists devised an “information ratchet” in 2007 to create a temperature difference in chemical systems that would otherwise be in thermal equilibrium. The following year, University of Oregon researchers devised an ingenious version using laser light to create a box, with two other lasers to serve as a trapdoor barrier and a sorting “demon,” respectively. Japanese physicists figured out how to coax a nanoscale bead up a spiral staircase in a 2010 paper in Nature, based on the concept of Szilard’s engine. And in 2013, German scientists built an experimental equivalent of Maxwell’s demon out of a pair of interacting quantum dots (tiny bits of semiconductors just a few nanometers wide).
The demon in the lab
“There have been many attempts to devise experimental systems the behave like a demon,” acknowledged David Weiss, team leader and co-author of the new Penn State paper. “There have been some successes at very small scales, but we have created a system in which we can manipulate a large number of atoms, organizing them in a way that reduces the system’s entropy, just like the demon.”
His team used three pairs of laser beams to trap and cool neutral (uncharged) cesium atoms to ultra-cold temperatures (a few degrees above absolute zero) in a 3D lattice with 125 positions (a 5×5×5 cube). They filled half the positions with atoms in random positions and then moved the atoms around by changing the polarization of the laser traps—the equivalent function of Maxwell’s demon, except using position to sort them rather than speed. By this means, they were able to create ordered 5×5×2 or 4×4×3 subsets within the originally disordered lattice, thereby reducing the system’s entropy.
The trick is in the ultra-cold temperatures, according to Weiss, because under those conditions, “the entropy of the system is almost entirely defined by the random configuration of the atoms within the lattice.” If the atoms were not super-cooled, rearranging them would have little effect on the system’s overall entropy. The Penn State scientists were able to decrease entropy in their experiment by a factor of about 2.4.
That makes this a promising option for building qubits. It’s a challenge to use neutral atoms for quantum computing, because their lack of charge means it’s hard to get them to interact sufficiently to become entangled. That’s typically achieved by flipping the state of one qubit depending on the state of a second qubit via a quantum version of a NOT gate.
But there is a high error rate with this method. Reducing the entropy within an atom trap makes it possible to create better quantum gates with fewer errors—and that’s just what Weiss and his team have achieved.